Design specification#

Sunwake SW-15 · SW-01 Rev 0 · August 2026

Companion to the design brief. This document records the engineering requirements behind the brief. Units are metric except for knots and nautical miles. Items marked [ENG] require calculation, CFD or FEA by the structural and hydrodynamic engineer of record before construction; the values shown are sizing assumptions, not approved figures. Any change to a bolded value or an [ENG] item requires a numbered revision. The design brief continues to govern intent.

SW-000 · General#

000.1 Principal particulars#

000.2 Rules, standards and certification basis#

• Structure: ISO 12215-5 (local scantlings) and ISO 12215-7 (global loads, multihull) — Design Category B.
• Stability and buoyancy: ISO 12217-1 Category B; inclining and lightweight survey at completion.
• Electrical: ABYC E-11 (AC/DC), E-13 (lithium, any voltage), E-30 (electric propulsion above 60 V DC), A-31 (chargers and inverters), TE-4 (lightning); high-voltage drivetrain practice per ISO 16315.
• Fuel and machinery: ABYC H-33 (diesel), P-1 (exhaust); CO detection A-24.
• Navigation lights per COLREGS; USCG carriage equipment for a 15 m vessel.
• Documentation deliverables: stability booklet, high-voltage one-line with arc-flash labelling, weight report at three stages (design, as-built, inclining), conformity statement, commissioning report, owner's technical manual. This list is also the provenance file the underwriter wants — see electrical and insurability.

000.3 Weight and centre-of-gravity policy#

Bottom-up estimate at Rev 0 in the design brief §8. Margins: 6 % on lightship mass and +50 mm on VCG, held until as-built weighing; margin releases only against weighed data. Target LCG at half load 8.45 m abaft station 0 (56.3 % LWL); TCG on the centreline within ±25 mm. Every vendor substitution above 20 kg routes through the weight report, and the substitution log records the speed consequence at the moment the trade is made rather than at the weighing. The exchange rate is about 0.11 kt at solar noon per 500 kg. [ENG: maintain a live weight ledger; calc/sunwake/weights.py is the working copy.]

SW-100 · Hull and structure#

100.1 Materials#

• Plywood: okoume BS 1088, all-okoume faces and core, in 6/9/12/15 mm. Use stock carrying a Lloyd's Register or BM TRADA-programme stamp; an edge marking that only says "conforms to BS 1088" is not sufficient. Use one vetted supplier and destructively test a sample before the volume order. Meranti below the chines is the only open variant, and its added weight must be considered with its lower cost: see materials takeoff.
• Reinforcement: stitched E-glass biaxial in epoxy, 300–600 g/m² by zone; woven where a fair surface matters more than in-plane stiffness.
• Resin: one epoxy system, drum-bought, hardener speed by season.
• Core, flat panels: PET or PVC foam where a panel wants stiffness without plywood mass — wet-deck and roof options, decided with the panel work.
• Fasteners below the waterline: none through the bottom skin except shaft and skeg hardware on G10 pads. Every hole drilled, filled and re-drilled.

100.2 Shell schedule (sizing basis, ISO 12215-5 Cat B) [ENG]#

Design pressures, sizing basis: bottom slamming forward about 35 kN/m², wet-deck slam about 28 kN/m² over a 0.25 m² patch, topsides 12 kN/m². [ENG: confirm per ISO 12215-5; FEA the wet-deck panel. The largest open scantling choice is topsides at 12 mm against 9 mm plus stringers — about 94 m², about 135 kg, and a row shift in the plywood schedule.]

100.3 Internal structure#

• Watertight bulkheads, each hull: crash bulkhead at 1.50 m; then 6.00, 9.90, 13.35 (machinery forward bulkhead) and 14.55 m; transom girder. All glassed to the shell both faces; penetrations below the DWL only through bonded watertight glands.
• Frames and webs at about 1.2 m nominal between bulkheads. Berth platforms, tank flats and battery trays are engineered as structure — they are the midship stiffening, not furniture.
• The chines are structure. Two taped, filleted longitudinal seams per side act as natural stringers and help the plywood monocoque meet the preliminary scantlings without exotic laminates.
• Cross structure: three primary plywood box beams — forward deck beam near x = 2.0 m (nets attach), main beam integrated with the house front bulkhead at x = 4.6 m, after beam at the house aft face near x = 11.8 m — plus the roof acting as a stressed panel between posts and the wet-deck panel as a shear web. [ENG: ISO 12215-7 global load cases; FEA a whole-bridge model including the roof. Beam webs may want 18 mm instead of doubled 15; that fork shifts about ten sheets.]
• Skegs: laminated plywood and glass fins with a sacrificial lower 150 mm on a G10 root flange, so a grounding damages the replaceable skeg tip before the hull shell. Skeg root doubler 4 × 600 biaxial over 600 × 400 mm.
• Chain locker: watertight, draining overboard, crash-bulkhead face padded; windlass foundation on 12 mm G10 over a 3 × 600 doubler.
• Flotation: foam in the bows and hull ends targeting unsinkable-by-calculation. [ENG: verify against 400.3.]

