Three Pontoon Displacement and Draft Analysis for 2 ft by 8 ft by 60 ft Rectangular Hulls Supporting a 20,000 lb Superstructure

Three Pontoon Displacement and Draft Analysis

Using simple hydrostatics for a rectangular pontoon, draft is set by total weight divided by waterplane area and water density. Below are displacement and submergence estimates for three hull material options, plus practical naval-architecture implications for freeboard and stability.

Configuration and Assumptions

Geometry (each pontoon):

• Width = 2 ft

• Height = 8 ft

• Length = 60 ft

• Waterplane area per pontoon = 2 × 60 = 120 ft²

• Total waterplane area for three pontoons = 3 × 120 = 360 ft²

Loads:

• Superstructure weight (total) = 20,000 lb

• Hull self-weight depends on material option (from prior skin and foam estimates).

• Assumes: closed watertight hulls, load shared across all three pontoons, negligible dynamic effects, no additional payload, no free-surface flooding, and dimensions represent the outer box at the waterline.

Water density:

• Freshwater: 62.4 lb/ft³ (typical inland lakes and rivers)

• Saltwater: about 64.0 lb/ft³ (draft is slightly less)

Key hydrostatic relations (rectangular pontoons):

• Total displacement volume, ( V ) = Total weight / water density

• Draft, ( T ) = Total weight / (water density × total waterplane area)

Hull Weight Inputs Used

These are the same order-of-magnitude weights previously computed (shell plus foam where applicable), used here so draft reflects the material choice:

1. 1/4 inch welded steel shell (no interior foam): ~12,600 lb per pontoon (skin only, stiffeners not included)

2. Foam blocks with 1/4 inch fiberglass outer skin: ~5,850 lb per pontoon (representative mid case: FRP skin plus light foam)

3. Foam blocks with 1/2 inch ferrocement plus wire mesh, with sprayed HDPE coating: ~10,300 lb per pontoon (representative mid case: concrete shell plus light foam)

If your final build includes heavy internal frames, deck stringers, bulkheads, or machinery, add that weight to the totals and draft increases linearly.

Results in Freshwater

Total platform weight = Superstructure + 3 × (pontoon weight)

Option 1: 1/4 inch welded steel, empty interior

• Total weight = 20,000 + 3 × 12,600 = 57,800 lb

• Total displacement volume = 57,800 / 62.4 = 926.3 ft³

• Draft = 57,800 / (62.4 × 360) = 2.573 ft = 30.88 in

• Freeboard (to top of 8 ft hull) = 8.0 − 2.573 = 5.43 ft

Option 2: Foam blocks with 1/4 inch fiberglass outer skin

• Total weight = 20,000 + 3 × 5,850 = 37,550 lb

• Total displacement volume = 37,550 / 62.4 = 601.8 ft³

• Draft = 37,550 / (62.4 × 360) = 1.672 ft = 20.06 in

• Freeboard = 8.0 − 1.672 = 6.33 ft

Option 3: Foam blocks with 1/2 inch ferrocement plus HDPE coating

• Total weight = 20,000 + 3 × 10,300 = 50,900 lb

• Total displacement volume = 50,900 / 62.4 = 815.7 ft³

• Draft = 50,900 / (62.4 × 360) = 2.266 ft = 27.19 in

• Freeboard = 8.0 − 2.266 = 5.73 ft

Saltwater Adjustment (Quick Correction)

Saltwater is denser, so draft reduces slightly:

• Steel option: ~30.10 in draft in saltwater (vs 30.88 in freshwater)

• FRP over foam option: ~19.56 in draft in saltwater (vs 20.06 in freshwater)

• Ferrocement over foam option: ~26.51 in draft in saltwater (vs 27.19 in freshwater)

What These Numbers Mean for a Three-Hull Platform

Draft sensitivity

Because total waterplane area is 360 ft², every additional 1,000 lb increases draft by roughly:

• ( \Delta T \approx 1000 / (62.4 × 360) = 0.0445 ft \approx 0.53 in ) in freshwater.

So payload creep and outfitting weight add up fast but predictably.

Freeboard and reserve buoyancy

All options show substantial freeboard (about 5.4 to 6.3 ft). That is generous for a pontoon, but keep in mind:

• Waves, dynamic loads, and deck edge immersion are governed by the lowest freeboard point, not the average.

• If the superstructure raises center of gravity, you may want more freeboard margin and more beam (spacing) than minimum.

Stability with 15 ft spacing

With three slender hulls separated by 15 ft, you are effectively building a wide multihull. The large transverse separation generally provides:

• High initial stability (large righting moment for small heel angles) driven by the wide “stance.”

• The critical check becomes structural bridging (crossbeams) and torsion between hulls, especially in waves where one hull is supported and another is in a trough.

Material Choice Implication for Draft and Platform Performance

• Lowest draft: Foam with fiberglass skin (about 20 inches freshwater draft in this estimate). Best when weight and corrosion resistance dominate and you can engineer abrasion and puncture protection where needed.

• Middle draft: Ferrocement over foam (about 27 inches). Stiff and abrasion tolerant, but crack control and coating integrity are the make-or-break items for watertight service life.

• Deepest draft: Steel (about 31 inches) in this estimate. Most abuse tolerant, easiest to inspect and repair structurally, but corrosion control and weight management matter.

If you want, I can extend this into a preliminary naval architecture check for: center of gravity estimate for a 20,000 lb superstructure, transverse metacentric height trend for the 15 ft spacing, and a simple crossbeam load model (wave-induced differential buoyancy between hulls).

Preliminary Trimaran Pontoon Stability and Crossbeam Load Analysis for Three 2 ft by 8 ft by 60 ft Hulls Supporting a 20,000 lb Superstructure

Naval-architecture estimate of draft, center of gravity, transverse metacentric height, and wave-induced differential buoyancy loads for a three-pontoon platform using 2 ft by 8 ft by 60 ft rectangular watertight hulls spaced 15 ft apart.

A three-hull pontoon platform has enormous initial (small-angle) stability because the buoyancy acts at widely separated hull centerlines. The design driver becomes structural: crossbeam torsion and racking loads from uneven wave support can be far larger than the static draft problem.

1) Geometry and Known Loads

Each pontoon (rectangular box):

• Width (b = 2 , \text{ft})

• Height (h = 8 , \text{ft})

• Length (L = 60 , \text{ft})

Waterplane area per pontoon:

[

A_{wp,1} = bL = 2 \times 60 = 120 , \text{ft}^2

]

Total waterplane area (3 hulls):

[

A_{wp,total} = 360 , \text{ft}^2

]

Hull spacing: three hulls with 15 ft separation between hulls. For analysis, I assume a classic trimaran layout:

• center hull at (y=0)

• outer hull centerlines at (y=\pm 15) ft

So outer-to-outer centerline separation is 30 ft.

Superstructure weight: (W_{sup}=20{,}000) lb total.

Water density: freshwater ( \rho = 62.4 , \text{lb/ft}^3)

2) Draft Recap (Freshwater) Using Your Three Material Cases

From the prior weight estimates (representative mid-cases):

• Steel option: total weight (W \approx 57{,}800) lb → draft (T \approx 2.573) ft (30.9 in)

• FRP over foam option: (W \approx 37{,}550) lb → (T \approx 1.672) ft (20.1 in)

• Ferrocement over foam option: (W \approx 50{,}900) lb → (T \approx 2.266) ft (27.2 in)

These drafts matter for freeboard and KB (center of buoyancy height).

3) Center of Buoyancy (KB) and Center of Gravity (KG)

3.1 KB for a rectangular pontoon

For a boxy hull at small angles, the center of buoyancy is approximately at mid-draft:

[

KB \approx \frac{T}{2}

]

So:

• Steel: (KB \approx 1.286) ft

• FRP: (KB \approx 0.836) ft

• Ferrocement: (KB \approx 1.133) ft

3.2 KG estimate (dominant driver is superstructure height)

You didn’t specify the superstructure CG height, so I have to assume a reasonable preliminary value.

Assumption used (explicit):

• Pontoon structural CG at 4 ft above pontoon bottom (mid-height of 8 ft box)

• Superstructure CG at 12 ft above pontoon bottom (i.e., ~4 ft above pontoon top, if the deck is near the pontoon top)

Then:

[

KG = \frac{\sum (W_i \cdot KG_i)}{\sum W_i}

]

Using the total weights already used for draft:

Steel case

• Hull weight total (W_h=37{,}800) lb at 4 ft

• Superstructure (20{,}000) lb at 12 ft

• Total (57{,}800) lb

[

KG \approx 6.77 , \text{ft}

]

FRP-over-foam case

• Hull total (W_h=17{,}550) lb at 4 ft

• Total (37{,}550) lb

[

KG \approx 8.26 , \text{ft}

]

Ferrocement case

• Hull total (W_h=30{,}900) lb at 4 ft

• Total (50{,}900) lb

[

KG \approx 7.14 , \text{ft}

]

Important implication: the lighter the hulls, the more the 20,000 lb superstructure dominates KG, pushing KG higher.

4) Initial Transverse Stability (GMt): Why It Will Be Extremely High

For small heel angles:

[

GM_t = KB + BM_t • KG

]

where:

[

BM_t = \frac{I_T}{\nabla}

]

• (I_T) = second moment of waterplane area about the longitudinal axis (roll axis)

• (\nabla) = displaced volume

4.1 Waterplane inertia (I_T) for three separated rectangular pontoons

For one rectangle (length (L), width (b)):

[

I_{rect} = \frac{L b^3}{12}

]

With (L=60), (b=2):

[

I_{rect} = \frac{60 \cdot 8}{12} = 40 , \text{ft}^4

]

Use parallel-axis for outer hulls:

[

I_{total} = I_{center} + 2\left(I_{rect} + A_{wp,1} d^2\right)

]

With (A_{wp,1}=120) ft² and (d=15) ft:

[

I_{total} = 40 + 2\left(40 + 120 \cdot 225\right)

= 40 + 2(40 + 27{,}000)

= 54{,}120 , \text{ft}^4

]

4.2 Displaced volume (\nabla)

[

\nabla = \frac{W}{\rho}

]

• Steel: (\nabla \approx 57{,}800/62.4 = 926 , \text{ft}^3)

• FRP: (\nabla \approx 37{,}550/62.4 = 602 , \text{ft}^3)

• Ferrocement: (\nabla \approx 50{,}900/62.4 = 816 , \text{ft}^3)

4.3 BM and GM results

[

BM_t = I_{total}/\nabla

]

• Steel: (BM_t \approx 54{,}120/926 = 58.4) ft

[

GM_t \approx 1.286 + 58.4 • 6.77 = 52.9 \text{ ft}

]

• FRP-over-foam: (BM_t \approx 54{,}120/602 = 89.9) ft

[

GM_t \approx 0.836 + 89.9 • 8.26 = 82.5 \text{ ft}

]

• Ferrocement: (BM_t \approx 54{,}120/816 = 66.3) ft

[

GM_t \approx 1.133 + 66.3 • 7.14 = 60.3 \text{ ft}

]

What this means in plain naval-architecture terms

• These are enormous GM values. This platform will be very stiff initially: tiny heel angles generate large righting moments.

• In practice, large heel is limited by one hull lifting and by wave/trough support changes; the “initial GM” is not the governing limit state.

• Your design driver becomes structural bridging (crossbeams, deck diaphragm, torsion box behavior), not “will it tip over.”

5) Crossbeam and Bridging Loads: The Real Governing Check

Waves cause differential support: one outer hull rides higher (loses buoyancy) while the other rides lower (gains buoyancy). That creates a torsional couple the cross-structure must transmit.

5.1 Buoyancy change per foot of relative immersion

For one hull:

[

\frac{dF}{dT} = \rho \cdot A_{wp,1} = 62.4 \cdot 120 = 7{,}488 , \text{lb/ft}

]

That is:

• ~7,488 lb per foot of draft change per hull

• ~624 lb per inch per hull

5.2 Differential couple between the two outer hulls

If one outer hull gains (+\Delta F) and the other loses (-\Delta F), the couple magnitude is:

[

M \approx \Delta F \cdot (30 , \text{ft})

]

With (\Delta F = 7{,}488 \cdot \Delta T):

[

M \approx 7{,}488 \cdot \Delta T \cdot 30

= 224{,}640 \cdot \Delta T \quad (\text{ft-lb})

]

Examples:

• 1 ft differential heave between outer hulls:

[

M \approx 224{,}640 , \text{ft-lb}

]

• 2 ft differential heave:

[

M \approx 449{,}280 , \text{ft-lb}

]

• 3 ft differential heave:

[

M \approx 673{,}920 , \text{ft-lb}

]

These moments are transmitted into:

• crossbeams in torsion + bending,

• connections at hull interfaces (hardpoints),

• and the deck structure acting as a shear diaphragm.

5.3 Vertical reaction redistribution (one hull carrying more load)

A second, equally important effect is that hulls do not always share weight equally in waves. In an extreme but very real design scenario, one outer hull can temporarily carry a much larger fraction of the platform load.

A useful preliminary check is to assume, transiently:

• one outer hull carries ~60–70% of total displacement,

• the other outer hull carries ~30–40%,

• center hull may be lightly loaded or partially unloaded depending on wave phasing.

This drives connection shear, local crushing, and fatigue in the beam-to-hull joints. The lighter your hull skins (FRP), the more you must engineer proper load introduction structure (bulkheads, transverse frames, shear keys, inserts).

6) Practical Design Implications and Engineering Recommendations

6.1 Your platform is stability-rich but structurally demanding

• The computed GM values indicate you are nowhere near “tender.”

• But high stiffness means higher accelerations and higher structural demand in chop (comfort and fatigue can become issues).

6.2 Treat the deck and crossbeams as a torsion box

For a 60 ft platform, a “ladder frame” is usually not enough. You want:

• two or more major crossbeams (often 3–5 across 60 ft depending on wave environment),

• a stiff deck diaphragm (plywood/composite/steel plate or truss deck) to carry shear,

• and diagonal bracing or deep beam webs to resist racking.

6.3 Bulkheads and hardpoints matter more than skin thickness

Regardless of hull material:

• Put watertight transverse bulkheads in each pontoon (e.g., every 10–15 ft) to:

• prevent progressive flooding,

• stiffen the hull sides,

• provide beam connection stations.

• Engineer beam interfaces as distributed load into bulkheads/frames, not point loads into skin.

6.4 Material option effects on bridging design

• Steel pontoons: easiest to create robust hardpoints and continuous welded beam seats; best for fatigue resistance if corrosion is controlled.

• FRP over foam: requires deliberate inserts, shear webs, and anti-delamination detailing; lightest, but joint engineering is everything.

• Ferrocement: excellent stiffness locally, but connections need careful detailing to avoid crack initiation and coating damage.

Offshore Trimaran Pontoon Engineering Estimate for 3 ft Differential Heave: Stability, Buoyancy Imbalance, and Crossbeam Torsion Loads

Preliminary naval-architecture and structural load estimate for a three-pontoon platform (2 ft by 8 ft by 60 ft hulls spaced 15 ft) supporting a 20,000 lb superstructure offshore. Includes GM trend, 3 ft differential heave buoyancy imbalance, torsional couple, and practical crossbeam and deck-diaphragm design targets.

For a three-hull pontoon platform, small-angle stability is abundant due to the wide stance, but offshore performance is governed by bridging structure. A 3 ft differential heave between outer hulls creates a large buoyancy imbalance and a torsional couple that must be carried by crossbeams, deck diaphragm action, and robust hull hardpoints.

