Half-Scale Seastead Prototype: First-Cut Weight, Range, Buoyancy, and Cost Estimate

Below is a rough engineering estimate for a half-scale prototype of your seastead/day-sailer concept. I am treating this as a conceptual feasibility estimate, not a final naval architecture design. There are enough unusual features here—small waterplane area, wing-shaped buoyancy pods, active stabilizers, distributed rim drives—that a proper design review by a naval architect and marine structural engineer would still be very important.

1. Assumed Half-Scale Geometry

You described the full-size version, then asked about a half-size version. I assumed all linear dimensions scale by 0.5. That gives:

Because the half-scale enclosed cabin would only be about 3.5 ft high, I assume the prototype is not enclosed as a true cabin, but instead is an open truss/deck platform with seats, netting, light solar frame, and maybe a small central fairing. That is consistent with your “day sailor” comment.

2. Buoyancy Estimate

2.1 Volume of each half-scale main float/leg

You gave the main leg as approximately a NACA 0030 foil, 5 ft chord, 1.5 ft max thickness, 9.5 ft span/length. A good approximation for a symmetric foil section area is about ~68% of chord × thickness. So:

• Section area ≈ 0.68 × 5 × 1.5 = 5.1 ft²
• Volume per leg ≈ 5.1 × 9.5 = 48.5 ft³

For 3 legs:

• Total enclosed displacement volume ≈ 145.5 ft³

Using seawater at about 64 lb/ft³:

• Maximum buoyancy if fully submerged ≈ 145.5 × 64 = 9,312 lb

2.2 Buoyancy at 50% submergence

At about half submerged:

• Displaced volume ≈ 72.8 ft³
• Buoyancy ≈ 72.8 × 64 = 4,656 lb

If your loaded weight is higher than that, the waterline rises above half-depth, which you already said may be acceptable. At roughly 60% submergence the buoyancy would be about:

• 0.60 × 9,312 ≈ 5,590 lb

At 70% submergence:

• 0.70 × 9,312 ≈ 6,520 lb

3. Rough Weight Estimate

This is the hardest part, because small custom aluminum marine structures often end up heavier than expected. I’ll give a light, medium, and conservative estimate.

3.1 Main legs/floats

For each float:

• Approx wetted shell area of one enclosed foil body ≈ 90–110 ft²
• Using marine aluminum plating and internal frames/bulkheads, a realistic finished structure might be 350–550 lb per float

So for 3 floats:

3.2 Triangle truss/frame

For a 35 ft / 35 ft / 17.5 ft triangular aluminum truss platform, open and lightly decked, not a true cabin:

A reasonable planning number is:

• Triangle truss/frame: 1,100 lb

3.3 Stabilizers and actuators

Three small active stabilizers, including shafts, bearings, brackets, actuator hardware, and control linkages:

• 150–300 lb total

Use:

• 225 lb

3.4 Propulsion

You suggested 2 Yamaha HARMO units. Depending on what parts are retained and how much custom mounting is required:

• 2 drive units + controls + cables + mounts ≈ 250–400 lb

Use:

• 320 lb

3.5 Batteries

For 50 kWh lithium battery:

• At 140–170 Wh/kg pack-level: about 650–790 lb

Use:

• 725 lb

3.6 Seats, net, railings, small solar frame, wiring, controls

Use combined:

• 500 lb

3.7 Total prototype dry weight estimate

A realistic total range is probably:

• Optimistic lightweight build: 3,600–3,900 lb
• Likely build: 4,100–4,500 lb
• Conservative: 4,800–5,400 lb

4. Payload / Extra Buoyancy

At 50% submergence, estimated buoyancy is about 4,656 lb. Comparing to the likely dry weight:

• Buoyancy at 50% draft: 4,656 lb
• Likely dry vessel weight: 4,220 lb
• Remaining payload at 50% draft: ~436 lb

That is only about:

• 2 adults plus small gear, or
• 3 lighter people with minimal gear

At 60% submergence:

• Buoyancy ≈ 5,590 lb
• Remaining payload ≈ 1,370 lb

That is much more practical for a day boat:

• 4–6 people + picnic gear + safety gear

That may still be acceptable if:

• the deck stays comfortably above the water,
• waterplane area is still small enough for a soft ride, and
• the active stabilizers can provide dynamic lift once underway.