100.4 Appendages#

• Rudders (2): balanced spades, NACA 0012, span about 0.80 m, mean chord 0.40 m, area about 0.32 m² each, balance 19 %. Stocks 60 mm 2205 duplex, self-aligning composite bearings. Emergency tiller fitting on each stock head, accessible through a cockpit sole plate.
• Skeg-mounted final shaft bearing with an integral rope guard ahead of each propeller.
• Anodes: bolt-on aluminium alloy for Loop and brackish water with zinc alternates for the salt season; reference-electrode monitoring point in each hull.

SW-200 · Propulsion#

200.1 Motors and drives#

• 2 × PMSM liquid-cooled traction motors, 350 V class: 60 kW peak (2 min), 25 kW continuous, base speed about 2,200 rpm. Inverters IP67, CAN (J1939/CANopen), regeneration used for shaft braking and dock crawl. Candidate families [ENG: select]: Danfoss Editron, Cascadia/Rinehart, ePropulsion I-series HV, Torqeedo Deep Blue class.
• Rating reference plane. Every power figure in this project is DC electrical input at the high-voltage bus. Bus power is tow power divided by a propulsive efficiency of 0.58 and a drivetrain efficiency of 0.90, and that is the basis of the speed tables, the operating points and the battery C-rate check. Vendor catalogues split: Danfoss Editron and Cascadia rate shaft output, ePropulsion rates DC input. On this boat's 128 kWh bank the difference is decisive — shaft-rated 60 kW drives push the two-minute sprint to about 133 kW of bank draw, 1.04C, over the ≤ 1C constraint. The RFQ must state the reference plane explicitly and the quote must answer on it. [ENG: pin at vendor selection; restate torque and base speed to match.]
• Reduction: single-stage 2.5:1 toothed belt (Gates Poly Chain class), motor above and abaft the shaft line, belt case in a ventilated dry compartment; helical gearbox as the alternate. Design propeller speed 700–900 rpm at cruise, 1,400 maximum.
• Mounting: motor and belt case on a common raft with dual-stage elastomeric isolation (raft to bearers, bearers to hull); thrust taken by a thrust bearing on the raft, not by the motor.
• Controller fouling logic — a written vendor requirement: torque-limit trip at +35 % over map within 50 ms; reverse-jog mode (±30 rpm, 100 Nm cap); dock-crawl mode with fine control from 0 to 120 propeller rpm; single-lever helm with a detented neutral. The controller sees a rope wrap in milliseconds where a diesel would grind it tighter.

200.2 Shafting and propellers#

• Shafts 45 mm Aqualoy 22, about 1.5 m long at 7–8° rake; dripless face seal with a spare bellows aboard; cutless bearings at the stern tube and skeg; rotary line cutters on both shafts (Shaft Shark, Spurs or Shaft Razor class) for Maine lobster gear.
• Propellers about 600 mm, four-blade nibral, DAR about 0.58, tip clearance at least 130 mm. One full spare set and a puller aboard. [ENG: final pitch and DAR from the motor map and a self-propulsion estimate.]
• Large, slow-turning propellers reduce propeller noise, one of the few remaining sound sources under electric propulsion.
• Toroidal propellers are a deferred running change. Commission conventional nibral first, then re-evaluate; the shaft taper stays SAE standard to keep the option open. The attraction is noise rather than efficiency — the big-slow-propeller specification has already harvested most of the tip-vortex gain.
• Recovery kit: because entering 10–16 °C water to clear a wrap carries a hypothermia risk, provide transom ladders deployable from the water, a pole-mounted serrated rope cutter, dive knife, fitted wetsuit and underwater camera.

200.3 Steering#

Hydraulic two-station (helm plus two autopilot pumps, port and starboard independent), cylinders on each rudder arm, synchronising bar deleted in favour of electronically synced independent circuits with a divergence alarm. Full-flow bypass for the emergency tiller. Wing docking stations with electric jog levers commanding both drives and the rudders. No bow thruster — screws 5.0 m apart turn the boat in its own length. [ENG: failure-modes review of the steer-by-wire overlay.]

