Design brief#
Sunwake SW-15 · SW-BRIEF Rev 0 · August 2026
This brief sets out what the boat is for and the choices that follow from that. The design specification covers the engineering requirements, and the lines plan defines the hull shape. Units are metric except for knots and nautical miles. Costs are in mid-2026 USD. The figures come from the calculation package in calc/; estimates and judgement calls are identified as such.
1. Vision#
Sunwake is intended to be a handsome, traditional-looking cruising catamaran for four. Propulsion is electric for day-to-day running, with diesel generation for long passages. The roof carries as much solar as the proportions allow, and the battery is sized around the output of the array and generators. The saloon is galley-up, quiet underway and open to the view.
Appearance is part of the brief. The boat should have a long, low, dark-hulled profile, with its traditional character coming from proportion and sheer rather than from imitating a monohull.
The owner will build the plywood-and-epoxy structure from a CNC-cut kit. Each hull has four developable strakes per side and can be assembled in a shed. Qualified professionals will engineer, install and commission the high-voltage electrical system, with staged sign-offs along the way. The design and build plan preserve that division of responsibility.
2. Mission profile#
| Home water | Coastal Maine |
| Seasonal range | US East Coast and the ICW, the Great Loop, Florida and the Bahamas |
| Silent cruise | 7.5–8 kt |
| Solar cruise | 7–7.5 kt |
| Passage speed on generators | 8–8.5 kt, 9–10 kt when schedule demands |
| Weather sprint | about 15 kt, two minutes |
| Long hauls | multi-day passages to 600 nm on generator |
| Nights | mostly at anchor; the boat must be energy self-sufficient on the hook |
| Climate | heating-dominant; one reversible seawater-source heat pump covers Maine heat and tropical sleeping-cabin cooling |
| Crew | four people, one head, galley-up saloon |
| Season | April to October afloat, hauled November to March |
3. Principal dimensions#
Computed from the lines plan at half load unless noted.
| Parameter | Value |
|---|---|
| LOA including spoon stem and integral stern platforms | 15.60 m |
| LWL, canoe body (the integration basis) | 15.00 m |
| LWL, effective (bottom running out into the platforms) | 15.30 m |
| Beam overall at the deck flare | 7.29 m |
| Hull centres | 5.00 m (s/L 0.33) |
| Demihull waterline beam | 1.404 m (L/B 10.7) |
| Volumetric slenderness L/∇^⅓ per hull | 8.17 |
| Draft, canoe body / to prop tips | 0.675 m / 0.95 m |
| Freeboard / bridgedeck clearance | about 1.35 m / 0.90 m (6.0 % LWL) |
| Air draft, mast folded | 3.53 m |
| Displacement light / half / full | 11.80 t / 12.70 t / 13.58 t |
| Prismatic coefficient Cp | 0.592 |
| LCB / LCF | 53.9 % / 58.1 % LWL abaft the bow |
| Wetted surface, canoe body, both hulls | 48.4 m² |
| Battery | 128 kWh LFP, two 64 kWh islands at 352 V, trays sized for 192 |
| Solar | 15 kWp on about 63 m² of roof |
| Generators | 2 × 20 kW variable-speed DC, HVO fuel, 600 L |
| Propulsion | 2 × 60 kW peak / 25 kW continuous, DC input at the bus, 2.5:1 belt, short shafts |
| Propellers | about 600 mm four-blade nibral, skeg-protected, line cutters |
| Charging | shore 2 × 12 kW · CCS DC (capped near 120 kW) · solar · generators |
| Accommodation | two queens in the hulls plus a convertible dinette, one head with shower |
| Deckhouse | 46 % of LOA; solar roof carried aft over the cockpit on posts |
| Construction | taped-seam plywood/epoxy, four developable strakes per side, two working chines |
4. Governing constraints#
These limits have shaped the design. The test suite in calc/tests/ checks each one, so a future change that crosses a limit will be caught during regeneration.
