Scaling down a semi-submersible/SWATH design by 50% for a prototype is an excellent engineering strategy. At 1/8th the volume, the "square-cube law" dictates that buoyancy drops faster than surface area. However, by substituting the heavy enclosed living area with lightweight marine catamaran nets and an open truss, this makes for a highly viable, extremely stable, and incredibly fun "day sailor."
Applying a 50% reduction to your full-scale dimensions gives us the following geometry for the prototype:
Because aluminum sheet thickness cannot simply be scaled down to 1/8th without sacrificing structural integrity, the relative weight of the hulls will be slightly higher. We overcome this by stripping out the heavy superstructure.
| Component | Details & Assumptions | Estimated Weight (lbs) |
|---|---|---|
| 3x Foil Legs | Constructed using 1/8" (3mm) marine 5083/5052 aluminum with internal ribs for stiffness. | 950 lbs |
| Triangle Truss Frame | Using off-the-shelf lighting/stage aluminum box truss (see section 5). Approx 120 linear feet including crossbeams at ~6 lbs/ft + custom plates. | 800 lbs |
| Batteries (50 kWh) | Marine-grade LiFePO4 server-rack style (10x 5kWh units) or bare cell banks. | 1,000 lbs |
| Propulsion | 2x Yamaha HARMO Rim Drives (no heavy outboard brackets required). | 250 lbs |
| Seats, Net, Steering, Hardware | Catamaran heavy-duty trampoline nets, basic marine seating for 4-6, console, wiring, bolts. | 300 lbs |
| Misc Build Gear | Stabilizer planes (actuated), 7ft small RIB dinghy (hauled outboard), solar panels for trickle charge. | 300 lbs |
| Total Lightship (Empty) Weight | ~3,600 lbs |
Buoyancy & Payload Capacity:
Since buoyancy at exactly 50% draft is 4,685 lbs, you have 1,085 lbs of payload capacity before dipping below the 50% mark. This accommodates roughly 5-6 adults plus day gear.
Even if you load it to 5,500 lbs (allowing for heavier building specs and more friends), the draft will merely increase from 4.75 ft to ~5.6 ft. This leaves nearly 4 feet of the legs above water, which is completely acceptable, especially considering the lifting foils (discussed below).
Because the foil shape ensures an incredibly low wave-making drag (small waterplane area principle), navigating through the water depends primarily on skin friction.
Conclusion: Let's say you go out for a 4-hour day sail—you will likely return to dock with over 70% of your battery remaining. There will be absolutely zero range anxiety.
Assuming you assemble it yourselves directly off a container in Anguilla (saving $20k+ in labor and yard fees), here is a conservative cost breakdown for bootstrapping the prototype.
| Item | Estimated Cost (USD) |
|---|---|
| Custom Aluminum Legs & Stabilizers (China Fab) | $12,000 |
| Off-the-shelf Aluminum Truss Frame | $4,500 |
| 50 kWh LiFePO4 Battery Bank | $12,000 |
| 2x Yamaha HARMO Rim Drives & Controllers | $10,000 |
| Netting, Seats, Custom Corner Brackets & Fasteners | $3,500 |
| 7ft Inflatable RIB Dinghy | $1,500 |
| Shipping: 40ft Container to Anguilla + Import Duties | $9,500 |
| Total Estimated Cost | ~ $53,000 |
Using standard trussing instead of custom fabricating the top triangle is a brilliant hack for a prototype.
Recommendation: Heavy-Duty Entertainment/Stage Truss
Entertainment box truss (from brands like Global Truss, Prolyte, or equivalent Chinese suppliers) such as standard F34 (11.4" square) or F44 (15.7" square) truss is manufactured using high-strength 6082-T6 or 6061-T6 Aluminum alloy. These alloys have fantastic baseline marine corrosion resistance.
At 1/8th the weight of the full vessel, your 1/4 size stabilizer airplane foils suddenly possess double the relative authority.
Even though they are primarily stabilizers, if you set the elevators to maintain a positive angle of attack, a 5-foot wing span traveling at 5 knots generates roughly 100-150 lbs of dynamic lift per leg (300-450 lbs total). This effectively "lifts" the boat by a couple of inches while moving, decreasing your submerged volume and dynamically lowering your skin friction drag!
This half-scale prototype is highly feasible, fits well within a 40-foot container, and looks likely to come in between $50k and $60k. Between the catamaran netting, the bolt-together structural truss, and utilizing Yamaha's electric RIM drives, it transforms a complex ship-building project into a massively enjoyable, adult-sized Erector Set. Testing this downwind of Anguilla will provide flawless validation precisely because the semi-submersible foil legs combined with oversized (relatively) stabilizer "airplanes" will negate whatever small chop makes it past the island's reef.