SW-300 · Electrical, high voltage#

300.1 Architecture#

Two mirrored propulsion islands, one per hull, cross-tied.

• Bank per hull: 64 kWh LFP, 110S nominal 352 V (window 308–394 V), prismatic cells in sealed, insulated, heated trays under the berth platform. Trays, cabling and protection sized for 96 kWh per hull (192 total). Modules removable through berth-platform hatches — no cabinetry demolition for service, and that is a hard requirement, not a preference.
• BMS: distributed, redundant pack controllers; contactor pairs and precharge on each bank; charge hard-lockout below +2 °C cell, discharge derate below −5 °C; overtemperature, cell ΔV and isolation faults open contactors.
• Cross-tie: rated bus tie (contactor, manual disconnect and Class aR fuse at both ends), normally closed, auto-opening on a differential fault so one hull's casualty never takes the second bank down.
• Protection: Class T or aR fusing at every source and bank; an insulation monitoring device per hull with a helm alarm; an HVIL interlock loop through every high-voltage connector and cover and through the two manual service disconnects (one per hull, labelled, glove-accessible); a crash signal from an inertia switch opening all contactors.
• Cabling: orange double-insulated 70 mm² mains, 35 mm² branches, segregated from low voltage by 100 mm or a barrier, in glanded bonded metallic trays.

300.2 Sources and charging#

The energy management supervisor (SW-320) enforces charge ceilings by temperature, solar-first dispatch, generator start rules (SOC below 25 % underway, below 15 % at anchor, or operator command) and the smart-load window: water heating, battery-bay heat and the optional watermaker follow solar surplus.

CCS is the least off-the-shelf item on the boat. Deleting it saves $15–25k but removes the ability to add half a charge during a lunch stop. Make the decision after quotations are received.

300.3 Low-voltage system#

• 24 V house, negative ground, fed by 4 × 100 A isolated DC-DC converters (two per hull, n+1), with 2 × 5 kWh 24 V LFP buffer batteries, one per hull, so navigation, communications, bilge, steering and lighting ride through any high-voltage shutdown for at least eight hours.
• 12 V taps through converters for electronics that need it. All low voltage per ABYC E-11; Class T fusing at the buffer batteries.
• Bilge pumps, navigation lights, VHF, insulation-monitoring alarms and CO and smoke detection sit on the protected bus, fed directly from the buffers.

SW-320 · Monitoring and control#

• Backbone NMEA 2000 plus an isolated CAN segment for traction; supervisor a marine PLC or Cerbo-class energy management system with dispatch logic specified openly — no cloud dependency for propulsion or charging.
• Helm displays: two 12″ multifunction displays for navigation plus one dedicated energy display showing SOC, net kW, solar, generator state and range-at-speed live from the calibrated power curve.
• Telemetry: LTE plus satellite on the folding mast; remote SOC, bilge, fire and shore-power-loss alerts; a winter-layup dashboard reporting bay temperatures and SOC drift.
• Alarm philosophy: three classes — advisory, act-soon, act-now — with distinct tones. Act-now is hardwired: high water, fire, high-voltage isolation fault, CO.

SW-400 · Ship systems#

400.1 Climate#

• Reversible seawater-source heat pump, 4 kW thermal, titanium seawater exchanger, hydronic loop at 35–40 °C to trench convectors along the saloon glazing, fan coils in each cabin and the head. Cooling mode serves the cabin fan coils only.
• Backup and boost: an HVO-fired hydronic heater, about 5 kW, on the same loop, for deep cold at the dock and for heat-pump service windows.
• Domestic hot water: 60 L calorifier with coils from the heat-pump loop and from generator coolant heat recovery, plus a 1 kW immersion element on the smart-load window. Generator heat supplies hot water during passages.
• Ventilation: every cabin gets a deck hatch and an opening port; the saloon four opening panes and two roof hatches; extractor in the head; recirculating hood plus an opening pane in the galley. [ENG: condensation review with the insulation specification — this is a Maine boat in April and October.]

400.2 Fresh, black and grey#

• Fresh water 2 × 200 L, one per hull at the LCG. Watermaker optional, energy-recovery type, plumbed for solar-window auto-run and winterisation.
• Hot and cold PEX manifolds; all through-hulls composite, accessible, labelled and double-clamped; seacock count minimised by manifolding.
• Black 200 L in the head hull with a fresh-flush macerating WC, deck pumpout and overboard where legal. Grey 120 L day tank with automatic overboard where legal and a hold mode for locks and the Great Lakes.