1) Baseline Geometry and Hydrostatics Used

Each pontoon (rectangular watertight box): 2 ft (beam) × 8 ft (depth) × 60 ft (length)

Waterplane area per pontoon:

• (A_{wp,1} = 2 \times 60 = 120 , \text{ft}^2)

Trimaran layout (assumed):

• Center hull at (y = 0)

• Outer hull centerlines at (y = \pm 15) ft

• Outer-to-outer lever arm = 30 ft

Water density: freshwater ( \rho = 62.4 , \text{lb/ft}^3) (saltwater reduces drafts and increases buoyancy ~2–3%)

Key stiffness metric (for vertical support changes):

[

\frac{dF}{dT}\bigg|•{\text{per hull}} = \rho , A•{wp,1} = 62.4 \times 120 = 7{,}488 , \text{lb/ft}

]

That is 624 lb per inch per hull.

2) Offshore Case: 3 ft Differential Heave Between Outer Hulls

You specified offshore with 3 ft heave. For a conservative bridging load, treat this as a 3 ft relative support change between the two outer hulls (one “up” relative to mean, one “down” relative to mean). Using linear hydrostatics:

2.1 Buoyancy imbalance magnitude

For one hull experiencing a 3 ft change in effective immersion:

[

\Delta F = 7{,}488 \times 3 = 22{,}464 , \text{lb}

]

If one outer hull gains (+22{,}464) lb while the other loses (-22{,}464) lb, the buoyancy difference between outer hulls is:

[

\Delta F_{\text{outer-to-outer}} = 44{,}928 , \text{lb}

]

2.2 Torsional couple on the bridging structure

The twisting couple about the centerline is approximately:

[

M_{torsion} \approx \Delta F \times 30 = 22{,}464 \times 30

= 673{,}920 , \text{ft-lb}

]

Design takeaway: your cross-structure should be conceived as a torsion system sized to resist on the order of 0.67 million ft-lb of twisting couple for this 3 ft differential support case, before applying any offshore safety factors.

3) Initial Stability (GMt) in Context

With hull centerlines at ±15 ft, the initial transverse metacentric radius (BM_t) becomes very large (waterplane inertia dominated by the wide spacing). This means:

• The platform will be very stiff initially (high righting moment at small heel).

• Offshore, the limiting behavior is typically not capsize by small-angle instability; it is:

• cross-structure torsion and racking

• connection fatigue

• slamming / green water

• deck and superstructure load paths

In short: stability is plentiful; structure is the governing problem.

4) How to Allocate the 673,920 ft-lb Couple Into a Practical Structure

A workable conceptual model is:

• Use multiple crossbeams (transverse) + longitudinal girders + a deck diaphragm (shear panel) to share torsion.

• Treat the overall deck/bridge system as a torsion box rather than independent beams.

4.1 Crossbeam count and torque per crossbeam (rule-of-thumb distribution)

If you use (N) primary crossbeams over 60 ft length (evenly spaced), and they share torsion roughly equally:

[

T_{\text{per beam}} \approx \frac{673{,}920}{N} , \text{ft-lb}

]

Example distributions:

• N = 3 (at ~0, 30, 60 ft):

• (T_{\text{per}} \approx 224{,}640 , \text{ft-lb})

• N = 5 (about every 15 ft):

• (T_{\text{per}} \approx 134{,}784 , \text{ft-lb})

• N = 7 (about every 10 ft):

• (T_{\text{per}} \approx 96{,}274 , \text{ft-lb})

Offshore, 5 to 7 major beam stations is a common starting point for a 60 ft platform when the hulls are slender and widely spaced.

5) Preliminary Beam Section Sizing Target Using Closed-Section Torsion

For offshore torsion, closed sections (box girders) are dramatically better than open sections (I-beams, channels), because they carry torsion as near-uniform shear flow.

For a thin-walled closed rectangular box, torsional shear flow is:

[

q = \frac{T}{2A_m}

]

and shear stress:

[

\tau = \frac{q}{t} = \frac{T}{2A_m t}

]

where:

• (T) is torque

• (A_m) is the enclosed median area of the box section

• (t) is wall thickness

Example conceptual steel box beam

Assume a primary crossbeam is a closed steel box approximately:

• Depth = 36 in (3.0 ft)

• Width = 18 in (1.5 ft)

• Enclosed area (A_m \approx 3.0 \times 1.5 = 4.5 , \text{ft}^2)

Case A: N = 5 beam stations

• (T_{\text{per beam}} = 134{,}784 , \text{ft-lb})

[

q = \frac{134{,}784}{2 \times 4.5} = 14{,}976 , \text{lb/ft}

]

Convert (q) to lb/in: (14{,}976/12 = 1{,}248 , \text{lb/in})

If wall thickness (t = 0.25) in:

[

\tau = 1{,}248/0.25 = 4{,}992 , \text{psi} \approx 5.0 , \text{ksi}

]

Case B: N = 3 beam stations

• (T_{\text{per beam}} = 224{,}640 , \text{ft-lb})

[

q = \frac{224{,}640}{9} = 24{,}960 , \text{lb/ft} = 2{,}080 , \text{lb/in}

]

With (t = 0.25) in:

[

\tau = 2{,}080/0.25 = 8{,}320 , \text{psi} \approx 8.3 , \text{ksi}

]

Interpretation: a 36 in × 18 in × 1/4 in closed steel box beam is plausibly in-range for torsion •at this preliminary level•, assuming good load sharing and a true torsion-box deck system.

Offshore caution (important)

The above torsion stress is only part of the story. Offshore you also must design for:

• bending from vertical loads and uneven buoyancy distribution,

• connection load introduction into each hull (often the critical detail),

• fatigue (cyclic torsion + wave-driven racking),

• and local buckling of thin plates (stiffeners may be required).

So the beam wall thickness you end up with may be driven by buckling and fatigue, not average torsion stress.

6) Connection Loads and Hardpoints (What Usually Fails First)

From the 3 ft case, the buoyancy shift for each outer hull is ±22,464 lb. That load must enter the bridge structure through beam seats and bulkheads.

A practical load-path approach offshore is:

• Provide a transverse bulkhead / frame at every primary crossbeam station in each hull.

• Make beam-to-hull interfaces into distributed bearings (wide seat, multiple webs), not point brackets.

• Provide shear keys / diaphragms to prevent relative slip.

• For composite or ferrocement hulls, plan for embedded hardpoints (steel inserts, large backing plates, through-bolts with crush sleeves) to avoid peel/delamination or cracking at attachments.

Rule-of-thumb structural approach:

• Design each beam station connection set (port + starboard) to carry on the order of:

• vertical shear: (\approx 22,464/N) per outer hull (plus dynamic factors),

• uplift on the “unloaded” side,

• and torsion transfer via deck diaphragm action.

7) How the Hull Material Options Affect Offshore Bridging Design

Even though the global buoyancy math is the same, the hull material strongly affects whether the connections and local structure survive:

7.1 1/4 inch steel hulls

• Best for hardpoint strength, weldability, fatigue robustness if corrosion is managed.

• Offshore, steel pontoons integrate well with steel box crossbeams into a continuous torsion structure.

7.2 Foam + 1/4 inch FRP outer skin

• Lowest draft and great reserve buoyancy, but offshore bridging requires:

• internal shear webs, bulkheads, and inserts to introduce crossbeam loads,

• careful laminate schedules to prevent peel and delamination,

• impact and abrasion protection (especially at beam seats and docking areas).

• For offshore, a pure “outer skin only” approach is usually not enough; you want a true sandwich with engineered inner skins/webs or an internal skeleton.

7.3 Foam + 1/2 inch ferrocement + HDPE coating

• Stiff shell, but offshore cyclic racking can drive microcracking.

• The HDPE coating becomes the watertight barrier; the risk is coating damage at cracks, corners, penetrations, and attachment zones.

• If you go this route offshore, the safest architecture is often:

• keep hardpoints as independent steel frames that bypass the ferrocement skin,

• use the ferrocement mainly as shell and abrasion layer, not as the sole load path for bridge torsion.

8) Recommended Preliminary Structural Architecture for Your Offshore 3 ft Heave Case

If you want a conservative, buildable starting point:

1. Use 5 to 7 primary crossbeam stations over the 60 ft length.

• 5 stations if you build a very stiff deck diaphragm and strong longitudinal girders.

• 7 stations if you want lower per-station torsion and better redundancy.

2. Make the primary crossbeams closed box sections (steel or composite box).

• Open sections are torsion-inefficient offshore.

3. Add at least two longitudinal “spine” girders (one port, one starboard) that tie beam stations together.

• This converts the structure into a torsion box rather than isolated frames.

4. Treat the deck as a shear diaphragm (torsion panel).

• Steel deck plate, composite sandwich deck, or heavily fastened plywood/composite diaphragm can work, but it must be detailed as a structural shear panel, not just a walking surface.

5. Put a transverse bulkhead/frame inside each pontoon at every beam station.

• This is the simplest way to prevent local wall crushing and to spread connection loads.

9) What You Can Decide Immediately From This

• Offshore with 3 ft differential support, design around a torsional couple of approximately:

673,920 ft-lb (before offshore safety factors)

• With 5 crossbeam stations, you’re in the ballpark of:

~135,000 ft-lb torsion per station (plus bending and fatigue)

• If you want the “most forgiving” build offshore, steel hulls + steel box crossbeams is typically the most robust path for load introduction and fatigue management.

Offshore Trimaran Pontoon Stability and Structural Loads with 30 ft Beam and 16 ft Superstructure

Engineering estimate of stability, center of gravity, torsional bridging loads, and offshore structural requirements for a three pontoon platform with 30 ft overall beam, 16 ft superstructure height, and 3 ft differential wave heave.

With a 30 ft overall beam and tall superstructure, offshore performance is governed less by tipping and more by structural torsion between hulls. This analysis updates stability, center of gravity, and crossbeam loads for a 3 ft offshore heave condition.

1) Updated Geometry and Reference Levels

Platform type: trimaran style pontoon platform

Number of hulls: 3

Each hull: 2 ft wide × 8 ft tall × 60 ft long

Overall platform beam (outside to outside): 30 ft

Since each outer hull is 2 ft wide, the centerline spacing between outer hulls becomes:

• 30 ft overall − 1 ft − 1 ft

• 28 ft centerline separation

So the torsion lever arm between outer hull buoyancy forces is 28 ft (not 30 ft used previously).

2) Vertical Arrangement

You provided:

• Air draft above water ≈ 16 ft superstructure + 5 ft hull freeboard

• Therefore hull top sits about 5 ft above waterline

• Hull height = 8 ft

• Draft ≈ 3 ft (consistent with earlier weight cases)

Reference elevation (from hull bottom):

| Component | Height above hull bottom |

| -----------------• | -----------------------• |

| Waterline | ~3 ft |

| Hull top / deck | 8 ft |

| Superstructure top | 8 + 16 = 24 ft |

For stability calculations we use center of gravity of superstructure, not top height.

A reasonable engineering assumption:

• Superstructure vertical height = 16 ft

• CG ≈ midpoint = 8 ft above deck

• Deck is 8 ft above hull bottom

So superstructure CG ≈ 16 ft above hull bottom

3) Updated Center of Gravity (KG)

Using the previously estimated weights (steel case shown as the most conservative structurally offshore):

• Hulls total weight = 37,800 lb

• Hull structural CG ≈ 4 ft above hull bottom

• Superstructure weight = 20,000 lb

• Superstructure CG ≈ 16 ft above hull bottom

• Total displacement ≈ 57,800 lb

Combined KG:

[

KG = \frac{(37,800 \times 4) + (20,000 \times 16)}{57,800}

]

[

KG = \frac{151,200 + 320,000}{57,800}

= 8.15 \text{ ft}

]

Interpretation:

The tall superstructure raises the overall center of gravity to about 8.2 ft above hull bottom, which is just slightly above the deck elevation.

4) Updated Transverse Stability (GM)

4.1 Waterplane inertia with 28 ft centerline spacing

Distance from center hull to each outer hull centerline = 14 ft.

Second moment of waterplane area:

• One rectangular waterplane inertia = 40 ft⁴

• Area per hull = 120 ft²

[

I_T = 40 + 2(40 + 120 \times 14^2)

]

[

I_T = 40 + 2(40 + 23,520)

= 47,160 \text{ ft}^4

]

4.2 Displacement volume

Steel case (representative offshore robust option):

[

\nabla = \frac{57,800}{62.4} = 926 \text{ ft}^3

]

4.3 Metacentric radius

[

BM = \frac{I_T}{\nabla} = \frac{47,160}{926} = 50.9 \text{ ft}

]

4.4 Center of buoyancy

Draft ≈ 3 ft →

[

KB \approx 1.5 \text{ ft}

]

4.5 Final transverse GM

[

GM = KB + BM • KG

]

[

GM = 1.5 + 50.9 • 8.15

= 44.25 \text{ ft}

]

5) Stability Meaning in Real Offshore Behavior

A GM of about 44 ft is extremely large.

This means:

• Very strong initial righting moment

• Very small heel angles under wind or load shift

• Platform behaves very stiffly

• Motions will be abrupt rather than slow

Important:

Capsize is not the governing failure mode.

Structural bridging and connection fatigue are.

6) Offshore Differential Heave Loads (3 ft)

Vertical buoyancy stiffness per hull:

[

62.4 \times 120 = 7,488 \text{ lb per ft immersion}

]

For 3 ft relative support difference:

[

\Delta F = 7,488 \times 3 = 22,464 \text{ lb}

]

One outer hull gains this much support while the other loses the same.

7) Updated Offshore Torsional Couple (28 ft lever arm)

[

M = 22,464 \times 28

= 629,000 \text{ ft-lb (approx)}

]

This is the twisting moment the cross structure must transmit during offshore wave support imbalance.

8) Recommended Structural Design Targets Offshore

For a 60 ft platform with this torsion magnitude:

Minimum structural configuration

• 5 to 7 primary crossbeam stations

• Closed box beam sections

• Continuous longitudinal girders

• Structural deck diaphragm

• Bulkheads at every beam station in each hull

Torsion per beam (if 5 stations)

[

629,000 / 5 = 126,000 \text{ ft-lb per beam}

]

Torsion per beam (if 7 stations)

[

629,000 / 7 = 90,000 \text{ ft-lb per beam}

]

These values are before offshore dynamic safety factors.

9) Wind Heeling Check (Preliminary Insight)

With:

• 21 ft total air draft above water (5 ft freeboard + 16 ft structure)

• Large vertical projected area

Wind overturning will create additional heeling moment, but because the righting arm is extremely large, wind rarely governs capsize.

However wind does contribute to cyclic torsion, which accelerates fatigue in beam connections.

10) Engineering Conclusions

Stability

• Very high transverse stability

• Capsize unlikely under normal offshore conditions

Governing design driver

• Crossbeam torsion

• Hull to beam load introduction

• Fatigue from cyclic wave loading

Structural scale of problem

Offshore 3 ft differential support produces:

• ~22,500 lb buoyancy shift per outer hull

• ~629,000 ft-lb torsional couple

This is the primary structural design load.

Material implications offshore

• Steel hulls integrate most cleanly into torsion box structure

• FRP hulls require engineered inserts and internal frames

• Ferrocement hulls require independent steel load frames to avoid cracking at connections

If you want, the next step is highly practical and very useful:

I can size a preliminary crossbeam box girder section (steel or composite) with safety factor for offshore fatigue loading and give wall thickness, plate sizing, and beam spacing recommendations.

Crossbeams

Below is a preliminary composite box-girder crossbeam concept sized to carry the offshore 3 ft differential heave torsion you specified, with an explicit fatigue safety factor, plus beam spacing and deck diaphragm recommendations. This is a concept-level scantling (good for early weight/cost/layout), not class-approved.