5. Could the Stabilizers Carry Some Weight?

Yes, potentially. Your half-scale stabilizer main wing is approximately:

• Span = 5 ft
• Chord = 0.5 ft
• Area per stabilizer = 2.5 ft²
• Three stabilizers total area = 7.5 ft²

Lift in seawater at 5 knots can be meaningful even for small foils. Very rough numbers:

• 5 knots = 8.44 ft/s
• Dynamic pressure in seawater is high enough that at moderate lift coefficient, total lift might reach roughly 200–600 lb for all three combined

That is highly dependent on:

• foil section,
• submergence,
• ventilation risk,
• control angle,
• wave encounter,
• drag penalty.

So yes: they may help unload the floats by a few hundred pounds at speed, and more importantly may help with pitch/roll damping.

6. Range Estimate with 50 kWh at 4–5 knots

This is very uncertain because there is no close standard hull analogy. Your vessel is not a normal displacement monohull, not a planing hull, and not exactly a normal trimaran either. The foil-shaped pontoons help reduce drag, but the structure and appendages add drag.

So the best way is to estimate required propulsion power at 4–5 knots.

6.1 Plausible propulsion power range

These numbers are for calm/sheltered water and include drivetrain losses in a rough sense.

6.2 Usable battery energy

From a 50 kWh pack, you normally do not want to use 100%. Assume:

• Nominal battery = 50 kWh
• Usable = about 45 kWh

6.3 Endurance and range

If the hull and appendages come out especially clean and efficient, maybe 30–45 nmi. If weight creeps up or the stabilizers and mounts add more drag than expected, maybe only 15–22 nmi.

7. Cost Estimate to Build

Assuming:

• Chinese fabrication for aluminum parts,
• container shipment,
• you do local assembly,
• no labor cost for your own assembly,
• you already have crane access.

7.1 Major cost buckets

If you aggressively simplify it—no solar initially, smaller battery, simpler seats, simpler controls, no decorative finishing—you might squeeze toward $60k–$80k.

8. Marine Aluminum Truss Parts: Off-the-Shelf?

There are off-the-shelf aluminum truss systems, but most are for:

• stage/lighting,
• trade show structures,
• temporary architecture,
• non-marine use.

These are usually not ideal for marine primary structure because of:

• fatigue concerns,
• connector looseness,
• galvanic corrosion at fittings,
• unknown wave slam load capacity.

What may be more practical is:

• 6061-T6 or 6082-T6 aluminum tube with custom bolted gusset plates, or
• marine alloy 5083/5086 plate and extrusions for the wet parts, plus
• bolted dry-structure modules above deck.

Some useful product categories to search for:

• marine aluminum extrusions
• aluminum space frame connectors
• bolted aluminum truss nodes
• architectural aluminum tube node systems
• offshore walkway aluminum trusses

For the actual marine primary structure, I would be cautious about using entertainment/stage truss unless it is dramatically overbuilt and carefully reviewed.

9. Overall Feasibility Judgment

Yes, I think the half-scale prototype could be a reasonable and fun day-sailer/test platform, provided you accept these points:

• It probably floats deeper than 50% at rest unless the build is very light.
• It is probably best as an open platform, not an enclosed cabin.
• Its likely useful range is around 20–30 nmi at 4–5 knots with 50 kWh.
• Dynamic stabilizer lift may improve comfort and payload margin underway.
• The largest design risks are motion behavior, foil ventilation, structural fatigue, and control tuning.

10. Summary

11. If You Want, Next Step

If you want, I can next produce one of these in HTML too:

• a more formal weight budget table you can edit,
• a hydrostatics table showing buoyancy at 40%, 50%, 60%, 70% draft,
• a propulsion/range calculator with inputs you can tweak, or
• a concept GA drawing description with dimensions laid out clearly for a fabricator.