Bottom Line Up Front
- A realistic drag coefficient for your fat (30% thickness) foil legs, based on frontal area and including finite-span, wave-making, and junction effects, is Cd ≈ 0.08–0.13; a good design value is 0.10. A circular cylinder of the same volume runs Cd ≈ 0.60–0.70.
- Each foil leg pushes water roughly like a 6½-inch-diameter pipe — not a 5-foot cylinder.
- Versus a similar-weight (≈ 36,000 lb) trawler or cat, expect roughly 1.5–1.8× their drag. Versus a similar-length (80 ft) trawler or cat that outweighs you 2–3×, you're roughly equal (0.9–1.3×).
- Your drag is friction-dominated and nearly linear with speed — no displacement "hump." A 40′ trawler hits a wall above ~7 kn; you don't.
- I have not seen this exact combination built or published. The nearest relatives are SWATH ships, the SLICE/TriSWATH multihull concepts, the ClubStead seastead study (round legs), and foil-section struts from hydrofoilers and shaft brackets. Every ingredient is proven; the combination looks genuinely novel.
All figures are desktop estimates (±30–50%), calm water, clean smooth paint, zero wind, legs at 0° yaw. See the disclaimer at the end.
The Design as I Understand It
Assumptions used throughout
- Symmetric NACA-0030-class section (t/c = 30%), chord 10 ft fore-aft, 3 ft thick athwartships, constant section over the 19 ft length.
- 9.5 ft of each leg immersed (your 50% spec) → wetted frontal area 3 × 9.5 = 28.5 ft² per leg, 85.5 ft² total.
- Displacement follows from your 50% immersion: 3 legs × 390 ft³ × 50% × 64 lb/ft³ ≈ 37,400 lb (16.7 long tons). If your real weight differs, immersion percent shifts — every +3,300 lb sinks you ≈ +10 inches (see stability section).
- Clean, smooth, unfouled surfaces; calm water; no wind; boat at zero yaw; appendages (junctions, ladder pockets, thruster blisters, stabilizer mounts) lumped as a +25% allowance.
What Drag Coefficient Would These Fat Foils Really Have?
Thirty percent thickness is very fat by airfoil standards (most sections are 12–18%), but underwater at your Reynolds numbers it's well-behaved. Here's the build-up:
| Contributor | Cd (frontal-area basis) | Basis |
|---|---|---|
| 2-D section alone, smooth, Re 6–9 million (NACA-0030 class) | 0.033 – 0.043 | Abbott & von Doenhoff section data (tested to 24%) extrapolated to 30% |
| + Finite span: wetted aspect ratio ≈ 0.95 (9.5 ft span ÷ 10 ft chord), one free tip, one end at the free surface | × 2 → 0.07 – 0.09 | Hoerner low-aspect-ratio strut data |
| + Wave-making (chord Froude 0.38 @ 4 kn, 0.56 @ 6 kn), leg-to-deck junction, ladder pockets, thruster blisters | +25–40% → 0.09 – 0.13 | Semi-sub / strut fairing experience |
| Design value used here | 0.10 | — |
| Circular cylinder, same Reynolds range (supercritical) | 0.60 – 0.70 | Classic cylinder data |
Why "fat" works fine here
- Gradual pressure recovery. A 30% section decelerates the flow slowly over a long tail; the boundary layer stays attached. Bluff-body separation — the thing that makes cylinders expensive — never happens.
- Big leading-edge radius. Thick sections tolerate speed changes, yaw, and fouling far better than skinny ones. Just don't sharpen the nose for looks.
- Fully turbulent flow. At Re 6–9 million there's no laminar-bubble drama; drag is predictable and roughness-tolerant.
- No cavitation concern. At 4–6 kn you're nowhere near cavitation speeds. Occasional ventilation (air sucked down) in steep wave crests is possible but minor at these velocities.
- The volume game is what you're winning. Going from 18% to 30% thickness maybe doubles the section's own drag coefficient — but it lets the same buoyancy live in a 3-ft-wide body instead of a 5.1-ft cylinder, and frontal area is what multiplies against q.
Headline Answer: Percentage vs an Equal-Volume Cylinder
Per foot of wetted span, per leg:
- At 4 knots: each foil leg costs about 9–11% of the drag of an equal-volume cylinder leg.
- At 6 knots: about 10–13% (wave-making grows a little faster than friction, nudging the ratio up).
- In plain terms: an ~89% drag reduction, or equivalently, each 3-ft-thick foil leg tows like a 6½-inch pipe.
This magnitude (80–90% reduction) matches what the offshore industry sees with fairings on risers and towed arrays: well-designed teardrop/foil fairings reliably cut cylinder drag by 70–90%. Your long 3.3:1 chord-to-thickness ratio is better than most commercial fairings, so the upper end of the reduction range is credible.