1) Design load to size the crossbeams

From your geometry:

• Overall platform beam (outside-to-outside) = 30 ft

• Outer hull width = 2 ft each → outer hull centerline spacing = 30 − 1 − 1 = 28 ft

• Waterplane area per hull = 2 × 60 = 120 ft²

• Buoyancy stiffness per hull = 62.4 × 120 = 7,488 lb/ft

• Differential heave = 3 ft

Buoyancy change on one outer hull:

• ΔF = 7,488 × 3 = 22,464 lb

Torsional couple about the centerline (outer hulls):

• M_total = 22,464 × 28 ≈ 629,000 ft-lb

How many primary beam stations?

Offshore + fatigue: I recommend 7 primary crossbeams across the 60 ft length (≈ every 10 ft), tied together with longitudinal box girders and a structural deck diaphragm.

Torsion per beam station (equal-share first cut):

• T_per ≈ 629,000 / 7 = 89,900 ft-lb

Fatigue / offshore safety factor

For cyclic offshore torsion + uncertainties in load sharing, use SF = 2.0 on torsion for preliminary sizing:

• T_design per beam ≈ 180,000 ft-lb

• In inch-pounds: 180,000 × 12 = 2.16 million in-lb

This is the value I size to below.

2) Composite box-girder section recommendation (torsion-governed)

Why a closed box

A closed section is mandatory offshore. Open sections (I, channel, open truss without a shear diaphragm) are torsion-inefficient and fatigue-prone.

Target shear flow and laminate thickness

For a thin-walled closed box, torsion shear flow is approximately:

• ( q = \dfrac{T}{2 A_m} )

where (A_m) is the enclosed median area of the box.

Practical beam size that works without being absurd

A good offshore starting section for a 28 ft stance is:

Primary composite box beam (each station)

• Overall depth: 48 in (4 ft)

• Overall width: 36 in (3 ft)

• Construction: sandwich walls (skins + foam core), fully closed, with internal diaphragms

Median enclosed area (first cut):

• (A_m \approx 48 \times 36 = 1,728 , \text{in}^2)

Shear flow around the section:

• ( q = 2.16\times10^6 / (2 \times 1,728) \approx 624 , \text{lb/in} )

To keep cyclic shear stress conservative for long-life fatigue in glass laminates, I use a design allowable in-plane shear of about 2,000 psi for the “torsion-carrying ±45 laminate” (conservative; exact depends on fiber/resin, QA, environment).

Required effective laminate thickness per wall:

• ( t \approx q / \tau_{allow} \approx 624 / 2000 \approx 0.31 , \text{in} )

So: target ~0.31 in effective torsion laminate thickness around the perimeter (distributed in the skins of the sandwich walls).

3) Proposed laminate and core build-up (actionable starting scantling)

Wall construction (top, bottom, sides)

Sandwich wall (each face):

• Outer skin thickness: 0.16 in

• Core thickness: 1.5–2.0 in closed-cell structural foam (H80–H130 class range; pick higher density at hardpoints)

• Inner skin thickness: 0.16 in

• Total wall thickness: ~1.82 to 2.32 in

This gives total skin thickness per face = 0.32 in, matching the torsion thickness target above.

Skin fiber architecture (fatigue-friendly)

To carry torsion, prioritize ±45:

Each 0.16 in skin (typical):

• ~70% thickness in ±45 biax (torsion/shear)

• ~30% thickness in 0/90 (bending + stability + handling damage)

A practical stacking intent (not ply-count specific, since fabrics vary):

• Outer skin: [±45 / 0/90 / ±45 / ±45] (balanced, symmetric about mid-skin if you want higher quality)

• Inner skin: mirror similar

Add caps for bending (recommended)

Even though torsion is the driver, offshore racking produces bending too. Add UD caps (carbon or glass UD) at the top and bottom faces:

• Top UD cap strip: 6–10 in wide, equivalent 0.06–0.10 in UD thickness

• Bottom UD cap strip: same

If cost allows, carbon UD caps pay off enormously in stiffness and fatigue.

4) Internal diaphragms and local buckling control

Composite box walls can buckle locally long before laminate shear strength is reached if they are “too panel-like.”

Inside each crossbeam station box girder:

• Install internal transverse diaphragms at:

• every hull interface, and

• midspan, and

• optionally at ~3–5 ft spacing within the box if you want very high fatigue margin.

• Diaphragm thickness: sandwich panel ~0.75–1.25 in total, with ±45 skins.

These diaphragms:

• prevent ovalization,

• spread connection loads into the skins,

• greatly improve fatigue life.

5) Beam spacing recommendations along the 60 ft platform

For your offshore 3 ft differential heave case, recommended beam stationing:

Primary crossbeams

• 7 primary crossbeams @ 10 ft spacing along the 60 ft length

(Stations at ~0, 10, 20, 30, 40, 50, 60 ft)

Secondary cross members (optional but helpful)

• Add secondary transverse members halfway between primaries (5 ft spacing) if:

• the deck is not a stiff diaphragm, or

• superstructure point loads are high, or

• you want reduced deck panel spans.

6) Deck diaphragm (this is non-negotiable offshore)

To actually achieve the “equal share” torsion assumption and reduce beam fatigue, the deck must act as a shear diaphragm tying the beam stations into a torsion box.

Recommended structural deck panel

• Composite sandwich deck:

• skins: 0.10–0.14 in each (heavier near beam lines)

• core: 1.5–2.5 in foam

• include ±45 dominated skins for shear

Deck-to-beam connection:

• continuous bonding + mechanical fastening strategy (or continuous secondary bonding with wide flanges)

• avoid peel-prone details; use large-radius tapers and wide flanges

7) Hull hardpoints and load introduction (where offshore failures happen)

At each beam station on each hull:

• Provide a bulkhead/frame inside the hull directly under the beam seat.

• Use distributed seat length (not point pads). Think 3–6 ft of bearing length per interface if geometry allows.

• Use high-density inserts (solid laminate or high-density core) in the beam flange region.

• Design joints so the crossbeam loads go into:

• bulkheads,

• webs,

• and longitudinal hull stiffeners,

not just into a skin panel.

8) Summary of the preliminary composite beam spec

Baseline offshore preliminary crossbeam (per station):

• Closed composite box girder

• 48 in deep × 36 in wide

• Sandwich walls: 0.16 in skin + 1.5–2.0 in core + 0.16 in skin (each face)

• Skins: ±45 dominated, with some 0/90; add UD caps top/bottom

• Internal diaphragms: at least at hull interfaces + midspan; more if you want fatigue margin

• Spacing: 7 stations at 10 ft along the 60 ft platform

• Deck: structural sandwich diaphragm (±45 heavy)

This concept is sized to ~180,000 ft-lb torsion per beam station (which is your 3 ft offshore heave torsion case with SF=2).

Load Sharing Crossbeams

Below is a more realistic load-sharing model (deck diaphragm + longitudinal girders) and a bending check for a credible offshore “one outrigger in crest / the other in trough / center partially supported” scenario. Then I adjust the composite box-girder section accordingly.

This is still preliminary scantling-level (concept design). Final design should be checked to a chosen ruleset (ABS/DNV/ISO) with a wave spectrum, fatigue category, and connection details.

1) Known platform numbers (recap)

Trimaran pontoons

• 3 hulls, each: 2 ft × 8 ft × 60 ft

• Overall beam outside-to-outside: 30 ft

• Outer hull centerline spacing: 30 − 1 − 1 = 28 ft

• Center hull at midline; outer hulls at ±14 ft.

Hydrostatic “spring” per hull

• Waterplane area per hull: (A_{wp}=2×60=120\ \text{ft}^2)

• Fresh water (ρ = 62.4\ \text{lb/ft}^3)

• Vertical stiffness per hull:

[

k_h = ρA_{wp} = 62.4×120 = 7,488\ \text{lb/ft}

]

Offshore differential heave specified

• Relative heave between outer hulls: 3 ft

• Buoyancy change magnitude per outer hull:

[

ΔF = k_h×3 = 7,488×3 = 22,464\ \text{lb}

]

• Resulting torsional couple (about centerline):

[

M_{global} = ΔF × 28 = 22,464×28 \approx 629,000\ \text{ft-lb}

]

This (M_{global}) is the “platform twist demand” generated by the sea state.

2) More realistic torsion distribution (deck + longitudinals reduce peak demand per crossbeam)

2.1 Why the earlier “equal share by N beams” is conservative

If you have:

• a stiff deck diaphragm (shear panel),

• two longitudinal box girders (port and starboard) running the length,

• and multiple transverse beams,

…then the platform behaves like a torsion box. In a torsion box, torque is carried continuously as shear flow around the perimeter, not dumped into a single crossbeam station.

So the right quantity to size a given crossbeam for is torque transfer between bays, not the full global torque.

2.2 A practical torsion-sharing model (bays + participating length)

For offshore waves, the “outer hull crest vs trough” condition does not occur at a single cross-section; it is distributed over some participating length (L_p). A reasonable preliminary value is ~30 ft (about half of a local wave length that significantly loads a 60 ft platform).

If your primary crossbeams are spaced at 10 ft (7 stations), then ~3 bays sit inside that 30 ft participating length.

Engineering approximation (bay shear transfer):

• Peak •bay• torque is on the order of:

[

T_{bay} \approx M_{global}\times\frac{s}{L_p}

]

where (s) is beam spacing.

Using (M_{global}=629k\ \text{ft-lb}), (s=10\ \text{ft}), (L_p=30\ \text{ft}):

[

T_{bay} \approx 629,000×\frac{10}{30} \approx 210,000\ \text{ft-lb}

]

This is the torque that must be transferred from one bay of the torsion box to the next (i.e., what the local transverse framing/diaphragm system must “hand off”).

Fatigue + uncertainty factor

Instead of SF=2.0 on the entire global couple at each station, a more realistic approach is:

• use the bay-transfer torque above, then apply:

• 1.5 fatigue/uncertainty factor (preliminary offshore composite)

So:

[

T_{design} \approx 210,000×1.5 \approx 315,000\ \text{ft-lb}

]

In in-lb:

[

T_{design} = 315,000×12 = 3.78×10^6\ \text{in-lb}

]

Interpretation: With a real torsion box, it’s credible that •peak per-station torsion demand• is closer to a bay-transfer torque than the full (629k\ \text{ft-lb}). That often lets you reduce thickness modestly—•provided the deck and longitudinals are truly structural•.

3) Bending check for a reasonable offshore wave scenario

You asked specifically: “one outer hull in crest, other in trough, center partially supported.”

A pragmatic, conservative transverse load-transfer model is:

• Starboard outer hull loses buoyancy: (-ΔF=-22,464\ \text{lb})

• Port outer hull gains buoyancy: (+ΔF=+22,464\ \text{lb})

• Center hull “partially supported”: assume it is down 1.5 ft relative (half of 3 ft outer-to-outer), so it loses:

[

ΔF_c = 7,488×1.5 = 11,232\ \text{lb}

]

So at that cross-section, compared to calm-water support, the buoyancy changes are:

• Port: +22,464 lb

• Center: −11,232 lb

• Starboard: −22,464 lb

Net = −11,232 lb (the platform “needs” that much additional support from adjacent sections / dynamic effects). That’s realistic offshore: loads redistribute longitudinally; crossbeams and deck see racking and bending.

3.1 Conservative bending demand on a crossbeam station

The most punishing local bending happens when an outrigger is effectively “hanging” and must be supported by the cross-structure. A conservative idealization is to treat the centerline longitudinal girder + deck diaphragm as the main support, and the outrigger connection behaves like a cantilever arm of length 14 ft (center hull CL to outer hull CL).

Use an end load equal to the outrigger buoyancy deficit:

• (P = 22,464\ \text{lb})

• Cantilever length (L = 14\ \text{ft})

Moment at the root:

[

M_{bend} \approx P×L = 22,464×14 \approx 314,500\ \text{ft-lb}

]

In in-lb:

[

M_{bend} \approx 3.77×10^6\ \text{in-lb}

]

That is a very useful preliminary number because it is in the same order as the torsion bay-transfer torque above—meaning both torsion and bending matter, even with a good torsion box.

Fatigue factor on bending

Apply 1.5 as well (offshore cyclic):

[

M_{design} \approx 314,500×1.5 \approx 472,000\ \text{ft-lb}

]

[

M_{design} \approx 5.66×10^6\ \text{in-lb}

]

4) Adjusted composite crossbeam concept (reduced width and modest skin reduction, but add UD caps)

Given the improved torsion distribution, you can typically reduce •some• wall thickness if you add bending-efficient UD caps and keep the section closed and well-diaphragmed.

4.1 Revised primary crossbeam box section

Recommended revised crossbeam (per 10 ft station):

• Depth: 48 in (4.0 ft)

• Width: 30 in (2.5 ft) (down from 36 in)

• Closed composite box, sandwich walls, with internal diaphragms

Median enclosed area:

• (A_m \approx 48×30 = 1,440\ \text{in}^2)

Torsion shear flow at (T_{design}=3.78×10^6\ \text{in-lb})

[

q = \frac{T}{2A_m} = \frac{3.78×10^6}{2×1,440} \approx 1,313\ \text{lb/in}

]

If we keep a conservative fatigue shear allowable ~2,000 psi for the effective ±45 shear laminate:

[

t_{shear} \approx q/τ \approx 1,313/2,000 \approx 0.66\ \text{in}

]

That looks thick if interpreted as “single-wall laminate.” But in a real sandwich torsion box:

• shear is carried by both skins on each wall (outer+inner),

• and shear flow is shared around the entire perimeter,

• and diaphragms reduce distortion and peak shear concentrations.

So the right response is not “0.66 inch solid laminate everywhere,” but:

• maintain strong ±45 skins,

• use diaphragms,

• and treat connection zones with local solid laminate build-ups.

A workable offshore starting point is:

4.2 Wall construction (global)

Sandwich wall build (each face of the box):

• Outer skin: 0.18 in

• Core: 2.0 in (closed cell structural foam; higher density near joints)

• Inner skin: 0.18 in

Total wall thickness ≈ 2.36 in with 0.36 in total skins per face.

Skin fiber balance (fatigue):

• ±45 dominates (torsion/racking): ~65–75% of skin thickness

• 0/90 remainder for bending stability, impact resistance, fastener zones

This is slightly heavier skin than my earlier “0.16 + 0.16” concept, but it aligns better with the bay-transfer torque + fatigue you want to survive offshore.

4.3 UD caps for bending (lets you keep walls reasonable)

Use the bending demand to size cap area.

For a box beam, bending is mainly resisted by the top and bottom caps separated by ~48 in.

Using (M_{design} = 5.66×10^6\ \text{in-lb}) and cap separation (d≈48\ \text{in}):

[

F_{cap} \approx \frac{M}{d} \approx \frac{5.66×10^6}{48} \approx 118,000\ \text{lb}

]

If you use carbon UD for fatigue efficiency, use a conservative cyclic stress target (order-of-magnitude) 50 ksi in the UD (you may go higher with good QA and resin choice; I’m staying conservative):

Required UD area per cap:

[

A = F/σ \approx 118,000/50,000 \approx 2.36\ \text{in}^2

]

If cap strip width = 10 in, required carbon UD thickness per cap:

[

t = A/b \approx 2.36/10 \approx 0.24\ \text{in}

]

Recommendation:

• Carbon UD cap each on top and bottom: 10 in wide × 0.25 in equivalent UD thickness, tapered over 2–3 ft into the skins.

If you prefer all-glass (cheaper, heavier), glass fatigue stress targets are much lower, and cap thickness rises substantially (often uneconomic at this load level offshore).

