1 · The problem — and the shape of the answer
Your requirements create three distinct regimes, and most hull forms are good at only one of them:
3–5 ft @ 3–5 s
15 ft @ ~15 s
cheap · containerized · redundant
- In short chop, a normal monohull of this size has a pitch natural period of ~4–5 s — dead-center in your 3–5 s wave band. It will never be comfortable.
- In long swell, a pure low-waterplane platform (oil-rig style) can get caught by crests — you asked for a hull that lifts and rides over like a boat.
- Economically, yachts fail because they are hand-built sculptures. The fix is a flat-pack, bolt-together kit with maximum solar real estate.
The answer: a “flared SWATH” catamaran. Take the proven comfort machine of the ocean — the Small Waterplane Area Twin Hull — and add two modifications:
- Deep heave-damping plates under each hull push the natural periods above the chop band and absorb nearly all residual motion.
- Flared reserve-buoyancy shoulders from +0.7 m up to deck level. In ordinary conditions they sit dry and add nothing. When a 15-ft swell arrives, they immerse progressively — a smoothly stiffening spring that lifts the platform and carries it over the crest exactly like a displacement hull, with no slam.
The result is a vessel that behaves like a floating building 95 % of the time and like a boat for the rare big-swell hours — with a 15 × 9.6 m roof for solar and a structure simple enough to ship in five 40-foot containers.
2 · The concept
Anatomy, zone by zone
- Lower hulls (foam-filled). Two 12.5 m streamlined bodies carry ~80 % of the buoyancy, 1.15 m deep, tops just below the waterline. Every void is packed with closed-cell foam — puncture one and it still floats.
- Struts. Only ≈ 0.30–0.35 m of waterline beam per side (≈ 3.5 m² total waterplane). Short waves push on this tiny area, so they generate almost no force. Watertight, with collision bulkheads.
- Heave plates. 3.5 × 2.2 m plates at 2.4 m depth act like dampers and flywheels: they add large added-mass and damping, pushing heave/pitch natural periods to ~7 s and flattening whatever resonance remains.
- Flared sponsons. The boat-behavior trick (see §3).
- Cross-deck + deckhouse. A box-girder deck at +2.6 m ties the hulls together and carries the house. Freeboard is deliberately generous: in a quasi-statically-followed 15-ft swell the relative crest height at the deck is small, because the platform rises with the wave.
- Solar canopy. 13.5 × 8.5 m of tilted, walkable, hail-rated glass over the whole ship — also your rain catcher and shade roof.
Materials & joining. Marine-grade 5083 aluminum throughout (6061-T6 extrusions for frames and canopy). All welding happens in the factory on jigs; every field joint is a machined flange with an EPDM gasket and torqued bolts — no field welding, fully inspectable, and repairable with hand tools.
3 · How it handles each wave regime
The governing equation is the natural period: Tn = 2π·√(m / (ρ·g·Awp)).
Shrink the waterplane Awp and add plate added-mass, and you choose where the ship
“sits” relative to the wave bands.
Regime A — 3–5 ft @ 3–5 s trade-wind chop (the daily condition)
With heave/pitch natural periods of ~7 s, these 3–5 s waves are above resonance: the platform physically cannot respond fast enough. Expected heave is only ⅓ to ½ of the wave height, and vertical accelerations at the deck stay under 0.07 g. A 4-ft chop becomes a ~1.3-ft lazy bob. Laptops stay open, bread rises, coffee stays in the cup. Keep the bow 20–30° off the seas with the pods and roll stays under ~4°.
Regime B — the transition (6–10 ft)
Mixed seas are a non-event: motion grows gradually, nothing resonates, secure the breakables and carry on.
Regime C — 15 ft @ ~15 s distant swell (rare)
A 15-second swell is slower than the ship’s natural period, so the response becomes quasi-static: the platform simply follows the sea surface, like a boat (response ratio ≈ 1.0–1.15). Vertical acceleration is ≈ 0.04–0.05 g — a slow elevator, not a fall. As the crest lifts the ship, the flared shoulders immerse progressively and add up to +55 % buoyancy — a smoothly stiffening spring, so there is no abrupt stop and no slam. Tactics: bow-on or bow-quarter to the swell, pods holding position, sea anchor ready, delicate work paused for a few hours. That’s exactly the trade you asked for.