| Constraint | Limit | This design | Margin |
|---|---|---|---|
| Erie Canal, low route, air draft | 4.57 m | 3.53 m | +1.04 m |
| Downtown Chicago River | 5.27 m | 3.53 m | +1.74 m |
| Bahamas banks draft | < 0.95 m | 0.95 m to prop tips | zero |
| Bridgedeck clearance | ≥ 6 % LWL | 0.90 m = 6.0 % | at target |
| Structure and stability | ISO 12215-5/-7, 12217-1 Category B | scantlings per construction [ENG] | — |
| Electrical | ABYC E-11/E-13/E-30/A-31/TE-4 and ISO 16315 | see electrical and insurability | — |
| LFP cold rules | no charge below +2 °C cell | insulated, heated trays, hard BMS lockout | — |
| Generator noise, one set at 16 kW | ≤ 55 dB(A) at 7 m, ≤ 50 in any cabin, ≤ 57 cockpit | acceptance test in the specification | — |
| Passage requirement | 600 nm | 934–1,031 nm at 8 kt on 600 L | +56–72 % |
| Sprint C-rate | ≤ 1C discharge | 0.94C on the DC-input rating basis | thin — see §7 |
| Propeller protection | Maine lobster gear | partial skegs plus rotary line cutters | — |
| Build | one jig, symmetric hulls, developable panels | four strakes per side, chine tax about 0.5 % | — |
Every Great Loop route is open, including downtown Chicago and the low Erie.
Draft is the binding constraint, with no margin. At 0.95 m to the propeller tips, the boat meets the stated limit for the Bahamas banks exactly. The prismatic coefficient, sectional area curve and power requirement are all acceptable at this loading. The lines plan lists three options: refair to a waterline beam of 1.45–1.50 m, reducing canoe-body draft to about 0.637 m and propeller draft to about 0.91 m without a measurable power penalty; arrive at the banks below half load; or keep the current lines. The current 0.675 m canoe-body draft is a half-load figure, so loading provides some control. This is the most important open decision in the design.
5. Hull form#
The demihulls are identical and symmetric, so both can be built on the same jig. That saves space, tooling and time.
Four developable strakes per side: bottom, bilge, upper bilge, topside. Two working chines, turning about 39° and 19° amidships, plus a soft boot seam at z = +130 mm turning about 4°, placed where the 1.9 m topside wants its longitudinal plywood joint anyway. The bottom panel carries 11° of deadrise amidships and warps to a steep 72° V at the entry.
The offsets produce the following characteristics:
- Sectional area curve hump-free and monotone about its maximum, which sits slightly abaft midships.
- Fine, hollow entry — about 6° of half-angle over the first 1.5 m of waterline.
- Straight run at 6.5°, dead straight over the after third, exiting into integral stern platforms.
- Transom at 58 % of maximum waterline beam, immersed 109 mm at rest, so it runs clean and ventilates dry above about 8 knots.
- Stem plumb at the waterline to maximise LWL, with a 0.30 m spoon above it and topside flare for reserve buoyancy and a dry ride.
- Sprung sheer: 1.97 m at the bow, 1.32 m at its low point 11.8 m aft, rising again to 1.40 m at the transom.
- Partial skegs immediately forward of each propeller, with sacrificial lower tips.
- No bulbous bow. At this Froude range a bulb is pure added wetted surface.
The hull form study explains the choice of four strakes. With two working chines, the model comes within a fraction of a percent of the hydrodynamic performance of the round-bilge strip-plank case. The extra cost over a single chine is about 150–250 build hours, without the moulds and additional skills required for the radius-chine alternative.
6. Performance#
Half load, calm water, clean bottom. Friction and air drag are computed from the hull the offsets generate; the residuary component is calibrated against the design basis curve, and an independent Michell thin-ship model is carried in calc/sunwake/michell.py as the physical check. Details and caveats in power and energy.
| Speed | Fn | Power at the bus | Energy |
|---|---|---|---|
| 4 kt | 0.17 | 1.9 kW | 0.47 kWh/nm |
| 5 kt | 0.21 | 3.5 kW | 0.70 |
| 6 kt | 0.25 | 5.9 kW | 0.98 |
| 6.5 kt | 0.28 | 8.1 kW | 1.25 |
| 7 kt | 0.30 | 10.6 kW | 1.51 |
| 7.5 kt | 0.32 | 12.5 kW | 1.67 |
| 8 kt | 0.34 | 14.9 kW | 1.86 |
| 8.5 kt | 0.36 | 19.7 kW | 2.32 |
| 9 kt | 0.38 | 25.6 kW | 2.84 |
| 9.5 kt | 0.40 | 33.9 kW | 3.57 |
| 10 kt | 0.42 | 43.5 kW | 4.35 |
| 11 kt | 0.47 | 59.7 kW | 5.43 |
| 12 kt | 0.51 | 74.4 kW | 6.20 |
| 15 kt | 0.64 | 108 kW | 7.20 |
Absolute power estimates carry roughly ±25–30 % uncertainty, with a wider band near the resistance hump. Comparisons between variants are more reliable. The final lines require CFD or tank testing before lofting.