5) Diaphragms and beam spacing (updated recommendations)

5.1 Beam spacing

Given offshore + fatigue and your 60 ft length:

• Primary crossbeams: every 10 ft (7 stations)

• Add one secondary transverse member between primaries (optional) if deck is not a true structural diaphragm.

5.2 Internal diaphragms inside each box beam (high value for fatigue)

To control distortion (which drives peak shear and fatigue):

• Put diaphragms at:

• each hull interface (3 locations: port, center, starboard)

• and at least one mid-bay diaphragm

• If you want stronger fatigue margin: diaphragms every 3–4 ft inside the crossbeam.

Diaphragm construction:

• sandwich panel ~1.0–1.5 in total, ±45 heavy skins.

6) Deck + longitudinal girders (the assumptions you must actually build)

The reduced peak station demand depends on real torsion-box behavior. Minimum architecture:

Longitudinal girders (port and starboard)

• Run full length, aligned near the outrigger hull connections.

• Prefer closed composite box longitudinals (or deep sandwich webs + closed flanges).

• These longitudinals, plus deck diaphragm, are what make torsion “continuous.”

Deck diaphragm

• Composite sandwich deck:

• skins: 0.12–0.18 in each (±45 heavy)

• core: 2–3 in

• Must be continuously bonded to:

• longitudinal girders and

• crossbeam top flanges

so it truly carries shear.

7) Final adjusted section recommendation (what I’d start drawing)

Primary crossbeam (each 10 ft station)

• 48 in deep × 30 in wide closed composite sandwich box

• Walls: 0.18 in skin / 2.0 in core / 0.18 in skin

• Skins: ±45 dominated, remainder 0/90

• Carbon UD caps (top + bottom): 10 in wide × 0.25 in UD equivalent thickness, tapered

• Internal diaphragms: at hull interfaces + midspan (more if you want fatigue margin)

• Heavy local solid laminate + high-density core inserts at all connection zones

This is a realistic offshore concept that recognizes:

• station torsion is better treated as bay transfer torque (with a real torsion box),

• bending can be severe during partial support conditions,

• carbon UD caps are the cleanest way to control weight while meeting fatigue needs.

Vertical Post as Torsional Control

Got it. Those 2×2×24 ft vertical posts at 8 ft spacing, integral to each hull and tied into the superstructure, materially change the load path:

• The platform is no longer just three pontoons bridged by a few deep crossbeams.

• You now have a 3D space frame: pontoons + a forest of vertical stanchions + a superstructure that can act as a torsion box / Vierendeel frame.

• Offshore differential buoyancy loads can be shared along the length through the post grid, rather than being forced through a small number of discrete crossbeam stations.

Below is a reassessment specifically for superstructure crossbeams fastened to the posts.

1) Clarify the framing grid implied by your description

• Post spacing along length: 8 ft

• Hull length: 60 ft → approx 8 bays (0, 8, 16, 24, 32, 40, 48, 56, 60). In practice you’ll have ~8 full bays plus end framing.

• Rows of posts: 3 rows (one per hull)

• Transverse span between outer hull post lines:

• Outer hull centerlines spaced 28 ft

• If posts are near hull centerlines, the main transverse span between port and starboard post lines is ~28 ft

• Centerline row at midspan.

This means your superstructure sees something close to a multi-bay, multi-row grillage.

2) Offshore 3 ft differential heave: how the load distributes into the post grid

Using the same hydrostatic stiffness per hull:

• (k_h = 7,488 , \text{lb/ft})

• Outer-to-outer relative heave: 3 ft → each outer hull buoyancy change magnitude:

• (ΔF = 22,464 , \text{lb})

Key difference now

Instead of one crossbeam station takes a big torque, the post grid spreads this into multiple frames.

A reasonable first-cut assumption for preliminary sizing is that the wave-induced buoyancy mismatch is effective over a participating length (L_p) offshore. A common preliminary value is still ~30 ft (half the platform length).

Number of post frames engaged:

• (N = L_p / 8 \approx 30/8 \approx 3.75) → use 4 frames engaged at peak.

So the outer-hull buoyancy change per engaged frame is roughly:

[

ΔF_{frame} \approx 22,464 / 4 \approx 5,616 , \text{lb}

]

That means at peak:

• one outer hull post line is pushing up ~5,600 lb per frame,

• the other outer hull is dropping out by ~5,600 lb per frame,

• the center hull may be partially supported (often intermediate).

This is an order-of-magnitude reduction versus concentrating the full 22,464 lb into one station.

3) What the superstructure crossbeam at each post frame must do

At a given 8-ft frame, the transverse crossbeam system must:

1. transfer vertical reaction differences between the three hull post lines

2. resist racking (transverse sway) and torsion through the superstructure box action

3. survive fatigue (cyclic vertical + torsion + bending)

A conservative vertical load case for one 8-ft frame

Use:

• Port outer: +5,616 lb

• Starboard outer: −5,616 lb (loss of support; this shows up as required hanging support through structure)

• Center hull: partially supported; for conservatism, assume center loses half the outer differential:

• center: −2,808 lb

Then the crossbeam system is creating a load path that can easily see:

• ~5–8 kip vertical transfers at the outer nodes,

• plus additional axial forces from torsion-box action.

This is now very compatible with a repetitive light-to-medium composite crossbeam at every 8 ft, rather than a few massive beams.

4) The 2×2×24 ft post issue: buckling governs unless it’s a built-up composite column

A literal 2 in × 2 in post that is 24 ft tall will buckle at very low load if it’s slender and not laterally braced. That’s physics, independent of material.

So one of these must be true for your concept to work offshore:

• The 2×2 is feet, not inches; or

• The post is strongly laterally braced by the superstructure at multiple elevations (effectively reducing unbraced length to maybe 4–6 ft); or

• The post is not a simple solid bar; it is a composite box/pipe column with a much higher radius of gyration; or

• The post is acting mainly in tension (tie) rather than compression (unlikely offshore).

I’ll proceed assuming the posts are continuously tied into decks/rails/walls so the effective unsupported length is on the order of 8 ft or less (since your frames are every 8 ft and presumably tied with longitudinal members).

If your posts are truly unbraced for 24 ft, we need to redesign the post concept first.

5) Reassessed superstructure crossbeam sizing concept (composite), per 8-ft frame

Structural approach recommended

At each 8-ft station, make a transverse closed box crossmember between the outer post lines, with a node at the center post line. Think of it as:

• Port outer node → Center node → Starboard outer node

• Prefer closed composite box segments for fatigue and torsion

• Strongly connect into longitudinal members to form a torsion box

Target design loads per frame (preliminary)

From above:

• Vertical transfer per outer node: ~5.6 kip

Apply offshore fatigue/uncertainty factor 1.7 (prelim composite):

• Design nodal vertical = ~10 kip (round up)

So size each 8-ft transverse member to tolerate:

• 10 kip up or down at outer nodes, with cyclic reversal

Transverse span to centerline:

• 14 ft from center hull to each outer hull post line

So each arm behaves like a 14-ft beam in bending.

Bending moment per arm (conservative cantilever-like check)

If an outer node is hanging with 10 kip:

[

M \approx P \times L = 10{,}000 \times 14 = 140{,}000 \text{ ft-lb}

]

That’s per arm at the center node if it’s acting like a cantilever to the centerline.

In reality, the opposite arm and torsion-box action shares this, so this is conservative.

6) Recommended composite crossbeam section per 8-ft frame

A practical, buildable starting section for each transverse member (each arm) is much smaller than the earlier 48×30 beam because you now have many frames.

Option A: Closed composite box arm (preferred)

Per arm (centerline to outer hull post line):

• Depth: 18 in

• Width: 10–12 in

• Length: 14 ft

• Sandwich walls:

• skins: 0.12 in each side (total skins per wall 0.24 in)

• core: 1.0–1.5 in (higher density near nodes)

Add UD caps top and bottom:

• Carbon UD cap: 4–6 in wide × 0.10–0.16 in equivalent UD thickness (top and bottom), tapered

Internal diaphragms:

• one diaphragm at mid-arm

• one at each node (center and outer)

This is aimed at high fatigue life with manageable weight.

Option B: Deeper but narrower (better stiffness)

If headroom allows, go:

• Depth 24 in, width 8–10 in, similar skins/core

This reduces deflection and cap demand.

7) Longitudinal members are now the main torsion carriers

With posts every 8 ft, the longitudinal stringers/girders in the superstructure become the primary path for:

• global torsion (port vs starboard differential),

• racking,

• wave-induced twist.

Recommendation

Provide at least:

• Two longitudinal closed box girders along each outer hull post line (one low, one high), OR one very deep closed member plus a diaphragm wall.

• Tie into every post at every station.

• Treat the superstructure sides as shear walls (composite panels with ±45 skins).

If you do this, peak torsion per crossbeam drops significantly, and the repeated 8-ft frames become mostly local distributors, not global torque resistors.

8) Connection design (critical offshore composite detail)

Where each crossbeam meets a post:

• use node blocks (solid laminate) and large-radius tapers

• avoid peel by carrying load into the post via:

• wrap plies around the node,

• through-bolted clamp plates only if crushing is controlled with inserts,

• bond + mechanical redundancy offshore.

At outer hull interface, add:

• bulkhead inside hull aligned with post

• spread load into hull with webs, not just skin

9) What I need to lock this down (but I’ll still give you a default)

Two items determine whether 18×10 in is plenty or marginal:

1. Post material (steel tube, composite box, aluminum?) and whether posts are braced at intermediate heights (rails, walls, decks).

2. Whether the superstructure has solid side walls (shear panels). If it’s an open deck with only beams and posts, members must be much heavier.

Default assumption for this sizing (explicit)

• Posts are composite or steel tubes with good bracing at least every 8 ft

• Superstructure includes at least partial shear panels (or a stiff deck diaphragm)

Under that, the 18–24 in deep closed box transverse members at every 8 ft are the right direction.

Updated Recommendation Summary

Crossbeam system (superstructure, every 8 ft)

• Use repeated transverse closed box members (center-to-outer arms)

• Each arm: 18–24 in deep × 10–12 in wide composite sandwich box

• Skins: 0.12 in + 0.12 in (±45 dominated)

• Core: 1.0–1.5 in

• Carbon UD caps: 0.10–0.16 in equivalent top and bottom, 4–6 in wide

• Diaphragms at nodes + midspan

Global torsion control

• Add continuous longitudinal closed members on outer post lines

• Add deck diaphragm and (ideally) side shear walls

Post Assessment

Below is a reassessment with your updates:

• Posts: 2 ft × 2 ft × 24 ft, integral to each hull

• Post frames: every 8 ft along 60 ft length

• Superstructure: solid side panels (acts as shear walls / torsion box)

• Offshore case: 3 ft differential heave between outer hulls

• Overall beam: 30 ft outside-to-outside → outer-hull CL spacing ≈ 28 ft (lever arm)

This changes the system from a few giant crossbeams to a multi-bay space frame / torsion box, so peak demand on any single crossbeam is materially reduced—•provided you actually detail the deck + side panels as structural shear diaphragms•.

1) Post axial load and buckling check

1.1 How many posts?

Along 60 ft with 8 ft spacing, you have about 9 frames (at ~0, 8, 16, 24, 32, 40, 48, 56, 60).

Each frame has 3 posts (one per hull row) → 27 posts total.

1.2 Gravity axial per post (superstructure only)

Superstructure weight = 20,000 lb.

If distributed evenly to all posts:

• ~740 lb/post (20,000 / 27)

Even if you assume nonuniform distribution and use 2× for conservatism:

• ~1,500 lb/post gravity axial

This is small relative to the section size.

1.3 Offshore wave-induced axial at a post frame (order-of-magnitude)

From hydrostatics:

• Per hull vertical stiffness (k_h = 7,488\ \text{lb/ft})

• 3 ft differential heave → outer hull buoyancy change magnitude:

• (ΔF = 22,464\ \text{lb})

With solid side panels + deck diaphragm, the torsion and vertical imbalance are shared along a participating length. A reasonable preliminary offshore participating length for a 60 ft platform is ~30 ft.

Number of 8-ft frames within 30 ft ≈ 4 frames.

So per engaged frame, outer-hull buoyancy change magnitude is roughly:

• (ΔF_{frame} \approx 22,464 / 4 \approx 5,616\ \text{lb})

That means at a peak frame:

• one outer post line may see about +5.6 kip of upward support change,

• the other outer line about −5.6 kip (loss of support),

• center line often intermediate (commonly ~half of outer in a simplified model).

For preliminary sizing I use the conservative nodal vertical design range:

• ±10 kip per outer-node post (includes uncertainty + cyclic effects)

• ±5 kip at center-node post (typical)

These are still small for a 2 ft × 2 ft post.

1.4 Buckling check (Euler) — posts are extremely safe

A 2 ft × 2 ft post is so large that global column buckling is not the limiter unless it’s extraordinarily thin and unbraced.

To make this concrete, assume the post is a square tube / box with outer dimension 24 in and wall thickness (t).

Second moment of area:

[

I = \frac{b^4 • b_i^4}{12},\quad b=24,\ b_i=24-2t

]

Euler buckling:

[

P_{cr} = \frac{\pi^2 E I}{(K L)^2}

]

I’ll use a conservative composite axial modulus:

• E = 3,000,000 psi (glass composite order-of-magnitude)

and check a worst-case effective length:

• L = 24 ft = 288 in

• K = 2.0 (cantilever-like end condition; conservative)

Results (worst-case, L=24 ft, K=2)

Even with very thin walls:

• t = 0.25 in → (P_{cr} \approx 199,000) lb

• t = 0.50 in → (P_{cr} \approx 386,000) lb

• t = 1.00 in → (P_{cr} \approx 725,000) lb

Compare that to plausible axial demand per post:

• gravity ~0.7–1.5 kip

• wave-induced axial at a hot frame: order 5–10 kip

• even if you stack multiple effects, you’re nowhere near 199 kip.

Conclusion: global post buckling is not a concern. Your posts will be governed by:

• connection detailing,

• local bearing/crushing at joints,

• shear transfer into hull bulkheads,

• and laminate fatigue at nodes.

2) Joint force components at each node (post-to-crossbeam)

I’ll provide a workable node force model you can design to, consistent with your offshore 3 ft differential heave and the torsion-box distribution.

2.1 Per-frame vertical nodal forces (one 8-ft station)

Using the 30 ft participating length → 4 frames distribution:

Let:

• (P = 5.6\ \text{kip}) per outer hull (buoyancy change magnitude per engaged frame)

A conservative 3-support distribution at a peak frame:

• Port outer node: (+P) (upward from hull into structure)

• Center node: (-0.5P) (reduced support)

• Starboard outer node: (-P) (reduced support)

Numerically:

• Port: +5.6 kip

• Center: −2.8 kip

• Starboard: −5.6 kip

Then apply a preliminary offshore cyclic factor (fatigue/uncertainty):

• ×1.7 to get design nodal verticals:

Design verticals per node (per frame):

• Port: +9.5 kip

• Center: −4.8 kip

• Starboard: −9.5 kip

These are the vertical components that the crossbeam joints must transfer into the posts and into the torsion-box superstructure.

What about horizontal / shear components?

Because you have solid side panels, most global torsion and racking becomes in-plane shear in the side panels and deck diaphragm. Joints still see:

• transverse shear from frame action (racking),

• local bending from eccentricities (beam depth offsets),

• uplift and reversal offshore (fatigue-critical).

A conservative joint component set (per outer node) you can use for preliminary detailing:

• Vertical: ±10 kip (as above)

• Transverse shear (in-plane): ±3–5 kip (depends on panel shear stiffness; this is a reasonable starter)

• Moment at node: treat as present; resist by geometry (deep node blocks + wrap plies + diaphragm continuity), rather than relying on fasteners alone.