| Condition | Expected motion (est.) | Life aboard |
|---|---|---|
| 3–5 ft @ 3–5 s chop | Heave ≈ ⅓–½ wave height; < 0.07 g; roll < 4° | Full work & cooking |
| 6–8 ft mixed | Gentle sway, no resonance | Secure breakables |
| 15 ft @ 12–16 s swell | Rides over like a boat; ≈ 0.04–0.05 g; no slam (flare engaged) | Pause delicate work; enjoy the show |
| Squall, 30–40 kt wind | Pods hold course to ~25 kt apparent; beyond that, run off | Normal storm procedure |
4 · Principal specifications (concept targets)
| Item | Value |
|---|---|
| Type | Flared-SWATH solar catamaran, all-electric |
| LOA / Beam / Draft | 15.0 m / 9.6 m / 2.4 m (air draft 4.9 m) |
| Displacement | 19 t light · 24 t full load |
| Waterplane area | ≈ 3.5 m² total (two struts) |
| Natural periods (est.) | Heave ≈ 7 s · Pitch ≈ 7.5 s · Roll ≈ 3.5 s (heavily damped) |
| Reserve buoyancy to deck edge | ≈ +55 % of displacement |
| Intact stability (est.) | GM > 4 m; downflooding angle > 60°; ISO 12217 Cat-A target |
| Construction | 5083 aluminum, jig-welded; foam-filled hulls; bolted flange field joints |
| Propulsion | 2 × 6 kW azimuthing electric pods, independent battery banks |
| Speeds | Cruise 1.5–2.5 kn (1.7–2.9 mph) · sprint 4 kn · holds course vs ≈ 25 kt wind |
| Solar | 18 kWp (15 canopy + 3 auxiliary), ≈ 70 kWh/day Caribbean average |
| Battery | 80 kWh LiFePO₄ (2 × 40 kWh independent), expandable to 160 |
| Water | 2 watermakers (300 + 150 L/d) + 1,200 L rain cistern + 600 L day tanks |
| Accommodation | Duo: sleeps 2, 55 m² enclosed + 40 m² outdoor · Familia retrofit: sleeps 6 |
| Shipping | 5 × 40′ high-cube containers; longest part 11.9 m; heaviest module 3.2 t |
| Assembly | 3–4 weeks, 4–5 people, no field welding |
| Design life | 30+ years structure (aluminum), 10–15 yr battery, 25 yr panels |
5 · Energy system — 100 % solar, with real margins
Harvest: 18 kWp × 5.2 peak-sun-hours × 0.78 system efficiency ≈ 70 kWh/day average in the Caribbean (trade-wind belt = famously sunny; squalls are short).
| Daily load | kWh/day | Notes |
|---|---|---|
| Propulsion @ ~1.8 kn, ~16 h | 12 | 0.4–1.5 kW draw; triples into a hard headwind |
| Refrigeration + freezer | 1.2 | DC compressor, 12 m³ total cold |
| Watermaker (300 L) | 4.0 | Efficient DC Clark-pump type; rain catchment offsets |
| Cooking (induction) | 2.5 | Any time of day — battery buffers |
| Hot water (200 L store) | 1.5 | PV-surplus diversion + element |
| Electronics, comms, lighting | 2.5 | Incl. Starlink |
| Ventilation fans | 0.8 | Cross-flow, low power |
| A/C (bedrooms, 2 h pre-cool) | 2.5 | Policy: evenings only; fans otherwise |
| Laundry & misc | 1.5 | Compact washer, line dry |
| Total typical | ≈ 28.5 | vs ≈ 70 harvested → ≈ 40 kWh/day surplus |
- Cloudy-day protocol: 80 kWh bank ≈ 3 days of reduced operations with zero sun. Sustained overcast is rare on this route; if it happens, you drift with the current and stop making water.
- DC-first architecture: fridge, watermaker, fans, lights run straight off 48 V DC. Only cooking and tools pass through the two redundant 5 kW inverters. Fewer conversions, fewer failures.
- Expansion: mounting rails and conduit sized for +3 kWp panels and +80 kWh battery (Familia version or future air-conditioning upgrade).
6 · Layout — “Duo” first, “Familia” later
| Space | Area | Notes |
|---|---|---|
| Saloon + galley + office nook | 22 m² | 270° view; desk faces forward; converts to kids’ bunks in Familia refit |
| Owner suite | 13 m² | Queen berth, hanging locker, workstation |
| Head with shower | 4.5 m² | Second head added in Familia pod |
| Cockpit / work deck | 28 m² | Fishing, dive gear, workshop, dinghy davit, outdoor galley |
| Foredeck | 12 m² | Auxiliary solar, anchors, lounging; Familia bunk-pod lands here |
| Below deck | — | Battery room (vented, cooled), engineering, 1,800 L water, stores, workbench |
Living aboard (the family part)
- Food: troll two lines at 2 kn (surprisingly productive at seastead speeds), solar dehydrator, sprouting rack, optional aquaponics barrel; provisioned for 60 days.