Speed available from each power source#
| Available power | Continuous speed |
|---|---|
| 11.7 kW — clear noon, 15 kWp array alone | 7.3 kt |
| 8.6 kW — ordinary bright day | 6.6 kt |
| 5.7 kW — haze or shoulder season | 5.9 kt |
| 16 kW — one generator at its sweet spot | 8.2 kt |
| 20 kW — one generator at rating | 8.5 kt |
| 32 kW — two generators at 16 kW each | 9.4 kt |
| 40 kW — two generators at rating | 9.8 kt |
| 52 kW — two generators plus noon sun | 10.5 kt |
| 50 kW — drives at continuous rating | 10.4 kt |
| 120 kW — sprint, two-minute peak | 15.5 kt |
Operating plan. Solar cruise is 7–7.5 kt. For a silent cruise at 7.5–8 kt, the battery supplies the difference between solar output and demand. One generator running at its quiet setting holds 8.2 kt; this is the expected speed for most offshore passages. Two generators support 9.5–10 kt. The drives' continuous rating sets the sustained limit at 10.4 kt. Fifteen knots is a two-minute weather sprint.
Weight has a direct and useful effect on performance: removing 500 kg adds about 0.11 kt at solar noon. The weight ledger in §8 therefore remains part of the performance model throughout the build.
7. Energy#
Battery#
The nominal battery capacity is 128 kWh LFP, split into one 64 kWh, 352 V island per hull (308–394 V operating window). Prismatic cells sit in sealed, insulated and heated trays below the berth platforms, close to the longitudinal centre of gravity. The usable window is 80 %, from 95 % to 15 % state of charge, or 102 kWh. Trays, cabling and protection are sized for a later increase to 192 kWh without structural changes.
C-rate remains an open item. A two-minute, 120 kW sprint draws 0.94C from a 128 kWh bank. That is within the limit, but with little margin, and it assumes the drives are rated by DC electrical input at the bus. This is the convention used for every operating point in this project. Some vendors instead publish shaft output. If the selected drives are shaft-rated at 60 kW each, the same sprint requires about 133 kW at the bus, or 1.04C. The drive RFQ must state the rating reference explicitly.
Silent endurance#
| Speed | Dark, battery only | With about 8.6 kW of bright-day sun |
|---|---|---|
| 5 kt | 29 h · 146 nm | effectively unlimited |
| 6 kt | 17 h · 104 nm | effectively unlimited |
| 7 kt | 9.7 h · 68 nm | runs past daylight, about 358 nm |
| 8 kt | 6.9 h · 55 nm | 16 h · 130 nm |
| 9 kt | 4.0 h · 36 nm | 6.0 h · 54 nm |
| 10 kt | 2.4 h · 24 nm | 2.9 h · 29 nm |
In ordinary summer sun, a full day at 7.5–8 knots leaves the battery at about half charge. The 128 kWh bank is not large enough for an overnight passage under battery power alone, so the generators are required for those passages.
Generators and fuel#
Two 20 kW variable-speed DC sets, one per hull, HVO throughout, 600 L in two 300 L epoxy-integral keel tanks at the LCG with crossfeed. Selection, noise strategy and the RFQ package are in generator selection.
| Speed | Energy | Generator range on 600 L |
|---|---|---|
| 7 kt | 1.51 kWh/nm | 1,149–1,268 nm |
| 8 kt | 1.86 | 934–1,031 nm |
| 8.5 kt | 2.32 | 751–828 nm |
| 9 kt | 2.84 | 612–675 nm |
| 10 kt | 4.35 | 400–441 nm |
The boat meets the 600 nm passage requirement with useful reserve at 8–8.5 kt and little reserve at 9 kt. Normal generator passages are therefore planned at 8–8.5 kt, with 9–10 kt reserved for shorter legs when needed. The keel tanks have room to increase capacity to 800–1,000 L at an estimated cost of a few thousand dollars and 160–320 kg of additional fuel.