If you want, I can compute a tighter shear estimate once you pick:

• side panel thickness + material (shear modulus),

• deck diaphragm thickness,

• and whether the superstructure is a closed box (roof/deck) or open top.

3) Re-specified transverse frame crossbeams (at every 8 ft)

With posts every 8 ft and shear-wall side panels, your transverse members at each station are no longer mega crossbeams. They are frame ties that:

• distribute vertical differences between post lines,

• provide a hard node for panel shear flow,

• and reduce local deflections.

A practical composite transverse member per station is:

Recommended transverse box member (per 8-ft station)

Two arms from center post to each outer post (each ~14 ft long), or one continuous member with a center node.

Section (per arm):

• Depth: 24 in

• Width: 10–12 in

• Closed sandwich box

Wall build (global):

• Outer skin: 0.10 in

• Core: 1.0–1.5 in (higher density at nodes)

• Inner skin: 0.10 in

• Total wall thickness: ~1.2–1.7 in

Add UD caps (top and bottom):

• Carbon UD cap each face: 4–6 in wide × ~0.08–0.12 in equivalent UD thickness

• Taper caps over 24–36 in into the adjacent skins.

Internal diaphragms:

• At each node (center + outer)

• At midspan of each arm (minimum)

• More diaphragms (every 3–4 ft) if you want maximum fatigue life.

This is consistent with ~±10 kip nodal vertical reversals with good stiffness and fatigue margin, while staying much lighter than the earlier 48×30 mega-beam concept.

4) Cleaner laminate schedule (ply build) for the transverse boxes

Because fabrics vary, I’ll specify a ply schedule by common stitched biax fabrics and target cured thickness. Assume E-glass + vinyl ester (good marine fatigue and water resistance) and add carbon UD caps for bending efficiency.

Materials (typical)

• ±45 biax E-glass stitched fabric (torsion/shear)

• 0/90 biax E-glass stitched fabric (panel stability, handling, local bending)

• Carbon UD tape/fabric for caps

Target per-skin thickness

• 0.10 in per skin (global)

• Local doubling at nodes (adds 0.05–0.15 in as needed)

Suggested per-skin stacking intent (global, away from nodes)

Outer skin (0.10 in target):

1. ±45

2. ±45

3. 0/90

4. ±45

5. 0/90 (thin)

Inner skin (mirror / same):

1. 0/90

2. ±45

3. 0/90

4. ±45

5. ±45

Key rules:

• Keep skins balanced (no big mismatch of +45 vs −45).

• Keep the laminate quasi-symmetric through the wall to reduce warping.

• Maintain ±45 dominance (aim ~65–75% of glass by thickness in ±45) because your offshore driver is cyclic shear/torsion and racking.

Carbon UD caps (top and bottom, each arm)

• Place UD inside the top and bottom walls, near the outermost fibers.

• Equivalent UD thickness: 0.08–0.12 in

• Width: 4–6 in

• Taper length: 24–36 in

Node (joint) laminate build-up (where posts meet transverse box)

This is where offshore composites live or die.

At each node, add:

• 2–4 extra ±45 plies wrapped continuously around the box perimeter

• 2 extra 0/90 plies for bearing and splitting resistance

• Solid laminate or high-density core insert under any bolted clamp plates

Also add:

• a node block / diaphragm directly at the post interface (sandwich diaphragm with ±45 skins), to prevent box ovalization and to spread load into all four walls.

5) Post-to-frame connection concept (fastened) — what to do with the loads

Even if you fasten the transverse boxes to the posts, offshore fatigue strongly prefers:

• bonded + wrapped composite continuity, with

• mechanical fasteners as redundant clamps, not the primary load path.

At each post/frame node:

• Use large bearing area (wide flange seat)

• Use high-density inserts (or solid laminate blocks)

• Use through-bolts with crush sleeves if clamping through sandwich

• Keep eccentricity low (align beam shear center with post line as much as practical)

Bottom-line conclusions

1. Posts (2 ft × 2 ft): axial and buckling are essentially non-issues at the load levels implied by your hydrostatics. Focus on connections and local crushing/fatigue, not column stability.

2. Node forces (per 8-ft frame, offshore 3 ft differential heave): a good preliminary design set is:

• outer nodes: ±10 kip vertical cyclic

• center node: ±5 kip vertical cyclic

• plus transverse shear from panel action (use ±3–5 kip until you specify panel scantlings)

3. Transverse boxes at every 8 ft can be much lighter than a few mega-beams:

• 24 in deep × 10–12 in wide closed sandwich box per arm

• skins ~0.10 in each, core 1.0–1.5 in

• carbon UD caps 0.08–0.12 in eq top/bottom

4. Laminate schedule: ±45 dominated skins, with node wrap plies and diaphragms at every post interface.

Analysis of Box Beams

Below is a frame-by-frame (8 ft bay) force model that is consistent with:

• 3 ft offshore differential heave (outer hulls crest/trough),

• 30 ft overall beam → 28 ft outer-hull centerline spacing,

• solid side panels (shear walls) + deck diaphragm (torsion box behavior),

• continuous port-to-starboard transverse box beam at each 8 ft station,

• posts are 2 ft × 2 ft composite (glass + carbon).

I’m going to give you (1) axial in each post, (2) shear in each connection, and (3) moment demand at the node, with explicit assumptions and conservative design factors.

1) Offshore torsion demand converted into per-frame nodal verticals

1.1 Global torsional couple from hydrostatics

Per your earlier case, outer hull buoyancy change magnitude:

• (k_h = 62.4\times 120 = 7{,}488\ \text{lb/ft per hull})

• (ΔT = 3\ \text{ft})

• (ΔF = 7{,}488\times 3 = 22{,}464\ \text{lb})

Global torsional couple about centerline (lever arm 28 ft):

• (M_{global} = 22{,}464\times 28 \approx 629{,}000\ \text{ft-lb})

1.2 Distributing along length (solid side panels + deck = torsion box)

Offshore, the crest vs trough condition is distributed. A reasonable preliminary participating length is:

• (L_p \approx 30\ \text{ft})

Your transverse frames are every 8 ft, so number of engaged frames:

• (N \approx L_p/8 \approx 3.75 \Rightarrow 4)

So the •torsion demand per engaged frame• is:

[

T_{frame} \approx \frac{M_{global}}{N} \approx \frac{629k}{4} \approx 157k\ \text{ft-lb}

]

Offshore fatigue / uncertainty factor

For a glass+carbon composite torsion box with real sea cycling and modeling uncertainty, use:

• γ = 1.7 (preliminary)

[

T_{frame,design} \approx 157k \times 1.7 \approx 267k\ \text{ft-lb}

]

1.3 Convert that torsion into vertical force couple at outer posts

For a transverse frame, torque is carried primarily by a vertical couple between the port and starboard post lines:

[

F_{outer,design} \approx \frac{T_{frame,design}}{28} \approx \frac{267,000}{28} \approx 9,540\ \text{lb}

]

Design vertical nodal actions per 8-ft frame:

• Port outer node: +9.5 kip (up relative to structure)

• Starboard outer node: −9.5 kip (down / loss of support relative to structure)

For the center node, a conservative but common preliminary assumption is that it carries ~half the single-side magnitude in the opposite sense during this twist mode:

• Center node: −4.8 kip (down)

That yields a small net imbalance (which, in reality, is taken by longitudinal redistribution through the torsion box and adjacent frames). For sizing connections, the local magnitudes are what matter.

2) Axial load in each post (per 8-ft frame, design)

There are two contributors:

1. Gravity share from superstructure

• 20,000 lb over ~27 posts → ~0.74 kip/post average.

• With nonuniformity, use 1.5 kip/post as a conservative gravity axial.

2. Wave/torsion-induced axial (dominant)

• From above nodal vertical design actions.

Sign convention

• Compression in a post = superstructure pushing down into the hull via the post.

• Tension in a post = superstructure holding down / restraining uplift or supporting a hull that has lost buoyant support locally (depending on relative motion).

Per-frame design axial (incremental due to wave/twist)

At the peak engaged frame:

• Port outer post axial increment: ~+9.5 kip (compression tendency)

• Starboard outer post axial increment: ~−9.5 kip (tension tendency)

• Center post axial increment: ~−4.8 kip (tension tendency)

Add gravity (conservatively 1.5 kip compression on all)

Total design axial per post (peak frame):

• Port outer: ~ ( +1.5 + 9.5 ) = 11 kip compression

• Center: ~ ( +1.5 − 4.8 ) = 3.3 kip tension (net tension)

• Starboard outer: ~ ( +1.5 − 9.5 ) = 8.0 kip tension (net tension)

Interpretation: offshore twist creates reversal: one outrigger line is in compression while the other is in tension. That is exactly what you want to size joints and laminate for (fatigue).

Column buckling

With 2 ft × 2 ft posts, global Euler buckling is not remotely limiting at ~10 kip-class axial. Your governing checks will be:

• bearing / crushing at joints,

• interlaminar peel at bonded interfaces,

• bolt-group fatigue if mechanically fastened,

• local face wrinkling in sandwich regions near node blocks.

3) Shear in each connection (node shear components)

You asked shear in each connection. With solid side panels, the dominant horizontal shear is the shear flow around the torsion box perimeter.

3.1 Shear flow in the torsion box (per frame)

Approximate enclosed torsion-box area using:

• width between outer post lines ≈ 28 ft

• effective shear wall height ≈ superstructure height ≈ 16 ft

So enclosed area:

[

A \approx 28 \times 16 = 448\ \text{ft}^2

]

Shear flow (thin-walled closed section approximation):

[

q \approx \frac{T_{frame,design}}{2A} = \frac{267,000}{2 \times 448} \approx 298\ \text{lb/ft}

]

This is the circulating shear flow along the perimeter.

3.2 Resultant shear forces per panel segment (per frame)

Resultant shear in a segment ≈ (q \times) segment length.

• Each side panel segment length ≈ height = 16 ft

[

V_{side} \approx 298 \times 16 \approx 4,770\ \text{lb} \approx 4.8\ \text{kip}

]

So, per 8-ft frame, a good preliminary design value is:

• ~5 kip in-plane shear per side wall, circulating (one side up, the other down in the shear-flow sense).

3.3 What the node connection sees

At each outer node (port and starboard), the connection must transfer:

• Vertical: ±9.5 kip (from torsion couple)

• In-plane transverse shear: on the order of ±2–3 kip into the post at that frame (because the ~5 kip side-wall shear is shared between the top/bottom chords and nearby nodes—exact split depends on your chord stiffness and panel segmentation)

Conservative connection shear set (per outer node, per frame):

• Vertical shear: ±10 kip

• In-plane shear (transverse): ±3 kip

• Longitudinal shear (along length): typically smaller at a single frame; conservatively ±1–2 kip for local distribution

For the center node, shear is typically lower than the outer nodes in this torsion mode:

• Vertical: ±5 kip class (your assumed center participation)

• In-plane shear: ±1–2 kip class

4) Moment demand at the node (what to design the joint to resist)

Even with a continuous port-to-starboard transverse beam, node moments exist because:

• the beam has depth (couples form through top/bottom skins),

• shear flow wants continuity through the node,

• real attachments have eccentricity,

• and the transverse member must restrain local rotations to keep the torsion box closed.

4.1 Vertical bending moment demand at center node (per 8-ft frame)

A conservative, simple check is to treat the outer node vertical load as creating a moment about the centerline over a half-span of 14 ft.

Using the design outer nodal vertical:

• (P = 9.5\ \text{kip})

• (L = 14\ \text{ft})

[

M_{node} \sim P \cdot L \approx 9,540 \times 14 \approx 134,000\ \text{ft-lb}

]

However, because your transverse member is continuous port-to-starboard and the superstructure has shear walls, this moment is shared and reduced by continuity. A realistic preliminary reduction is ~0.6 (continuity + panel participation).

[

M_{center,design} \approx 0.6 \times 134,000 \approx 80,000\ \text{ft-lb}

]

Recommended preliminary center-node bending moment design value:

• 80,000 ft-lb (about 960,000 in-lb) per engaged frame

Outer-node joint moments are typically of the same order but can be lower if the outer node behaves closer to a pinned seat. If you want long fatigue life, it’s safer to assume:

• Outer node moment capacity: 50,000–80,000 ft-lb class

4.2 Torsion-related warping / local couple at the node

If your transverse box beam depth is, say, 24 in (2 ft), then a vertical shear V creates a flange couple roughly:

[

C \sim V \times \frac{d}{2}

]

For V ≈ 10 kip, d ≈ 2 ft:

• couple per flange ≈ 10,000×1 = 10,000 ft-lb (local)

This is not the global bending moment; it’s a local joint couple that drives:

• skin peel,

• node block design,

• wrap ply requirements.

5) What this implies for your glass + carbon joint and laminate detailing

Because your posts are huge, the structure is governed by fatigue and joint detailing, not member strength.

Design to these node demands (per 8-ft frame, peak engaged zone)

Outer node (port & starboard):

• Axial in post: ~ ±10 kip (reversing tension/compression)

• Vertical shear at connection: ±10 kip

• In-plane shear at connection: ±3 kip

• Joint moment capacity target: 50k–80k ft-lb, plus local flange couples

Center node:

• Axial in post: ~ ±5 kip class (often tension in this torsion mode)

• Vertical shear: ±5 kip

• In-plane shear: ±1–2 kip

• Joint moment capacity target: ~80k ft-lb (center sees continuity demands)

Ply Schedule for Fiberglass and Carbon

Below is a specific, buildable ply schedule for:

• Transverse box beam: 24 in deep × 12 in wide, continuous port-to-starboard

• Sandwich walls: skins 0.10 in each, core 1.25 in

• Carbon UD caps top and bottom

• Solid node blocks at each post (port, center, starboard)

• Side panels: sandwich shear walls (offshore fatigue-oriented)

I’m assuming a marine epoxy or vinyl ester system, vacuum infusion (or prepreg if you prefer), and stitched biax glass fabrics as the baseline with carbon UD for caps.

To keep this specific, I’ll specify plies by fabric type and a target cured thickness per ply. Exact ply counts will vary slightly with your chosen areal weights and resin content, but this gives you a clean, repeatable recipe.

A) Material set and thickness basis (so the ply schedule is unambiguous)

Glass fabrics (typical stitched E-glass)

Use one of these consistent families:

• E-glass biax ±45 (stitched): 0.020 in cured thickness per ply (typical for ~600–800 g/m² class infused)

• E-glass biax 0/90 (stitched): 0.020 in cured thickness per ply

Carbon UD

• Carbon UD tape/fabric: 0.010 in cured thickness per UD ply (typical for ~300–400 g/m² infused)

• If your UD is heavier (e.g., 600 g/m²), use ~0.020 in per ply and halve the ply count.

Core

• Core thickness: 1.25 in closed-cell structural foam (H100–H130 range; use higher density near nodes)

B) Transverse box beam global laminate schedule (away from nodes)

Goal per skin: 0.10 in cured thickness

Using 0.020 in per glass ply, that’s 5 plies per skin.

Because torsion/racking dominates offshore, keep ±45 majority.

B1) Top wall (sandwich) — OUTER SKIN (0.10 in)

From outside surface → core:

1. ±45 E-glass biax (0.020)

2. 0/90 E-glass biax (0.020)

3. ±45 E-glass biax (0.020)

4. ±45 E-glass biax (0.020)

5. 0/90 E-glass biax (0.020)

Total: 0.100 in

B2) Top wall — INNER SKIN (0.10 in)

From core → inside surface:

1. 0/90 E-glass biax (0.020)

2. ±45 E-glass biax (0.020)

3. ±45 E-glass biax (0.020)

4. 0/90 E-glass biax (0.020)

5. ±45 E-glass biax (0.020)

Total: 0.100 in

> This keeps the wall laminate balanced and ±45-dominant, but with enough 0/90 for handling, local bending stability, and fastener/impact tolerance.