- Kids: rail netting, door alarms, wearable PLBs, and Starlink-powered schooling. The Familia pod adds a proper 4-berth cabin + second head.
- Comfort: shade canopy everywhere, cross-ventilation, evening bedroom A/C, outdoor showers off the swim platforms.
7 · Safety & redundancy — designed for “new and unproven”
You said it yourself: this is new, so things will go wrong. The design assumes failures and pairs everything.
| System | Primary | Backup |
|---|---|---|
| Propulsion | Pod A on Bank A | Pod B on Bank B — either alone steers and makes 3 kn |
| Energy storage | 40 kWh LFP bank A | 40 kWh bank B, separate BMS, fused, thermally isolated, vented overboard |
| DC bus | 48 V bus 1 | 48 V bus 2 + independent 12 V critical bus (comms, alarms, nav) |
| Bilge | 2 pumps on bus 1 | 2 pumps on bus 2 + high-water alarms to satellite |
| Flooding | Foam-filled hulls (unsinkable sections) | 6 watertight volumes; any one flooded → afloat; +55 % flare reserve |
| Comms | Starlink | Iridium Certus + VHF/DSC ×2 + AIS + 2 EPIRBs + 3 PLBs |
| Anchoring | 35 kg plow, 100 m chain | Stern anchor + 200 m rope + storm drogue/sea anchor |
| Water | 300 L/d maker | 150 L/d backup maker + 1,200 L rain cistern |
| Fire | All-electric ship (no fuel aboard) | LFP chemistry, battery off-switches, smoke/heat/thermal-runaway detection, 5 extinguishers |
| Lightning | Bonded mast + ground plate | Surge protection on every antenna and PV string |
| MOB | 1.1 m rails + netting | MOB buttons, auto-hold autopilot, throw lines, ladder |
Storm doctrine (please read this twice)
- Primary strategy is avoidance — which is exactly what your route does: June–November is spent south of ~12°N, where hurricane tracks are climatologically rare.
- If a system threatens: run to a bail-out harbor (§8), double anchor scope, deploy chafe gear, close storm shutters, prepare the grab bag.
- Tortuga 15 is not designed to survive a direct hurricane strike at a dock or exposed anchorage. No small craft is. The design goal is: never be where the storm is.
8 · The Caribbean loop — and why this hull likes it
| Leg | Distance | Current & wind notes |
|---|---|---|
| ① East, north of Cuba | ~600 nm | Variable, generally benign; winter cold fronts worth watching |
| ② South, Lesser Antilles | ~500 nm | Crosses the west-setting Caribbean Current; brisk easterlies — comfortable angles |
| ③ West, north of South America | ~1,200 nm | Riding the Caribbean Current westward — free 0.5–1.5 kn; south of 12°N for the whole hurricane season |
| ④ North, Central America | ~900 nm | Coastal flows assist; frequent sheltered stops (San Blas, Bocas, Bay Islands) |
| ⑤ Return, Yucatán → Windward Passage | ~700 nm | Variable; plenty of bail-outs (Islas Mujeres, Cayman, Jamaica south coast) |
- At 1.8 kn average the full loop is ~90 sea-days — a 5–7 month annual circuit with long stops. Propulsion for the whole loop costs ≈ 1,100 kWh — about 16 sunny days of harvest, spread over months.
- Bail-out harbors en route: Grenada · Curaçao · Cartagena · San Blas · Bocas del Toro · Roatán/Utila · Isla Mujeres · Grand Cayman · south-coast Jamaica · Santo Domingo.
- Draft of 2.4 m opens up the shallow, well-protected anchorages that deep-keel yachts can’t use — a genuine quality-of-life advantage on this route.