Total Drag Estimate — the 3 Legs, Then Everything
| Drag budget item | @ 4 kn (lbf) | @ 6 kn (lbf) | Note |
|---|---|---|---|
| 3 foil legs — profile + skin friction (Cd 0.10) | 385 | 870 | Core answer to your question |
| Wave-making, junctions, appendages (+25%) | 100 | 220 | Grows fastest with speed |
| 3 legs, subtotal | ≈ 485 | ≈ 1,090 | ±30% |
| Towed 14′ RIB (riding in your lee) | 110 | 220 | Displacement-mode tow |
| Structure windage (moving through still air) | 30 | 60 | Truss + house + rails |
| TOTAL, calm conditions | ≈ 625 | ≈ 1,370 | ±30–40% |
Headwinds add fast: a 20-kt headwind while making 6 kn adds roughly 700–900 lbf of windage on the big truss. In weather, you slow down anyway — your drag curve punishes speed mildly, wind punishes it hard.
Comparisons: Similar Weight and Similar Length
Benchmarks are derived from typical published fuel burns (a diesel at ~35% engine efficiency and ~55% propulsive efficiency delivers roughly 0.9 lbf of thrust per 0.7 gph at 6 kn — i.e., a 40′ trawler burning 3 gph is pushing against ~750 lbf).
| Vessel | Displacement | Drag @ 4 kn | Drag @ 6 kn | Comment |
|---|---|---|---|---|
| Foil-leg seastead (this study) | ~37,400 lb | 500–750 | 1,100–1,650 | Legs + dinghy + calm-air windage |
| Same volume in cylinder legs (thought experiment) | ~37,400 lb | 4,500–6,000 | 10,000–12,500 | Why the foil shape is the whole ballgame |
| 40′ full-displacement trawler | ~36,000 lb | 320–480 | 600–900 | Burns ~2.5–3.5 gph @ 6.5 kn |
| 45′ cruising catamaran | ~36,000 lb | 340–520 | 660–1,000 | Slender hulls, big wetted area |
| 80′ full-displacement trawler | ~120,000 lb | 480–720 | 960–1,440 | 3× your weight, similar drag |
| 80′ power catamaran | ~80,000 lb | 420–640 | 860–1,300 | 2× your weight, similar drag |
Reading the ratios honestly
- Versus similar weight (≈ 36 klb): you'll burn roughly 1.5–1.8× the fuel of a purpose-built 40′ trawler or 45′ cat at the same speed. Those are racing-snake efficiency machines with almost no payload area. You are not — you're an 80 × 40 ft island.
- Versus similar length (80 ft): you're at 0.9–1.3× the drag of vessels that outweigh you 2–3×. That's the real headline: 80-foot-platform livability at 40-to-50-foot-vessel drag.
Two lenses on efficiency (both @ 6 kn)
| Metric | Foil seastead | 40′ trawler | 80′ trawler | 80′ power cat |
|---|---|---|---|---|
| lb of drag per long ton | ~82 | ~47 | ~22 | ~30 |
| lb of drag per 1,000 ft² of platform footprint | ~430 | ~1,700 | ~680 | ~570 |
Per ton, everyone beats you — displacement haulers get relatively more efficient as they grow. But per square foot of usable platform, you're the most slippery thing on the table, and for a seastead, area is the payload. That's the metric that matters for your mission.
The missing "hump"
A 40′ trawler meets its hull-speed wall near 7–7.5 kn and drag goes vertical. Your legs' wave-making starts from a tiny base and grows gradually — pushing to 7–8 kn costs you a superlinear but gentle penalty, not a wall. If you ever want to outrun weather, the foil-leg layout is unusually forgiving about it.
Power, Thrusters, and the Solar Story
- Six rim drives is a good fit: cruise on 2–4 units at their sweet spot, fire all six for 6+ kn, maneuvering, and station-keeping. Differential thrust fore/aft gives you steering without rudder drag; the legs' own side-force helps turns.
- Solar budget: 1,600 ft² of roof realistically carries ~25–30 kWp (flat-mount, walkway gaps). In decent sun that's ~90–120 kWh/day. Minus ~20 kWh for hotel loads, you can motor at 4 kn for roughly 8–10 hours daily on sunshine alone in sunny latitudes.
- 6 kn is a battery event, not a solar cruise: ~31 kW means you'll want a 100–200 kWh bank for bursts, cloud gaps, and night arrivals — or accept 4 kn as your solar cruise and 6 kn as your generator/battery sprint.
- Nice property: your propulsion power scales almost with V² (friction-dominated), so slowing from 6 → 4 kn cuts power by ~65%.
Stability & Motions Snapshot (rough — verify with real weights)
| Quantity | Value | Meaning |
|---|---|---|
| Waterplane area | ≈ 62 ft² | A 40′ monohull has ~280 ft². Yours is genuinely "small oil platform" class. |
| Tons per inch | ≈ 330 lb/in | +3,300 lb of supplies = +10 in sinkage. Weight budget discipline matters. |
| Heave period | ≈ 19 s | Above nearly all ocean swell energy (8–14 s typical, storms to ~18 s). This is the big seakeeping prize. |
| Roll period | ≈ 3 s | Very stiff (legs spread 40 ft → huge transverse metacentric height). Quick but small-amplitude; your active stabilizers are the right tool to damp it. |
| Pitch period | ≈ 2 s | 80 ft longitudinal spread → very stiff, small amplitude in matched seas. |
| Crest clearance | ≈ 9.5 ft | Occasional green water under the deck in big seas — design the underside and railing for slam loads. |
| Max buoyancy (legs flooded to top) | ≈ 75,000 lb | Keep operating displacement ≤ ~55–60k lb for reserve. More weight = deeper immersion = more drag. |
Small waterplane also means low heave damping — the platform will glide through long swells beautifully but needs the active stabilizer planes (and/or an anti-roll tank) to kill residual oscillation. Your little airplanes with servo elevators are exactly the right architecture: small actuator, big plane, lift balanced on the pivot.
