```html Half-Scale Seastead Prototype – Feasibility & Cost Estimate

Half-Scale Seastead Prototype – Analysis

This is a rough-order feasibility study for a ½‑scale day‑sailor prototype of the semi‑submersible trimaran seastead. The goal is to validate software, motion control, and bolt‑together kit construction in sheltered Caribbean waters (Anguilla) while keeping the build light enough for a 40 ft shipping container and on‑site assembly with minimal tools (no field welding).

1. Half-Scale Key Specs

Triangle deck frame: two 35 ft sides, 17.5 ft transom, 3.5 ft deep (floor‑to‑roof). Open net / day‑sail layout.
Legs / Foils (×3): 9.5 ft total length, 5 ft chord, 1.5 ft max thickness (NACA 0030, extruded vertically). Designed waterline: ~50% submerged (≈4.75 ft).
Stabilizers (×3): 5 ft span, 0.5 ft chord, 3 ft fuselage, 1 ft tail span.
Propulsion: 2× Yamaha HARMO RIM‑drive thrusters (electric rim motors, transom or leg‑mounted).
Energy: 50 kWh LiFePO₄ battery bank; roof solar in place but not assumed for propulsion range.
Tender: 7 ft lightweight inflatable, towed on short bridle off the transom center.

  Scaling insight you spotted: Displacement drops by 1/8th, but the stabilizer foil area only drops by 1/4th.  
  Because lift scales with area while vessel inertia scales with mass (volume), the half‑size boat has roughly twice the lifting authority per unit of boat weight. That makes the small prototype more responsive, not less—exactly the behavior you want for testing in moderate swells.

2. Displacement & Buoyancy

Using the exact NACA 00xx profile area formula (integrated), a NACA 0030 with 5 ft chord has a cross‑sectional area of roughly 0.2055 × c² ≈ 5.14 sq ft. Extruded over a 9.5 ft length, each leg displaces about 48.8 ft³. With 50% submergence at rest:

Submerged volume per leg: ~24.4 ft³
Total submerged volume (3 legs): ~73.2 ft³
Seawater displacement: 73.2 ft³ × 64 lb/ft³ ≈ 4,685 lb

This is a true small‑waterplane‑area (SWATH) concept: the waterplane area is only the three foil outlines (~15.4 ft² total), so the boat sinks about 1 ft for every 1,000 lb of added load. That gives the soft, long‑period heave you are looking for.

3. Estimated Weight Budget

Item	Est. Weight	Notes
3 × Foil leg shells (6061‑T6, 3/16" skin + ribs)	1,300 lb	Welded tanks from China; ladders & thruster mounts pre‑fitted
Triangle truss frame (bolted 6082/6061 tube)	800 lb	Top/bottom chords, diagonals, node plates; no heavy house walls
Net, seats & minimal deck	100 lb	Trampoline net, 4 sling seats, small console
50 kWh LiFePO₄ battery bank	350 lb	~6.5–7 lb/kWh; 48 V nominal pack with BMS
2 × Yamaha HARMO RIM drives + mounts	200 lb	Standard outboard units adapted to leg struts
3 × Stabilizers & micro actuators	120 lb	Wing + elevator + servo; bolt onto leg trailing edge
Solar array + frame	80 lb	Semi‑flexible or framed panels on roof
Controls, wiring, nav lights, fenders	200 lb	Waterproof displays, joystick, cabling
7 ft inflatable dinghy + tow bridle	60 lb	Light RIB or roll‑up, towed on short line
Fasteners, gaskets, contingency	150 lb	316 stainless bolts, Delrin bushings, sealant
Dry Weight Subtotal	3,360 lb
Total Available Displacement	~4,685 lb
Payload Margin (people + gear + water)	~1,325 lb	Safe for 5–6 adults or 4 adults + gear

Construction note: Because the prototype is a day sailor, we drop the full‑scale glass house. Replacing walls with a net and using thin‑walled, bolted tube trusses is what keeps the weight closer to 1/6th of full scale rather than 1/8th, giving you a generous 1,300+ lb payload cushion despite the non‑scaling aluminum skin.

4. Range Estimate (50 kWh Battery)

The legs have very low wetted area compared with a conventional hull, but the thick NACA 0030 shape carries form drag. At the prototype’s speeds (not Froude scaled), drag is dominated by viscous + pressure drag on the three struts and any lift‑induced drag from the stabilizers if they are carrying load.

Mode	Est. Electrical Load	Endurance (50 kWh)	Theoretical Range	Practical Range (80% DoD)
4 knots (cruise)	~2.5 – 3.5 kW	14 – 20 h	56 – 80 NM	45 – 65 NM
5 knots (brisk)	~5.0 – 7.5 kW	7 – 10 h	35 – 50 NM	28 – 40 NM

  Bottom line for a “few hours of fun”: Even at 5 knots the prototype can
  motor out 10–12 NM, anchor or drift for lunch, and return on a single 50 kWh charge
  with plenty of reserve. For all‑day cruising at 4 knots, you have enough juice for
  40–60 NM in real‑world conditions (wind, small chop, stabilizer load).