Solar and the Maine season#
The array is rated at 15 kWp and occupies about 63 m² across the house and cockpit roof. A painted fascia makes the array appear as one dark roof plane. Strings are divided among high-voltage MPPTs so that mast and boom shading costs no more than 2 kWp. Estimated clear-noon output is 11.7 kW. Daily harvest uses monthly average irradiation for coastal Maine at 44° N and a 0.80 annual-energy derate. The base hotel load is 11.5 kWh/day, plus seawater heat-pump heating.
| Month | Solar | Load incl. heat | Net | Free miles/day |
|---|---|---|---|---|
| April | 57 kWh | 23.4 | +34 | 14 nm |
| May | 64 | 15.5 | +48 | 21 nm |
| June | 68 | 12.7 | +56 | 24 nm |
| July | 67 | 11.5 | +56 | 24 nm |
| August | 60 | 11.5 | +49 | 21 nm |
| September | 49 | 13.2 | +36 | 15 nm |
| October | 35 | 18.5 | +16 | 7 nm |
Free miles are calculated at 8 kt with a 25 % allowance for weather and fouling. At anchor, the model shows an energy surplus from April through October. It turns negative in November, when the boat is scheduled to be ashore.
Maine summer solar production is within a few percent of the Bahamas. Longer days at 44° N largely offset the lower sun angle, and cooler air improves panel efficiency. Coastal Maine seawater remains at 6–16 °C during the season, giving the seawater-source heat pump a modelled COP of 3.6–4.4, compared with 2.4–3.0 for an air-source unit.
8. Weight#
Mass control is a design requirement. The lightship estimate carries a 6 % margin until the completed boat is weighed, and every vendor substitution over 20 kg must be entered against its weight group. A 500 kg change is worth about 0.11 kt at solar noon, so the ledger must stay current through the build.
| Group | Mass (kg) |
|---|---|
| Hull structure, two hulls (plywood/epoxy/glass, taped seam) | 3,150 |
| Bridgedeck, wet deck, plywood box beams | 1,400 |
| Deckhouse, glazing and solar roof structure | 1,100 |
| Battery, 128 kWh installed (trays, heating, BMS) | 1,150 |
| Motors, reduction, shafts, propellers, rudders | 600 |
| Generators (2), enclosures and exhaust | 640 |
| Solar array and MPPTs | 330 |
| Electrical: HV cable, LV, chargers, isolation transformers | 480 |
| Systems: heat pump, plumbing, pumps | 470 |
| Interior fit-out (two hull cabins, saloon, head) | 1,050 |
| Deck gear, rails, ground tackle | 460 |
| Paint, fairing, miscellaneous | 300 |
| Sum | 11,130 |
| Margin, 6 % | 668 |
| Lightship, design | 11,798 |
Half load is lightship plus four crew, 300 L of fuel, 200 L of water and stores: 12.70 t. Full load, with all tanks, cruising stores, tender and spares, is 13.58 t. Integrating the lines gives an independent displacement estimate within 0.2 % of the weight ledger. [ENG: ledger to Rev 1 with vendor masses, and the power model re-run behind it.]
9. Accommodation and styling#
Main-deck layout. The cockpit, saloon and side decks share one level. The aft wall of the saloon opens to the cockpit. The galley is aft to port, the dinette is forward to port, and the main helm is forward to starboard behind glass with about 300° visibility. Small wing stations at both aft quarters serve docking. There is no flybridge.
The two queen berths are in the hulls, above the battery trays, with the head and shower forward in the starboard hull. This keeps the deckhouse to 46 % of LOA rather than about 60 %. At the foot of each 2.03 × 1.52 m berth, clear width is 1.65 m, giving 131 mm of margin. The accommodation study covers the alternatives and trade-offs.
Standing headroom is not required throughout the hulls. Full height is provided at the cabin door and wardrobe, with sitting headroom over each berth. Raising the hull coachroofs for full standing headroom would make the topsides taller and spoil the low profile.
Exterior finish. The topsides are deep navy, the deckhouse cream, with a gold cove stripe and matching boot-top stripe. The sprung sheer is capped by a varnished rail. The glasshouse sits well aft of the stems, behind a long foredeck, and has an aft-raked windshield, radiused corners and narrow painted mullions. A gently crowned fascia frames the solar array and continues aft over the cockpit on slim posts. Other details include oval hull ports, a folding signal mast with a burgee, gilt transom lettering and an arched bridgedeck fairing.