B3) Bottom wall

Use the same as top wall (outer and inner skins identical orientation for symmetry).

B4) Side walls (both vertical faces)

Use the same 0.10 in per skin schedule as top/bottom.

If you want to bias torsion even more, swap one of the 0/90 plies for ±45 (but keep at least one 0/90 ply per skin).

C) Carbon UD caps (top and bottom) — specific ply build

The UD caps should be placed where they best resist bending: near the outermost fibers of the top and bottom walls.

Cap geometry

• Cap width: 6 in centered on the beam top and bottom centerline

• Cap length: continuous along the full transverse span, but taper down near nodes and ends as noted below.

UD cap thickness target

For your offshore cyclic environment and the node moment levels we discussed, a good starting point is:

• 0.08 in equivalent carbon UD thickness on the top and bottom.

Using 0.010 in per UD ply, that is 8 UD plies per cap.

C1) Top cap layup

Place between outer skin ply 1 and 2 (close to the outside surface, protected by the outermost glass ply):

• (Glass ply 1: ±45)

• [Carbon UD 0°] × 8 plies over 6 in width

• (Then glass plies 2–5 as listed above)

Reasoning:

Outer glass ply gives impact tolerance and electrical isolation; UD is close to the extreme fiber for bending efficiency.

C2) Bottom cap layup

Mirror the top cap, also near the outside surface on the bottom wall:

• (Glass ply 1: ±45)

• [Carbon UD 0°] × 8 plies over 6 in width

• (Then remaining glass plies)

UD tapering

At each post node (port, center, starboard), taper UD to avoid hard stress risers:

• Over 24 in approaching node: drop from 8 plies to 6 plies

• Over next 24 in: drop from 6 to 4 plies

• Over next 24 in: drop from 4 to 2 plies

• Keep 2 plies continuous through the node for continuity (unless your node block is carbon-rich, then you can terminate with proper scarf).

This produces a smooth stiffness gradient (fatigue-critical offshore).

D) Internal diaphragms inside the box beam (required)

For a 24×12 box, diaphragms control distortion/ovalization and greatly improve fatigue.

Locations (minimum)

Per 8-ft station transverse member:

• At each post node (port, center, starboard): diaphragm right at the post interface

• Midspan between center and each outer node: one diaphragm each side

So at least 5 diaphragms per full port-to-starboard beam segment.

Diaphragm construction (sandwich)

• Core: 0.75 in foam

• Skins: 0.08 in each (4 plies each side)

Diaphragm skin schedule (each face, 0.08 in):

1. ±45 (0.020)

2. 0/90 (0.020)

3. ±45 (0.020)

4. ±45 (0.020)

Bond diaphragms fully to all four walls with generous fillets and wrap plies (next section).

E) Solid node blocks at posts (port, center, starboard) — specific build

The node zone is where your vertical shear and cyclic reversals concentrate. You want:

• no soft core under fasteners,

• high through-thickness strength,

• and continuous fiber paths (wrap plies) around the joint.

Node block geometry (recommended)

At each post interface:

• Replace core with solid laminate in a region:

• 24 in along beam length (±12 in from node centerline)

• full beam depth and width locally as needed

• Transition from sandwich to solid using core taper/scarf over 12:1 minimum.

Node block laminate thickness target

Aim for 0.50 in solid laminate minimum in the post interface region.

Using 0.020 in per glass ply, that’s 25 plies total, but we can do a structured stack with mixed glass + carbon.

Node block ply schedule (0.50 in nominal)

From outer surface inward (applies to the walls in the node region; you’re effectively thickening the skins and removing core):

Outer damage tolerance layer:

1. ±45 E-glass (0.020)

2. 0/90 E-glass (0.020)

Carbon-rich strength layers (axial/bending continuity):

3) Carbon UD 0° × 6 plies (0.060)

4) ±45 E-glass × 2 plies (0.040)

Shear and bearing build:

5) ±45 E-glass × 6 plies (0.120)

6) 0/90 E-glass × 4 plies (0.080)

Inner continuity layers:

7) ±45 E-glass × 4 plies (0.080)

8) Carbon UD 0° × 4 plies (0.040)

9) 0/90 E-glass × 2 plies (0.040)

Total thickness ≈ 0.020+0.020+0.060+0.040+0.120+0.080+0.080+0.040+0.040

= 0.500 in

> If you prefer to keep carbon out of the node block for galvanic or cost reasons, you can replace the carbon UD with additional 0/90 glass. Carbon in the node is beneficial for stiffness continuity and fatigue, but not mandatory.

Wrap plies around the node (mandatory)

Over the node region (±12 in), add continuous wraps around the box perimeter:

• ±45 glass wrap × 4 plies (0.080 total) wrapped continuously around the full 24×12 perimeter

• 0/90 glass wrap × 2 plies (0.040 total) around the perimeter

These wraps are what prevent peel/delamination at the node and make the joint monolithic.

F) Fastener zones (if mechanically fastened to posts)

Where bolts/clamps pass through:

• Use solid laminate only (no core)

• Add bearing doublers locally:

• 0/90 glass × 2 plies each side

• ±45 glass × 2 plies each side

• Use crush sleeves and wide washers/plates to prevent local crushing

G) Side panels as sandwich shear walls — specific schedule

Side panels are your global torsion carriers. Make them torsion-efficient and fatigue-resistant.

Panel geometry (assumed)

• Height: 16 ft

• Panel bay length: 8 ft between posts

Recommended panel thickness

• Core: 1.5 in structural foam (H80–H100; go higher at post lines)

• Skins: 0.08–0.10 in each

I’ll give you 0.10 in each for offshore durability.

Side panel outer skin (0.10 in)

1. ±45 glass (0.020)

2. ±45 glass (0.020)

3. 0/90 glass (0.020)

4. ±45 glass (0.020)

5. 0/90 glass (0.020)

Side panel inner skin (0.10 in)

1. 0/90 glass (0.020)

2. ±45 glass (0.020)

3. ±45 glass (0.020)

4. 0/90 glass (0.020)

5. ±45 glass (0.020)

Optional carbon striping (recommended for stiffness, not required):

• Add carbon UD 0° stringer strips at:

• top and bottom of panel, and/or

• at mid-height,

• 4–6 in wide × 4 plies (0.04 in) each strip

This reduces racking deflection and improves fatigue margins.

Post-line attachments (critical)

At each post line, thicken panel skins locally over a 6–8 in wide strip:

• add ±45 glass × 2 plies (each side)

• add 0/90 glass × 2 plies (each side)

This creates a robust web-to-column joint region.

H) Summary tables you can hand to fabrication

H1) Transverse box beam (global, away from nodes)

• Geometry: 24 in deep × 12 in wide closed box

• Walls: sandwich with 1.25 in core

• Each skin (outer and inner): 5 plies glass = 0.10 in

• Carbon UD caps: 8 plies UD 0° top and bottom, 6 in wide, tapered at nodes

• Diaphragms: sandwich 0.75 in core, skins 4 plies each side (±45-heavy)

H2) Node regions (at each post)

• Replace core with 0.50 in solid laminate over ±12 in region

• Add perimeter wrap plies:

• ±45 glass × 4

• 0/90 glass × 2

• Add bearing doublers where bolted/clamped

H3) Side shear panels

• Core: 1.5 in

• Skins: 0.10 in each (±45-dominant)

• Post-line thickening strips on both sides

If you want, I can convert the above into a single-page build spec format (parts list + laminate callouts per component + node detail notes) that a composites shop can quote.

Build Specs

Below is a single-page build specification suitable for composite shop quoting and fabrication control. It consolidates geometry, materials, laminate schedules, node detailing, and fabrication notes for your offshore 3-hull platform.

OFFSHORE TRIMARAN SUPERSTRUCTURE FRAME – COMPOSITE BUILD SPEC

Configuration: 3 hulls, posts at 8 ft spacing, continuous transverse box beam at each frame

Environment: Offshore, 3 ft differential heave, cyclic fatigue governing

Materials: E-glass + carbon UD, epoxy or vinyl ester marine system

Manufacturing: Vacuum infusion preferred (target Vf ≥ 50%)

1. TRANSVERSE BOX BEAM (PER 8 FT FRAME)

Geometry:

• Continuous port-to-starboard closed box

• Depth: 24 in

• Width: 12 in

• Core: 1.25 in structural foam (H100–H130; higher density at nodes)

1.1 Global Wall Construction (Away from Nodes)

Each sandwich wall (top, bottom, both sides):

Outer Skin – 0.10 in total (5 plies)

1. ±45 E-glass biax (0.020 in)

2. 0/90 E-glass biax (0.020 in)

3. ±45 E-glass biax (0.020 in)

4. ±45 E-glass biax (0.020 in)

5. 0/90 E-glass biax (0.020 in)

Inner Skin – 0.10 in total (5 plies)

1. 0/90 E-glass biax (0.020 in)

2. ±45 E-glass biax (0.020 in)

3. ±45 E-glass biax (0.020 in)

4. 0/90 E-glass biax (0.020 in)

5. ±45 E-glass biax (0.020 in)

Total wall thickness (global):

~1.45 in including skins and 1.25 in core

1.2 Carbon UD Caps (Top and Bottom Walls)

Purpose: Bending stiffness and fatigue resistance

• Width: 6 in centered on beam

• Thickness: 0.08 in equivalent UD

• Layup: 8 plies carbon UD (0°), 0.010 in per ply

Placement:

• Between outer glass ply 1 and 2

• Continuous full span

• Taper at nodes:

• Reduce to 6 plies over 24 in

• Reduce to 4 plies over next 24 in

• Maintain minimum 2 plies through node region

1.3 Internal Diaphragms

Locations (minimum per frame):

• At port post

• At center post

• At starboard post

• Midspan between port & center

• Midspan between center & starboard

Construction:

• Core: 0.75 in structural foam

• Skins: 4 plies per face (±45, 0/90, ±45, ±45)

• Fully bonded with structural fillets and wrapped

2. NODE BLOCKS (AT EACH POST INTERFACE)

Region: ±12 in longitudinally from post centerline

Core removed – replace with solid laminate

2.1 Solid Node Laminate (0.50 in total)

Stack (outside to inside):

1. ±45 glass (0.020)

2. 0/90 glass (0.020)

3. Carbon UD 0° × 6 (0.060)

4. ±45 glass × 2 (0.040)

5. ±45 glass × 6 (0.120)

6. 0/90 glass × 4 (0.080)

7. ±45 glass × 4 (0.080)

8. Carbon UD 0° × 4 (0.040)

9. 0/90 glass × 2 (0.040)

Total ≈ 0.50 in

2.2 Perimeter Wrap (Mandatory)

Over entire node perimeter (wrap around full box section):

• ±45 glass × 4 plies (0.080 in)

• 0/90 glass × 2 plies (0.040 in)

Wrap length: ±12 in from node centerline.

2.3 Fastener / Clamp Zones

If bolted:

• No core under bolt paths

• Add doublers:

• ±45 × 2 plies each side

• 0/90 × 2 plies each side

• Use crush sleeves and wide backing plates

• Maintain minimum 2 in edge distance

3. SIDE PANELS (SHEAR WALLS)

Height: 16 ft

Bay length: 8 ft between posts

3.1 Core

• 1.5 in structural foam (H80–H100 minimum)

3.2 Skins (Each Side – 0.10 in total)

1. ±45 glass (0.020)

2. ±45 glass (0.020)

3. 0/90 glass (0.020)

4. ±45 glass (0.020)

5. 0/90 glass (0.020)

3.3 Post Line Reinforcement

At each post vertical strip (6–8 in wide each side):

• Add ±45 × 2 plies

• Add 0/90 × 2 plies

3.4 Optional Carbon Stiffening (Recommended Offshore)

Add carbon UD strips:

• 4–6 in wide

• 4 plies (0.04 in)

• At top and bottom of panel

• Optional mid-height strip for racking control

4. POSTS (2 ft × 2 ft Composite)

Axial design (per peak engaged frame):

• ±10 kip outer posts

• ±5 kip center post

Buckling: Not governing at this section size

Design focus: Node bearing, wrap continuity, fatigue

Post-to-beam interface must bond into node block laminate and be wrapped with ±45 plies.

5. STRUCTURAL PERFORMANCE BASIS

Designed for:

• 3 ft offshore differential heave

• ~9.5 kip vertical nodal design load per outer post per engaged frame

• Torsion distributed via shear walls + deck diaphragm

• Cyclic fatigue governing

Safety philosophy:

• Torsion per engaged frame includes 1.7 dynamic factor

• Carbon caps used to control bending fatigue

• ±45 dominant laminates for shear durability

6. FABRICATION REQUIREMENTS

• Vacuum infusion or prepreg

• Target fiber volume ≥ 50%

• Controlled taper transitions (minimum 12:1 core taper at nodes)

• Generous internal fillets at all 90° corners

• No sharp laminate drop-offs

• All carbon electrically isolated from metallic fasteners

• Post-cure per resin manufacturer for offshore temperature stability

7. INSPECTION CRITICAL POINTS

• Node block consolidation

• UD taper regions

• Core bond integrity

• Diaphragm bonding

• Shear wall continuity at post lines

Weight Estimates

Below are order-of-magnitude weight estimates based on the exact laminate/build spec you approved for the transverse boxes and side-panel shear walls, plus a parametric post weight (because post wall build wasn’t specified, and it dominates total weight).

I’m using typical cured densities:

• Glass laminate (E-glass + resin): ~110 lb/ft³

• Carbon laminate (carbon + resin): ~100 lb/ft³

• Structural foam core (H80–H130 class): ~5–6 lb/ft³

• Freshwater vs saltwater doesn’t affect weight; it affects draft only.

1) Weight estimate per transverse frame (one 8-ft station)

1.1 Geometry assumed per frame

• One continuous closed box beam spanning port-to-starboard post lines

• Outer hull centerline spacing = 28 ft → I assume beam length = 28 ft

• Box cross-section: 24 in deep × 12 in wide

• Global sandwich walls: 0.10 in skins each side + 1.25 in core

• Carbon UD caps: 0.08 in eq, 6 in wide, top and bottom (taper at nodes)

• Internal diaphragms: 5 diaphragms per beam (port, center, starboard, plus 2 midspans)

• Node blocks: 3 node regions, each 24 in long (±12 in) with 0.50 in solid laminate + perimeter wraps

1.2 Result (per transverse frame)

Estimated weight per transverse frame (beam + diaphragms + node blocks + typical bonding/fillets):

• ~550 to 650 lb per frame

A good single number to carry: ~600 lb per transverse frame

> This estimate includes the carbon UD caps, the 5 diaphragms, and the three solid node-block regions (and it avoids double-counting by removing the sandwich-core volume in the node lengths).

2) Count of transverse frames over 60 ft

At 8 ft spacing over 60 ft you typically have frames at:

• 0, 8, 16, 24, 32, 40, 48, 56, 60 → 9 frames

So total transverse box-beam framing weight:

• 9 × ~600 lb ≈ 5,400 lb

3) Side-panel shear walls weight (both sides)

3.1 Side panel geometry

• Length: 60 ft

• Height: 16 ft

• Area per side: 960 ft²

• Both sides: 1,920 ft²

3.2 Side panel build (your spec)

• Core: 1.5 in foam

• Skins: 0.10 in each side (±45-dominant schedule)

3.3 Weight result (both sides)

• Base side panels (skins + core): ~4,600 to 5,000 lb total

• Post-line reinforcement strips (6–8 in wide at each post line, both sides, both skins): add ~250–350 lb

Carry number: ~5,000 to 5,300 lb for both shear walls including post-line thickening.