5. Component Cost Estimate

Assuming free owner assembly, an on‑site crane, and a Chinese kit supplier shipping a 40 ft container to Anguilla.

Component / System	Low Estimate	Realistic Estimate	Notes
3 × Welded foil legs (incl.测试)	$7,500	$12,000	Largest fab item; plate, weld, fairing, test
Triangle truss (3 sides + joints)	$3,500	$6,000	CNC cut tubes, bolted flanges, anodize optional
Stabilizers (3 wings + actuators)	$1,500	$2,500	Small milled parts; actuators off‑the‑shelf
2 × Yamaha HARMO drives	$8,000	$10,000	Retail units; alternative leg mounts per your plan
50 kWh LiFePO₄ pack + BMS	$8,000	$14,000	Chinese cells/prismatic; price swings with lithium market
Solar array + roof frame	$800	$1,500	Semi‑flexible panels, marine cable entry
Controls, displays, wiring	$1,500	$3,000	Weatherproof screens, joystick or AP integration
Hardware, fasteners, net, seats	$800	$1,500	316SS bolts, spectra net, seat slings
7 ft tender + tow rig	$400	$800	Basic inflatable or RIB
40 ft container shipping + customs	$2,500	$4,500	China → Anguilla; insurance & port fees
Contingency / nice‑to‑have	$1,500	$3,500	Spare props, tools, paint, upholstery
Total Kit Cost (excl. local labor)	$35,500	$59,300

Reality check: If you are clever with used cells, smaller batteries, or simpler brushed actuators, you can approach the low end. If you want decorative fit‑and‑finish, redundant dissimilar thrusters, or a full navionics suite, budget toward the high end (≈$65k–$75k).

6. Off‑the‑Shelf Truss & Aluminum Parts

You asked if suitable marine truss exists off the shelf. Short answer: true “marine grade” truss is not common as a catalog item, but you have three practical sourcing paths:

Adapted Entertainment Truss (6082‑T6): Brands like Eurotruss, Prolyte (H30V / H40V), or Global Truss (F34 / F44) are 6082‑T6 aluminum, made in long sticks with bolted spigots. They are engineered for bending loads over stage spans. You would need to:
- Size for a 35 ft unsupported span (use HD series or double‑up chords).
- Replace mild‑steel spigots with 316SS and add Teflon/gaskets at joints.
- Specify a marine‑grade anodizing or epoxy barrier after machining.
Pros: Cheap, fast supply. Cons: Joints can be sloppy/hard to seal; not designed for cyclical wave loading.
Structural T‑Slot Framing: 80/20, Bosch Rexroth, etc. Great for prototyping internal racks, but very heavy and expensive for a 35 ft triangle.
Custom Chinese Kit Chords (Best Fit): A Chinese aluminum boat builder (common in Qingdao, Guangdong, or near Ningbo) can extrude or brake‑form your exact top/bottom chords with integral bolt plates, then ship them as 35 ft sticks. You add cross bracing with standard 2" × 2" × 1/8" 6061‑T6 tube from any metal distributor. This is the sweet spot for a one‑off: you get marine alloy, welded flanges, and container‑friendly lengths without paying for a full custom space‑frame design.

7. Build & Operational Notes

No welding in Anguilla: All joints are bolted or clamped. The foil legs arrive as sealed, internally‑framed tanks with an upper bolted flange. The triangle arrives in three 35 ft chord segments and one 17.5 ft stern segment. You bolt chords to node plates, tension the net, and step the legs.
Draft increase under load: With a SWATH geometry, adding 4 adults (~800 lb) will settle the legs roughly 10 inches deeper. That still leaves the top of each leg and the ladder well above water in the sheltered Anguilla lee.
Stabilizers as lifting foils: At 5 knots each 5 ft × 0.5 ft wing can generate several hundred pounds of lift with modest angle of attack. Using them to offload even 600 lb from the legs reduces draft slightly and can tune pitch, though it adds induced drag (factored into the range table above).
Testing envelope: This is a coastal / lagoon day sailor. Do not cross open ocean. The containerized kit concept is perfect for a beach club or cove‑to‑cove demo.

8. Summary

  Dry weight: ~3,400 lb
Displacement reserve: ~1,300 lb (5–6 adults + gear)
Range on 50 kWh: ~45–65 NM at 4 knots or ~28–40 NM at 5 knots (practical 80% discharge)
Build budget: ~$35k–$60k for the core kit delivered to Anguilla
Stabilizer authority: Effectively double per unit weight vs. full scale — ideal for software tuning

The concept looks both buildable and financable as a prototype. The biggest cost drivers are the custom foil leg weldments and the battery bank. If you can keep the day‑sailor interior minimal (net, seats, no house systems), the half‑scale model will float high, move efficiently on RIM drives, and give you a stable platform to prove the ride control algorithms before scaling up to the full 70 ft living space.

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