Real-World Watch-Outs (these move the numbers)
- Biofouling is enemy #1. A stationary seastead grows barnacles fast. Weeks of fouling can add 20–50% drag; months can double or triple it. Budget a cleaning ROV or diver cadence and foul-release coating. This — not the foil shape — will decide whether you get the numbers on this page.
- Junction drag. Where each leg meets the deck underside, horseshoe vortices form. Generous fillets are cheap insurance (worth 10–20% of total drag).
- Ladder placement. Keep the ladders recessed into pockets, not protruding from the leading edge — even above the waterline they live in the spray zone and trip the flow at the worst spot.
- Storm windage. A 40-kt blow on an 80 × 40 truss is 1,500–2,500 lbf. Rig a drogue/sea anchor and a storm heading strategy; the water stabilizers can't help you in air.
- Yaw discipline. Thick sections are tolerant, but sustained sideslip beyond ~5–8° gets draggy. Steer gently with differential thrust; consider small fixed skegs behind the aft legs for cheap course-keeping at cruise.
- Rim-drive hygiene. Grate the intakes against lines and bags, keep the units flush-faired, and design for diver/ROV inspection — fishing net wrapped in a rim motor is a mission-ending event offshore.
- Coatings & corrosion. The waterline band on each leg works hardest (wave slap, oxygen, sunlight). Spec a premium coating system there and inspect annually.
Have I Seen This Idea Before? Short Answer: No — But Its Parts Are Proven
I'm not aware of any built or published vessel that combines all three: (1) small-waterplane-area floats at the edges of a wide platform, (2) the floats being thick NACA-section legs that are the entire buoyancy, and (3) the whole package designed to transit efficiently under power. Searching patent and concept literature under "TriSWATH," "SWASH-type hull," "faired column semi-submersible," and "seastead" would be worthwhile — but to my knowledge this specific combination is novel, and it's a clever one.
The family tree
- SWATH ships (1970s–present, e.g., Radisson Diamond, Navy ocean-surveillance ships) — prove the tiny-waterplane seakeeping magic. But their buoyancy lives in fully-submerged torpedo pods; the struts are thin and carry no buoyancy.
- SLICE (Pacific Marine, 1990s) and TriSWATH (Navatek) — three- and four-strut small-waterplane multihulls. The closest published geometry to yours, still with submerged pods doing the floating.
- Semi-submersible oil platforms — your stated inspiration. Always round/rectangular columns, because they were never meant to go anywhere fast.
- ClubStead (Seasteading Institute, 2009) — a mobile seastead on four cylindrical legs. The closest mission cousin, held back by exactly the bluff-body drag you've engineered away.
- Foil-section underwater struts — thoroughly proven: rudders, keel fins, propeller shaft brackets, Moth/AC75 hydrofoil arms, towed-sonar fairings (70–90% drag reduction vs bare cylinders). You're applying a proven tool to a new job.
Verdict: every ingredient is flight-proven; the recipe looks original. The concept — "a small oil platform whose legs are wings" — is a genuinely interesting niche: station-keeping comfort of a semi-sub, transit economics approaching a small trawler, and a solar field the size of a tennis court.
Suggestions to Tighten the Design
- If buoyancy margin allows, try 26–28% thickness instead of 30% — worth another ~10% drag reduction for modest volume loss.
- Keep the NACA-0030 nose shape with its generous leading-edge radius. Fat sections stall abruptly if someone "streamlines" the nose too sharp.
- Add fillets (12–18 in radius) at every leg-to-deck junction.
- Surface finish is real money at Re 10⁷: a rough or fouled leg can cost 30–50% of these gains. Smoothness is your cheapest horsepower.
- Give the stabilizer planes a proper control law (heave-rate + roll feedback, plus slow trim mode) — with three independent planes you also get redundancy and can counter heel from supplies or wind.
- Before cutting metal: a 1:7 scale model (leg chord ≈ 17 in) tow-tested at a university tank, or an OpenFOAM free-surface simulation, would firm up these ±30% numbers quickly and cheaply.
- Log thruster torque vs GPS speed from day one — within a month you'll have a real drag curve of your own vessel, and fouling will announce itself numerically.
Disclaimer
All values on this page are desktop engineering estimates assembled from standard references (Hoerner's Fluid-Dynamic Drag, Abbott & von Doenhoff section data, ITTC friction lines, and published fuel-consumption data for comparable vessels). They carry ±30–50% uncertainty, ignore your specific structural details, and are not a substitute for CFD, model testing, or review by a licensed naval architect — particularly for stability, structural slamming loads, and classification requirements.