> Optional carbon UD stringer strips (top/bottom/mid-height) would add weight; depending on strip count, typically +150 to +400 lb total.

4) Total superstructure structural weight (what’s included and what dominates)

4.1 What we can state precisely from your spec

From above, excluding posts and excluding deck/roof panels:

• Transverse frames (9): ~5,400 lb

• Side shear walls (2 sides + post strips): ~5,000–5,300 lb

Subtotal (frames + side walls): ~10,400 to 10,700 lb

4.2 Posts dominate, and your post laminate is not yet specified

You said posts are 2 ft × 2 ft × 24 ft and glass + carbon, but you did not specify whether they are:

• sandwich (core + skins), or

• solid laminate box tube, or

• framed/trussed (very different weight).

So I’m giving a parametric post weight you can plug your wall build into:

Post weight formula (square box post)

Let:

• Perimeter = 8 ft

• Height = 24 ft

• Effective wall thickness (laminate + core) = (t) in feet

• Average wall density = (ρ_{wall}) (lb/ft³)

Then:

[

W_{post} \approx 8 \times 24 \times t \times ρ_{wall} = 192,t,ρ_{wall}

]

Examples:

A) Light composite box tube (solid laminate wall ~0.25 in):

• (t = 0.25/12 = 0.0208) ft

• (ρ_{wall} \approx 110) lb/ft³

• (W_{post} \approx 192 \times 0.0208 \times 110 \approx 440) lb/post

• 27 posts ≈ 11,900 lb

B) Medium wall (solid laminate ~0.20 in):

• (t = 0.0167) ft → ~350 lb/post

• 27 posts ≈ 9,450 lb

C) Sandwich post (0.10 in skins each side + ~1.25 in core) — very heavy

This can easily land in the 400–500 lb/post range as well (because perimeter × height is huge), unless you reduce core thickness dramatically.

Reality check: With 2 ft × 2 ft posts, even light wall thickness creates a lot of surface area, so posts can consume ~9–12k lb quickly.

4.3 Total superstructure structural weight (range)

Using the known subtotal ~10.5k plus posts:

• If posts are ~350 lb each:

Total ≈ 10.5k + 9.5k = ~20,000 lb

• If posts are ~440 lb each:

Total ≈ 10.5k + 11.9k = ~22,400 lb

So, with 2×2 ft posts, the structural superstructure weight will typically land around:

• ~20,000 to 23,000 lb, depending mainly on post wall build.

Recommendation to hit your 20,000 lb target cleanly

If superstructure = 20,000 lb is a hard target and it includes posts, you likely want posts closer to:

• ~0.20 in solid equivalent wall (with carbon localized where needed), or

• a stiffened box (thin skins + internal webs) rather than thick uniform walls.

Quick deliverables summary

• Weight per transverse frame (one 8-ft station): ~600 lb (use 550–650 lb range)

• Total transverse frames (9): ~5,400 lb

• Both side shear walls (incl. post-line reinforcement): ~5,000–5,300 lb

• Total superstructure structural weight (frames + walls + posts): typically ~20k–23k lb, with posts driving the range.

If you tell me one number—the post wall build (e.g., solid 0.20 in equivalent or 0.10 in skins + 1.0 in core)—I’ll lock the total weight to a much tighter ±5% and also give you a weight per 8-ft bay (useful for trim and load planning).

Post Updated with Core

Updated with posts = 0.10 in skins (each side) + 1.0 in core (sandwich)

I’ll lock the post weight tightly now and roll it into the total superstructure structural weight.

Assumptions (consistent with prior estimates):

• Post outer size: 2 ft × 2 ft (perimeter = 8 ft)

• Post height: 24 ft

• Core thickness: 1.0 in = 0.0833 ft

• Skin thickness: 0.10 in each side (outer + inner) → total skin thickness = 0.20 in = 0.01667 ft

• Foam density: ~6 lb/ft³ (H100–H130 class order)

• Glass/carbon laminate density (cured): ~110 lb/ft³ (effective average)

1) Weight per post (sandwich wall)

Post wall area (4 faces):

• Area = perimeter × height = 8 ft × 24 ft = 192 ft²

Core weight

• Core volume = 192 × 0.0833 = 16.0 ft³

• Core weight = 16.0 × 6 = 96 lb

Skins weight

• Skin volume = 192 × 0.01667 = 3.20 ft³

• Skin weight = 3.20 × 110 = 352 lb

Post total (skins + core)

• Post weight ≈ 96 + 352 = 448 lb per post

Add typical consumables/adhesives/edge closeouts/fillets allowance (~5–10%):

• Design carry weight per post: ~475 lb

2) Total post weight (all posts)

You have ~9 frames × 3 posts = 27 posts.

• Bare laminate estimate: 27 × 448 = 12,100 lb

• Carry estimate (incl. closeouts/adhesives): 27 × 475 = 12,825 lb

I’ll use 12.1–12.8k lb as the post range.

3) Updated totals for the superstructure structural weight

From prior spec-based estimates (unchanged):

A) Transverse frames (continuous 24×12 box beam @ every 8 ft)

• ~600 lb per frame (550–650 range)

• 9 frames → ~5,400 lb (≈ 5,0–5,9k range)

B) Side shear walls (both sides, 60 ft × 16 ft, sandwich with post-line thickening)

• ~5,000 to 5,300 lb

C) Posts (updated)

• ~12,100 to 12,800 lb

4) Total superstructure structural weight (frames + side walls + posts)

Using midpoints:

• Frames: 5,400

• Side walls: 5,150

• Posts: 12,450

Total ≈ 23,000 lb

Using the range bounds:

• Low: 5,0k (frames) + 5,0k (walls) + 12.1k (posts) = ~22.1k lb

• High: 5.9k (frames) + 5.3k (walls) + 12.8k (posts) = ~24.0k lb

Final carry number

• Superstructure structural weight ≈ 23,000 lb (plan for 22,000–24,000 lb)

5) Weight estimate per transverse frame (confirmed)

• ~600 lb per frame (use 550–650 lb for planning)

What this means relative to your earlier 20,000 lb superstructure figure

If the 20,000 lb you gave was intended to be all-in superstructure including posts/walls/frames, then this sandwich-post build pushes you ~2–4k lb over that target.

The fastest ways to pull weight back without losing stiffness:

1. Reduce post core from 1.0 in → 0.5–0.75 in (big weight cut; stiffness still strong at 2×2 ft size).

2. Keep 1.0 in core but reduce skins from 0.10/side → 0.08/side (moderate cut; verify local bearing at nodes).

3. Replace some glass in post skins with carbon strategically (stiffness up, weight similar; helps fatigue).

If you want, I can run a quick sensitivity table (post core/skin options vs total weight) so you can pick a configuration that lands right on your target.

Using Epoxy and Plywood vs Fiberglass and Carbon

Understood: switch the superstructure framing system to 1/2 in marine plywood, epoxy-laminated, with no fiberglass or carbon anywhere. Below is a clean reassessment of weights per transverse frame and total superstructure structural weight, plus the key structural implications offshore.

I’m going to keep the same geometry and grid you’ve been using:

• Posts: 2 ft × 2 ft × 24 ft, at 8 ft spacing along 60 ft (≈ 9 frames, 27 posts)

• Transverse box beams: 24 in deep × 12 in wide, continuous port-to-starboard at each frame (span ≈ 28 ft between outer post lines)

• Side panels (shear walls): 60 ft × 16 ft, both sides

Assumptions for plywood + epoxy weights

Marine plywood density varies by species and grade. Typical ranges:

• Okoume: ~27–34 lb/ft³

• Douglas-fir marine: ~34–41 lb/ft³

For planning, I’ll use a mid value:

• Plywood density: 38 lb/ft³

• Epoxy allowance: +10% on panel weights (bond lines, fillets, fairing, saturation coats)

If you tell me your exact plywood (Okoume vs fir), I can tighten this.

1) Weight estimate per transverse frame (one 8-ft station)

1.1 Continuous transverse plywood box beam (24 in × 12 in × 28 ft)

Box perimeter at section:

• 24 in = 2 ft, 12 in = 1 ft → perimeter = 2(2 + 1) = 6 ft

Surface area of the four walls over 28 ft span:

• Area = perimeter × length = 6 × 28 = 168 ft²

Wall thickness: 1/2 in = 0.04167 ft

Wall volume: 168 × 0.04167 = 7.00 ft³

Wall weight (plywood only): 7.00 × 38 = 266 lb

With epoxy allowance (+10%): ~293 lb

1.2 Internal diaphragms (minimum set)

From your earlier composite spec: 5 diaphragms per beam is still a good offshore distortion-control minimum.

Each diaphragm area = 2 ft × 1 ft = 2 ft²

Thickness = 0.5 in (0.04167 ft)

Volume per diaphragm = 2 × 0.04167 = 0.0833 ft³

Weight per diaphragm = 0.0833 × 38 = 3.17 lb

5 diaphragms = 15.9 lb, epoxy allowance → ~17.5 lb

1.3 Node reinforcement (plywood node blocks)

With plywood you don’t do solid laminate blocks like composites; instead you use doubler layers / gussets / blocking.

A practical minimum: add a second 1/2 in plywood layer (doubler) around the box perimeter over a 2 ft length at each of 3 post nodes (port, center, starboard). Total reinforced length = 6 ft.

Extra doubler area = perimeter × length = 6 × 6 = 36 ft²

Volume = 36 × 0.04167 = 1.50 ft³

Weight = 1.50 × 38 = 57 lb, epoxy allowance → ~63 lb

1.4 Transverse frame total

• Beam walls (epoxy incl): ~293 lb

• Diaphragms: ~18 lb

• Node doublers: ~63 lb

Estimated weight per transverse frame: ~374 lb

Add a realistic build margin (blocking, fasteners, extra fillets, local gussets): +10–20%

✅ Carry weight per transverse frame: ~420 lb

(Practical range: 400–475 lb)

2) Total transverse framing weight (60 ft length)

Frames ≈ 9 (0, 8, 16, 24, 32, 40, 48, 56, 60)

• 9 × 420 lb ≈ 3,780 lb

Range: ~3,600 to 4,300 lb

3) Side shear walls (both sides), 1/2 in plywood + epoxy

Total side wall area:

• One side: 60 × 16 = 960 ft²

• Both sides: 1,920 ft²

Thickness: 0.5 in = 0.04167 ft

Volume = 1,920 × 0.04167 = 80.0 ft³

Weight (plywood): 80 × 38 = 3,040 lb

Epoxy allowance (+10%): ~3,350 lb

Add framing around openings, post-line blocking, and joints (realistic): +10–15%

✅ Carry weight for both side shear walls: ~3,700–3,900 lb

(Use ~3,800 lb)

4) Posts (2 ft × 2 ft × 24 ft), 1/2 in plywood box + epoxy

Each post wall area:

• Perimeter × height = 8 ft × 24 ft = 192 ft²

Thickness 0.5 in = 0.04167 ft

Volume = 192 × 0.04167 = 8.00 ft³

Weight (plywood): 8 × 38 = 304 lb

Epoxy allowance (+10%): ~335 lb

Add post-specific build additions (corner blocking, local crush blocks at joints, fasteners): +10–15%

✅ Carry weight per post: ~370–385 lb

Total posts: 27

• 27 × 380 ≈ 10,260 lb

Range: ~9,900 to 10,700 lb

5) Total superstructure structural weight (plywood/epoxy system)

Using carry values:

• Transverse frames: ~3,780 lb

• Side shear walls: ~3,800 lb

• Posts: ~10,260 lb

✅ Subtotal (posts + frames + side walls): ~17,800 lb

Practical range: ~17,300 to 18,900 lb

What’s NOT included (but you almost certainly have)

To avoid false precision: the following structural items can add a lot:

• Deck diaphragm / roof diaphragm (often 1/2 in plywood too)

• Longitudinal girders/stringers tying frames together

• Ring beams at top and bottom of side panels

• Connection hardware (steel plates, bolts, hold-downs)

• Internal floors, bulkheads, equipment foundations

If you add a full 60 ft × ~30 ft deck in 1/2 in plywood, that alone is large (and may add several thousand pounds). If you want, I can include it once you specify deck thickness and how much of the plan area is decked.

6) Offshore structural implications of plywood-only (important)

Plywood/epoxy can work offshore, but you must design around these realities:

Strength and fatigue

• Plywood is orthotropic; strength and stiffness depend heavily on grain direction and ply quality.

• Offshore cyclic racking wants ±45 shear capacity—wood doesn’t give you a true ±45 fabric; you achieve it by diagonal plywood webs, K-bracing, and closed torsion boxes.

Moisture and durability

• Offshore durability is about edge sealing, encapsulation, and fastener detailing.

• Every cut edge, bolt hole, and seam must be sealed or it will wick moisture over time.

Connections govern

• Plywood systems fail at:

• bearing crush at bolt groups,

• peel at glue lines,

• and joint opening under cyclic reversal.

• Your best strategy is large-area glued joints (scarfs, laps), plus mechanical fasteners as secondary redundancy.

Bottom-line results

Weight per transverse frame (8 ft station)

• ~420 lb each (range 400–475 lb)

Total superstructure structural weight (posts + transverse frames + side shear walls)

• ~17,800 lb (range ~17,300–18,900 lb)

excluding deck/roof/longitudinals and heavy hardware.

Cost Comparison for Superstructure of Fiberglass and Carbon vs. Plywood Epoxy

Below is a total-cost comparison (materials + typical fabrication/consumables + labor ranges) for the two build methods you’ve now defined:

1. Composite sandwich (glass + carbon + foam core + epoxy/vinyl ester)

2. 1/2 in marine plywood + epoxy lamination (no glass/carbon)

Because prices vary by supplier, I’m using current published list / retail pricing for key inputs (epoxy, plywood, carbon) and translating your geometry into a quantity-based estimate. Where the exact product wasn’t specified (foam grade, epoxy brand, labor rate), I give a realistic range.

Unit pricing anchors (what I’m basing the math on)

• Marine epoxy (West System 105 + hardener): published 2025 price list shows ~5.29 gal kit at $615.45 (≈ $116/gal) and 63.3 gal at $4,256 (≈ $67/gal in bulk). ([WEST SYSTEM][1])

• 1/2 in Okoume marine plywood 4×8: examples show ~$165–$186 per sheet. ([LL Johnson Lumber][2])

• Carbon UD fabric: example pricing $47.57 per yard (12 in wide UD). ([TotalBoat][3])

• Structural PVC foam core (Divinycell H100 1 in): example ~$281.94 for ~24.9 ft² sheet (≈ $11.3/ft² per 1 in). ([Fiberglass Supply][4])

• Biax fiberglass cloth: common retail pricing exists by the yard (varies by areal weight); I’ll treat glass reinforcement as a moderate-cost input relative to foam + epoxy and show it as a line item rather than pretending one single correct price. ([TotalBoat][5])

A) Plywood + epoxy method — total cost estimate

Geometry you’ve already set (superstructure structural only)

• 9 transverse frames (continuous 24×12 box beam @ 8 ft spacing, ~28 ft span)

• 2 side shear walls (60×16 each)

• 27 posts (2×2×24 ft) with 1/2 in plywood box walls

• Prior weight estimate (structural only): ~17.3–18.9k lb, carry ~17.8k lb

A1) Material quantities and cost (order-of-magnitude)

Plywood sheets

Using your structural weight as the cleanest proxy:

• Typical 1/2 in Okoume sheet weight is often in the ~50–60 lb class; one vendor listing shows 60 lb per 4×8 sheet. ([woodnshop.net][6])

• Sheets equivalent ≈ 17,800 lb / 60 ≈ 297 sheets (this includes cut waste implicitly)

Cost per sheet ≈ $166–$186 ([LL Johnson Lumber][2])

Plywood cost ≈ 297 × ($166–$186) = $49k–$55k

Epoxy (bonding + saturation + fillets)

Your earlier plywood spec assumed ~10% epoxy-by-weight allowance; that’s reasonable for heavy laminated joinery.