9 · Build, shipping, assembly, cost
Container manifest (5 × 40′ high-cube)
| Container | Contents | Max piece | Weight |
|---|---|---|---|
| C1 | Port hull assembly (11.9 m center body + bow/stern plugs nested inside) | 11.9 m | 3.1 t |
| C2 | Starboard hull assembly | 11.9 m | 3.1 t |
| C3 | Deck-grid halves, sponson flare wedges, heave plates | 5.9 m | 2.8 t |
| C4 | Deckhouse flat-pack, glazing, interior kit | 5.9 m | 2.2 t |
| C5 | Energy & propulsion kit: panels, battery boxes, pods, inverters, anchors, comms, spares | 2.4 m | 2.4 t |
All parts ≤ 2.30 m wide, ≤ 3.2 t — standard container handling, standard trucks, a rented 20–25 t mobile crane for three days, or assembly on a slipway and a roller launch.
Assembly sequence (no welding)
- Set hulls on parallel ways, 7.2 m apart, with the supplied alignment jig.
- Bolt on bow and stern plugs (machined flanges, gaskets, torque wrench).
- Mount struts to hulls — factory-drilled, numbered holes.
- Attach heave plates and sacrificial wear strips.
- Float hulls (or launch later); lower deck-grid halves onto struts, bolt the center splice.
- Fit sponson flare wedges around the perimeter.
- Erect deckhouse panels, fit glazing and doors.
- Install canopy posts and solar array; wire strings to MPPTs.
- Install battery boxes, inverters, pods, plumbing; torque-and-tag every connection.
- Commissioning checklist: leak test, BMS config, pod calibration, anchor/MOB drills, 48-hour burn-in.
Crew: 4 handy people + 1 factory supervisor (fly-in for weeks 1 and 3). Duration: 3–4 weeks. Everything is numbered, illustrated, and video-call supported.
Cost (Duo version, USD, ±25 % until RFQs return)
| Line item | Low | High |
|---|---|---|
| Aluminum structure, fabricated & jig-welded (China) | $95k | $140k |
| Solar 18 kWp marine + MPPT | $14k | $20k |
| Batteries 80 kWh LFP + BMS | $16k | $26k |
| Pods, inverters, controls, wiring | $22k | $32k |
| Plumbing, watermakers, interior fitout | $22k | $36k |
| Safety, anchors, comms, spares | $16k | $24k |
| Freight (5 containers to Caribbean) | $12k | $20k |
| Local assembly labor + crane | $18k | $30k |
| Total | $215k | $328k |
Roughly ¼ to ⅓ the price of a traditional yacht with similar living space — and near-zero operating cost (no fuel).
Familia retrofit: +$30–45k (foredeck bunk pod, second head, +2 kWp, +40 kWh) — shipped later in a single container and bolted on in a week.
10 · Requirements traceability
| # | Your requirement | How Tortuga 15 meets it | Confidence |
|---|---|---|---|
| 1 | Family-scale MVP (couple first) | 55 m² Duo layout; Familia retrofit adds 4 berths in one container | High |
| 2 | Caribbean loop at 1–3 mph | 0.4–1.5 kW cruise draw; 4 kn sprint; holds course vs ≈ 25 kt wind; current-assisted loop | High |
| 3 | Totally solar | 18 kWp + 80 kWh; ≈ 70 kWh/day harvest vs ≈ 28.5 kWh/day use | High |
| 4 | Gentle in 3–5 ft @ 3–5 s | SWATH waterplane + heave plates; heave ≈ ⅓–½ wave height; < 0.07 g | Medium — tank test pending |
| 5 | Ride over 15 ft @ 15 s, no slam | Quasi-static follow (ratio ≈ 1.0–1.15); +55 % flare reserve; 2.6 m freeboard; sea anchor | Medium — tank test pending |
| 6 | Much cheaper than a yacht | $225–315k vs $0.9–1.5M+ comparable space | Med-High — RFQ pending |
| 7 | China fab, 40′ containers, easy assembly | 5-container bolt-together kit; 3–4 weeks; no field welding | High |
| 8 | Very safe: redundancy & reliability | Twin everything; foam-filled hulls; triple comms; storm-avoidance doctrine | High by design; class review pending |
11 · Alternatives considered (and why they lost)
| Configuration | Chop comfort | Big swell | Solar area | Cost | Kit-ability | Simplicity | Verdict |
|---|---|---|---|---|---|---|---|
| Displacement monohull | 2 | 4 | 2 | 3 | 3 | 4 | Pitchy in 3–5 s chop — fails Regime A |
| Sailing catamaran | 3 | 4 | 3 | 3 | 3 | 3 | Rig = cost, shading, skill, failure modes |
| Pure SWATH | 5 | 3 | 3 | 2 | 3 | 2 | No reserve buoyancy — buries in big swell |
| 4-column semi-submersible | 5 | 3 | 4 | 2 | 3 | 2 | Doesn’t “ride over”; deck-wetness risk; pricey |
| HDPE pontoon barge-cat | 2 | 2 | 5 | 5 | 4 | 5 | Cheap & sunny but unsafe in swell |
| Tortuga 15 (flared SWATH) | 5 | 4–5 | 5 | 4 | 4 | 3–4 | The balanced winner |
The insight: don’t pick between “platform” and “boat” — build a platform that becomes a boat exactly when the sea demands it. That’s the flare.