Brightwork is limited to cap rails, handrails and a transom badge, with an estimated maintenance burden of one weekend a year.
10. Construction and effort#
Taped-seam plywood and epoxy over permanent bulkheads: panels pulled to the bulkheads and tack-welded, every seam taped both sides with staggered biaxial, everything sheathed inside and out, plywood box beams, foam flotation targeting unsinkable-by-calculation. Full detail in construction; quantities in materials takeoff.
Work is divided by system. The owner builds the wooden structure and handles the low-voltage work: hulls, bridgedeck, house, fairing, paint, deck hardware, 24 V systems, plumbing and interior. Professionals engineer, install and commission the battery, drives, generators, MPPTs, chargers, CCS equipment and high-voltage protection. Survey checkpoints are documented for the underwriter.
| Phase | Hours (estimate) |
|---|---|
| Hulls, 2 × about 775 | 1,550 |
| Join, beams, bridgedeck | 800 |
| House, trunks, decks | 650 |
| Fairing and paint | 950 |
| Owner-side systems (LV, plumbing, mounts) | 450 |
| Interior including both hull cabins | 700 |
| Commissioning support, rig-out, margin | 400 |
| Total | about 5,500 (range 4,600–6,400) |
Working full-time, the owner could complete the build in an estimated 2.5–3 years. At 20 hours a week, the estimate is 5–6 years. The preferred shed is at least 17 × 9.5 m with 4 m clearance. The hulls could instead be built one at a time in a smaller bay and joined in rented space.
11. Cost#
The project carries two cost estimates. The first excludes owner labour and overhead, and puts materials and systems at $0.57M–$0.97M. The professional high-voltage package accounts for 41–44 % of that total, reflecting the cost of the 352 V architecture.
The second includes a professional boatbuilder working alongside the owner. It runs from about $0.80M to $1.25M, with a central estimate of $1.00M and launch in month 26. The phase-by-phase estimate is in build timeline and cost.
12. Alternatives considered and rejected#
- A monohull. Attractive and practical, but estimated to use 30–40 % more energy per mile while offering half the solar roof area.
- A longer catamaran. More length raises the natural cruising speed, but not economy. Once structural weight is scaled with length, the shorter boat uses less energy per mile at its own cruising speed.
- A shorter catamaran. It would be cheaper and more efficient per mile, but would have less roof area for solar and less bridgedeck clearance. Fitting four berths, a head, the battery and machinery would also become difficult.
- Full planing. The installed battery mass makes a planing hull a poor fit.
- Hydrofoils. Hull effective lift-to-drag is about 85 at 8 knots against 12–18 for any foil system; the crossover sits above this boat's envelope. A stern foil remains worth one CFD question.
- Round-bilge strip plank. It has a smoother finish and is worth about half a percent in the model compared with four strakes, but would require more specialist yard work. See the hull form study.
- Radius-chine tortured plywood. Flat four-strake panels give comparable performance without moulds or a new construction method.
- Asymmetric demihulls. Marginal at this spacing and speed, and symmetry means one jig.
- Steerable pods and saildrive legs. They are deeper, more vulnerable and more expensive, and require specialist servicing. Widely spaced twin screws already provide good low-speed control.
- NMC batteries. Energy density is not worth the safety and cycle-life trade when low weight placement is desirable.
- Full carbon construction. It saves weight, but material costs are three to four times higher. It also performs poorly for sound and thermal insulation and complicates lightning protection and repair.
- Aluminium. Heavy at this size, prone to condensation, hard to shape into classic curves, and stray-current corrosion is a poor match for a boat living on shore chargers.
- A donor hull or a stalled amateur project. This would be the fastest and cheapest route to the water, but would surrender control over hull form, clearance, weight and appearance.
13. Where the design goes next#
The full engineering register is in open questions. Five items must be resolved before the design can progress:
- Naval architect of record. Paid design reviews from two or three candidates, then a staged contract. The plan is in naval architect engagement.
- The Bahamas draft decision (§4), because it moves the lines and everything downstream of them.
- The underwriter's written conditions, especially on battery certification, before any high-voltage hardware is ordered — see electrical and insurability.
- The generator RFQ, which can issue immediately; the requirement set is settled.
- The drive rating reference plane, pinned in writing, because the C-rate check passes on one basis and fails on the other.