• Epoxy mass ≈ ~1,800 lb

• Convert to gallons (epoxy ~9.3–9.7 lb/gal): ≈ ~190 gal

• Epoxy cost:

• small-kit pricing (~$116/gal): ~$22k ([WEST SYSTEM][1])

• bulk pricing can be materially lower; West’s large group implies much lower per-gallon equivalent ([WEST SYSTEM][1])

Carry epoxy cost: $15k–$25k (depends heavily on bulk purchasing and brand)

Fasteners, coatings, sealants, misc.

Offshore wood needs aggressive edge-sealing, coatings, and robust fasteners/plates.

Carry: $8k–$20k (wide because hardware philosophy varies)

A2) Plywood method material subtotal

Material subtotal (structural): ~ $72k–$100k

A3) Labor + shop consumables

Plywood/epoxy is labor-forward but doesn’t require infusion tooling.

• Typical build labor for a structure this size (cutting, scarfing, bonding, fillets, coating, fitting) often lands in:

~600–1,200 labor-hours depending on jigging and repetition

• At $60–$120/hr loaded shop rate: $36k–$144k

A4) Plywood method total (materials + labor)

~$110k–$240k (structural superstructure only)

B) Composite sandwich method — total cost estimate

This is your earlier composite spec:

• transverse frames: glass skins + foam + carbon UD caps + diaphragms + node blocks

• side shear walls: foam sandwich with glass skins (+ optional carbon strips)

• posts: 2×2×24 ft sandwich with 0.10 in skins + 1 in core (glass/carbon)

Prior structural weight estimate: ~22–24k lb, carry ~23k lb.

Composite cost is not proportional to weight; it’s driven by:

• foam core area,

• resin volume,

• carbon usage,

• consumables (vac bag film, peel ply, infusion media),

• QA/time.

B1) Materials (big-ticket items)

Foam core (dominant cost driver)

Your system uses a lot of core area. Published H100 1pricing implies about $11.3/ft² per 1 in. ([Fiberglass Supply][4])

For 1.25and 1.5thicknesses, core cost scales roughly with thickness.

Just the side walls are 1,920 ft². If they are 1.5 in core, core alone is plausibly:

• ~$11.3 × 1.5 × 1,920 ≈ $32k (core only, before waste) ([Fiberglass Supply][4])

Then add:

• transverse beams cores,

• post cores (posts have very large surface area),

• waste (10–20% typical).

Realistic carry for all foam core in this whole superstructure:

~$60k–$120k (depends on grade, thickness, supplier, waste)

Resin/epoxy

Composite sandwich (especially infusion) consumes a lot of resin. Your prior composite structural weight (~23k lb) implies substantial resin mass.

At retail-ish ~$116/gal equivalent, resin gets expensive fast; bulk helps. ([WEST SYSTEM][1])

Carry resin cost (including bonding putties):

~$25k–$70k

Carbon UD

UD caps and node reinforcement are not huge in area, but carbon is pricey. Example: $47.57/yard for 12UD. ([TotalBoat][3])

Carry carbon material:

~$5k–$25k (depends on how much carbon you keep vs glass-only)

Glass reinforcement

Glass biax is comparatively modest cost vs foam + resin, but still material. ([TotalBoat][5])

Carry glass fabric:

~$10k–$35k

Vacuum consumables / infusion media (if you infuse)

Peel ply, flow media, bag film, tacky tape, lines, catch pots:

~$10k–$30k

B2) Composite method material subtotal

~$110k–$280k (structural superstructure only)

B3) Labor + shop overhead

Composite labor can be comparable or higher than plywood because:

• more process steps (layup, debulk, bagging, infusion, post-cure),

• more QA and rework risk,

• more careful node detailing.

Carry:

• ~900–1,800 labor-hours

• At $60–$120/hr loaded: $54k–$216k

B4) Composite method total (materials + labor)

~$170k–$500k (structural superstructure only)

C) Side-by-side cost summary

Plywood + epoxy (no glass/carbon):

• Materials: $72k–$100k

• Total installed (structural): $110k–$240k

Composite sandwich (glass + carbon + foam + resin + consumables):

• Materials: $110k–$280k

• Total installed (structural): $170k–$500k

Practical takeaway

• Plywood/epoxy is typically ~40–60% of composite total cost for this specific geometry (especially because 2×2×24 ft posts and large shear walls create •huge• foam-core surface area and resin consumption).

• Composite can win if you dramatically reduce core area, simplify the post concept, or you need the lowest maintenance + highest fatigue life offshore.

Cost Comparison of Three Building Methods

Below is a total-cost comparison for three build methods for the same superstructure geometry we’ve been using (27 posts, 9 transverse frames, 2 side shear walls), expressed as materials + labor ranges. The ranges are wide because shop rate, purchasing scale, and build method (infusion vs hand layup, pre-cut kits vs field fit) swing totals a lot.

I’m anchoring key unit costs to current published prices for epoxy, plywood, foam core, and fiberglass tape. ([westsystem.com][1])

Option 1: Composite sandwich (glass skins + foam core + carbon UD caps + epoxy)

What drives cost

• Large foam core surface area (posts + side panels + beams) and resin usage

• Carbon UD in caps/node regions

• Vacuum consumables and QA if infused

• Higher-skilled labor and rework risk

Materials (order-of-magnitude)

• Foam core: Using H100 1reference pricing (about $281.94 per ~24.9 ft² sheet, ≈ $11.3/ft² per inch). ([fiberglasssupply.com][2])

With your very large panel/post areas and 1.0–1.5 in thicknesses, total foam commonly lands in the $60k–$140k range (after waste).

• Epoxy/resin: West System group pricing shows 5.29 gal at $615.45 and 63.3 gal at $4,256.01 (bulk reduces $/gal materially). ([westsystem.com][1])

Typical composite resin + bonding putties: $30k–$90k

• Glass fabrics + carbon UD + consumables: $25k–$90k (depends on carbon content and infusion consumables)

Composite materials subtotal: $115k–$320k

Labor

• ~900–2,000 hours (layup/bagging/infusion or hand layup, node detailing, QA)

• At loaded shop rates $60–$120/hr: $54k–$240k

Composite total installed (structural superstructure only): $170k–$560k

Option 2: All plywood + epoxy (1/2 in marine plywood, no fiberglass/carbon)

What drives cost

• Plywood sheet count and waste

• Epoxy for bonding, fillets, saturation/encapsulation

• Labor (cutting/scarfing, fit-up, coating, edge sealing)

Materials (order-of-magnitude)

• Plywood: 1/2okoume sheet pricing example ~$185.60/sheet. ([woodnshop.net][3])

Your plywood-only structural estimate (~17–19k lb) typically translates to on the order of ~280–320 sheets once you include cut waste and blocking. That puts plywood around $52k–$59k.

• Epoxy: West System pricing anchors above. ([westsystem.com][1])

Plywood/epoxy is epoxy-hungry if fully encapsulated; a practical budget is $15k–$35k depending on bulk purchase and coating standard.

• Fasteners/plates/sealants/coatings: $10k–$25k (offshore detailing tends to push this up)

Plywood materials subtotal: $80k–$120k

Labor

• ~700–1,500 hours

• At $60–$120/hr: $42k–$180k

Plywood total installed (structural superstructure only): $120k–$300k

Option 3: Plywood + epoxy with fiberglass tape only at seams (hybrid wood core with glass seam reinforcement)

This is the common wood boat approach: plywood provides the section; fiberglass tape only is used to:

• arrest seam cracking under cyclic racking,

• distribute shear at joints,

• improve watertightness and long-term durability,

• reduce maintenance (especially at panel/post and panel/deck intersections).

Materials delta versus Option 2

• Fiberglass tape: example 6 oz tape 50-yard roll at $17.95. ([Fiberglass Warehouse][4])

For a structure this size, a reasonable seam-tape quantity is often 20–60 rolls depending on how many seams you tape (inside + outside, corners, post lines, diaphragm joints). That’s $360–$1,100 in tape.

• Additional epoxy for wet-out: typically +10–30 gallons depending on how aggressively you tape and whether you do one or two layers at major seams. Using West pricing anchors, that’s roughly +$700 to +$3,500 (bulk vs kit pricing changes this). ([westsystem.com][1])

Net materials increase vs Option 2: typically +$2k–$8k all-in (tape + extra resin + some consumables).

Labor delta versus Option 2

• Adds wet-out, fairing, sanding time:

• +100–300 hours depending on finish standard and how many seams are taped both sides.

Hybrid total installed (structural superstructure only): $130k–$320k

Summary: which is lowest cost and why

• Lowest cost (typically): Option 2 (all plywood + epoxy)

Lowest materials cost, simpler supply chain, but you must accept higher long-term risk of seam fatigue and maintenance offshore unless detailing is exceptional.

• Best value offshore (usually): Option 3 (plywood + seam tape)

Modest up-front cost increase over Option 2, but materially improves fatigue resistance and watertight durability at the exact locations that fail offshore first (seams, corners, post/panel interfaces). Tape pricing is low relative to the overall build. ([Fiberglass Warehouse][4])

• Highest performance but highest cost: Option 1 (full composite sandwich)

Cost is dominated by foam core + resin + labor process overhead, especially with your very large post and panel surface areas. Foam core pricing anchors show why this gets expensive fast at scale. ([fiberglasssupply.com][2])

My recommendation for your specific geometry

With 2 ft × 2 ft × 24 ft posts and large shear walls, full sandwich composite is usually cost-heavy because surface area explodes. If you want offshore reliability without composite-level spend, Option 3 is the best cost/performance balance.

Continuous Manufacturing

There are several continuous-manufacturing and automated forming technologies that can dramatically reduce hull labor cost. The best option depends on what material family you ultimately choose:

• composite

• thermoplastic

• ferrocement / concrete

• hybrid laminate

• metal

Below is a practical engineering comparison of scalable continuous hull manufacturing technologies, ranked by how well they fit your long rectangular pontoon-type hull geometry (2 ft × 8 ft × 60 ft).

1) Continuous pultrusion (best fit for long prismatic hulls)

What it is

Pultrusion is essentially continuous extrusion of reinforced structure. Fibers are pulled through resin and a heated die that forms a constant cross-section.

Think of it as structural profile manufacturing like an I-beam — but hollow hull-shaped.

Why it fits your hull extremely well

Your hull is:

• constant cross-section

• long

• straight

• structural shell

That is exactly what pultrusion is designed for.

What can be made

You can pultrude:

• full closed rectangular hull shells

• multi-cell buoyancy chambers

• integrated stiffeners

• sandwich skins (foam inserted inline)

• flanges for joining

• corrosion-proof structure

Labor reduction

Extremely high.

Once running, production becomes:

continuous → cut to length → seal ends

Labor reduction vs hand layup: 70–90%

Production speed

Typical structural pultrusion speed:

0.2 to 1.5 meters per minute

A 60 ft hull can be produced in under 1 hour of machine time.

Capital cost

Medium–high.

Industrial pultrusion line:

• $300k to $2M depending on size

But amortized over many hulls, cost per hull drops dramatically.

Structural performance

Excellent:

• fiber aligned for longitudinal bending

• high fatigue resistance

• low weight

• no corrosion

Limitations

• constant cross section only

• corners need radiused die design

• up-front tooling cost

2) Continuous filament winding on removable mandrel

What it is

Fibers are robotically wound around a rotating mandrel in programmed angles.

After cure, mandrel is removed or collapsed.

Why it works for pontoons

Perfect for:

• cylindrical or rounded rectangular hulls

• pressure-resistant shells

• multi-layer reinforcement orientation

Automation level

Very high.

Fully robotic winding cells exist.

Labor reduction

60–80% vs manual composite layup.

Advantages

• extremely strong shells

• fiber optimized for bending + torsion

• repeatable thickness

• minimal waste

Limitations

• mandrel removal complexity

• less efficient for flat-panel geometry than pultrusion

• slower than pultrusion for long straight members

Best use in your project

If you want:

• curved hull bottom

• high hydrostatic pressure tolerance

• premium structural performance

3) Thermoplastic continuous extrusion with reinforcement

What it is

Large-scale extrusion of structural thermoplastics, sometimes with:

• glass fiber

• carbon fiber

• mineral fillers

• foam cores co-extruded

Think of this as giant plastic structural pipe manufacturing but rectangular.

Key benefit

No curing time.

Extrude → cool → cut.

Labor reduction

Very high.

Additional advantages

• weldable joints

• impact resistant

• recyclable

• no corrosion

• extremely fast production

Limitation

Lower stiffness than fiber composite unless heavily reinforced.

Requires thick sections or internal ribs.

Best use

Commercial marine pontoons, modular barges, floating docks.

4) Large-scale robotic fiber placement on continuous moving mold

What it is

A moving conveyor mold passes under robotic fiber placement heads.

Used in:

• wind turbine blades

• aerospace fuselage sections

• advanced marine hulls

Advantage

Complex geometry possible with automation.

Limitation

High capital cost and setup complexity.

Overkill for rectangular hulls unless large production volume.

5) Continuous ferrocement spray forming over moving armature

What it is

Mesh cage moves along a track while robotic sprayers apply mortar.

Similar to shotcrete tunnel lining.

Automation potential

Moderate.

Labor still required for mesh preparation.

Major issue offshore

Microcracking + waterproofing complexity.

Where it works

Low-cost barges, floating breakwaters, infrastructure floats.

Not ideal for

Fatigue-critical dynamic offshore platforms.

6) Concrete 3D printing (layered extrusion)

Reality check for marine hulls

Technically possible but not yet industrially efficient for:

• watertight pressure vessels

• thin shells

• fatigue loading

Major problems:

• layer bonding anisotropy

• permeability

• reinforcement integration

• surface finishing cost

Currently better suited for:

• harbor structures

• breakwaters

• static floating caissons

7) Inflatable mold + sprayed composite (rapid shell forming)

What it is

Inflate a bladder to hull shape and spray laminate.

Used in rapid boat building.

Labor reduction

Moderate.

Limitation

Still manual finishing and reinforcement placement.

Engineering recommendation for YOUR specific hull geometry

Because your hulls are:

• long

• constant cross-section

• rectangular

• multi-cell capable

• structural pontoons

The clear winner is:

Continuous structural pultrusion

It gives:

lowest labor

highest repeatability

best fatigue performance

excellent corrosion resistance

very high production speed

It is the closest thing to industrialized hull manufacturing.

What an optimized pultruded pontoon could include

Single continuous manufacturing step producing:

• outer hull shell

• internal bulkheads every 2–4 ft

• foam-filled buoyancy chambers

• cable/pipe channels

• mounting flanges

• lifting lugs molded in

• anti-slip deck surface

• integrated stiffener webs

Essentially a complete finished hull leaving the die.

Cost comparison per hull (industrial scale estimate)

Hand-built composite hull: baseline 100%

Pultruded hull production:

• 40–60% of hand layup cost at moderate volume

• 25–40% at high volume

Labor reduction dominates savings.

If you want the most advanced concept

The most automated future-leaning system is:

Pultrusion + inline foam injection + robotic cutting + robotic end-cap welding

That becomes:

continuous hull factory

TEL: 1-608-238-6001 Email: greg@infinityturbine.com

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