12 · Honest limitations & top risks
- Unverified motion numbers. The §3 estimates follow standard SWATH physics but must be confirmed by tank test before fabrication. This is the gating item.
- Strut fatigue & impact. Thin waterline sections work hard structurally. Mitigation: generous fore-aft strut length, 8–10 mm plating, full-penetration fatigue welds, dye-pen QC, FEA with 10⁸-cycle targets, replaceable bow caps, foam fill.
- Grounding. 2.4 m draft + plates demand chart discipline. Mitigation: sacrificial wear strips, foam fill, lifting eyes (hauls out at ~13 t), insurance.
- Windage in squalls. A 15 × 9.6 m sail area of structure. Mitigation: low-profile canopy, faired rails, 12 kW of pod thrust — holds course to ~25 kt apparent; beyond that you run off.
- Marina incompatibility. 9.6 m beam means mooring balls, med-moor, or your own moorings — which suits the route anyway.
- Regulatory & insurance novelty. Flag as a pleasure yacht; pursue CE-A / class survey to unlock insurers. Early adopters should expect higher premiums and paperwork.
- Biofouling. Warm Caribbean water fouls plates fast. Budget ~4 h/month of dive cleaning or specify foul-release coating.
- Supply chain. Pod motors and premium cells are the fragile links; the spares kit and a second-source RFQ strategy address this.
- Human factors. Security planning (route intelligence, AIS management, hardened doors), medical telemedicine links, and storm-avoidance discipline are operational requirements, not options.
13 · Development roadmap
| Phase | Months | Content | Budget |
|---|---|---|---|
| 0 — Design freeze | 0–6 | Naval-architecture package, structural FEA, 1:7 tank-test campaign, China yard RFQs (3 yards), cost lock | $40–70k |
| 1 — Prototype | 6–14 | Build unit #1, ship, assemble, 3-month shakedown on the actual loop route; instrument motions and publish data | $250–350k |
| 2 — Pilots | 14–24 | 3 units with pilot families; iterate interior, energy policies, Familia pod; insurance track record | Per-unit |
| 3 — Series | 24+ | 10 units/year, Familia variant, owner network, shared spares pools across the Caribbean | — |
14 · FAQ
Can it survive a hurricane at sea?
No — and neither can any small craft. The design philosophy is avoidance: your route already places you south of the hurricane belt in season, with a week’s notice to reach a bail-out harbor. At a sheltered anchorage with doubled scope and storm gear, it is a disciplined survivor; at an exposed dock in a direct strike, nothing about it is hurricane-proof.
What is the service life?
Aluminum structure 30+ years with anode and coating care; LFP batteries 3,000–5,000 cycles (10–15 years, replaceable in a weekend); panels 25 years; pods overhaul every ~5 years.
How much weekly maintenance?
4–6 hours: dive-clean the plates and hulls, rinse solar, check filters and anodes, glance at BMS logs. Quarterly: pod gearbox oil. Annual: haul for wear strips and coatings.
Could it cross an ocean?
It is designed for the Caribbean: protected-sea hops with weather windows, not open-ocean passages. The structure targets ISO 12215 Category A, but the operational envelope is the basin you described.
Why no sails or a wind generator?
At 1–3 mph, solar covers propulsion with a 2.5× margin. A rig would cost money, shade the array, demand skill, and add the exact failure modes you’re trying to engineer out. (A towing kite is a possible future option — the stern is designed for it.)
What about security and piracy?
Route intelligence, AIS transmission management where lawful, steel-core doors, shuttered glass, a safe, satellite tracking shared with family, and — most powerful — the convoy effect of an owner network on the same loop.
Is the 15-ft-swell claim guaranteed?
It is a physics-based estimate pending the tank test. The mechanism (quasi-static follow + progressive flare) is textbook; the specific tuned numbers for this size are exactly what Phase 0 buys you.