400.3 Bilge and damage control#

Per hull: four compartments each with a 2,500 L/h automatic pump and float alarm, plus one 8,000 L/h 24 V crash pump with a strum box and camlock hose reachable to any compartment, plus a manual diaphragm pump in the cockpit locker. High-water alarms on the protected bus, repeated to telemetry. Collision-bulkhead voids foam-filled. Unsinkable-by-calculation is the target: reserve buoyancy in the sealed forepeak, after void and foam at least equal to the flooded hull mass. [ENG: verify, and run the downflooding review under ISO 12217-1.]

400.4 Fire#

• Battery trays: per-tray smoke and heat sensing, aerosol suppression per bay, a thermal-runaway vent duct from each tray to a topside clamshell — never into the accommodation — and automatic high-voltage isolation on detection.
• Generator enclosures: automatic clean-agent flood with fuel shutoff and shutdown, plus a port for manual discharge.
• Galley: blanket and a 2 kg ABC extinguisher. Boat total four ABC plus a 2 kg CO₂ in the cockpit. Smoke and CO detection in every cabin and the saloon.

400.5 Galley#

Induction, no propane aboard: two flush 3.7 kW zones managed to 5 kW combined by the energy management system, a 3 kW combination convection microwave, 130 L fridge and 75 L freezer drawers on 24 V compressors with keel-cooled condensers [ENG: consider sharing the heat-pump loop], and counters on a weight budget of 60 kg.

SW-500 · Generators and fuel#

• 2 × 20 kW variable-speed DC generator sets (1,200–3,000 rpm), HVO-rated common-rail diesels, PM alternator with active rectifier onto the high-voltage bus. Selection, candidates and the RFQ package are in generator selection.
• Installation: after machinery bay in each hull (x 13.35–14.55), set on a raft with double-elastic mounting inside an enclosure lined with 50 mm class-A foam and a mass barrier; underwater exhaust with a gas/water separator and an above-waterline idle relief; flexible sections in every service and no rigid bridges; drip-free daily service points through a cockpit sole hatch.
• Cooling: heat exchanger with a dedicated scoop and strainer. Keel cooling is not catalogued on the variable-speed line, so the raw-water circuit becomes a noise-design item. Coolant loop tapped for the calorifier (400.1).
• Fuel: 2 × 300 L epoxy-integral keel tanks at the LCG, crossfed, with dual Racor 500 filters per engine, a timed polishing loop, HVO-compatible elastomers throughout, fills on both side decks and an overfill catch. About 635 L of tank volume is available below z = −0.25 m, so growth to 800–1,000 L is real if passage planning ever asks for it.
• Noise acceptance, one set at 16 kW at anchor: ≤ 55 dB(A) at 7 m abeam, ≤ 50 dB(A) in any cabin, ≤ 57 dB(A) in the cockpit. Tested per SW-800. Specify measured vibration limits at the bearers in the installation contract, not just dB(A) in the cabin: the structure-borne path is what reaches a berth 1.5 m forward of the bulkhead.

SW-600 · Deck and equipment#

• Ground tackle: 33 kg Rocna or Spade class primary, 80 m of 10 mm G40 chain, 1,500 W 24 V windlass with helm and wing-station control and a chain counter, snubber bridle to both bows on chafe-sleeved Dyneema legs to the forward beam padeyes. Secondary Fortress FX-23 with 12 m of chain and 80 m of octoplait in the after locker. A light kedge as third.
• Docking: eight 300 mm stainless cleats including midship spring cleats at the deck gates both sides, fairleads at all of them, deck gates port and starboard for float boarding, fender racks under the cockpit seats.
• Rails: 610 mm perimeter rail on the foredeck with gates; cockpit coaming rail; a hand-hold every metre from helm to berth to head, inside and out.
• Tender: about 3.1 m aluminium-hull RIB with a 3 kW electric outboard sharing the 24 V charging, 85 kg all up, on platform chocks with a roof-boom and electric-winch launch — the folding mast doubles as the boom king-post.
• Swim and boarding: integral stern platforms with moulded side steps in the transom face, plus a fold-down ladder reaching 1.2 m below the waterline and deployable from in the water (the fouling-recovery requirement), and a hot-and-cold transom shower to starboard.
• Covers: cockpit shade panels between the roof posts; winter frame and cover in the layup kit.

SW-700 · Interior and finishes#

• Palette: white and off-white painted panels in satin two-part polyurethane; varnished khaya or cherry trim on fiddles, handrails, table edges and companion steps; an oiled light-oak engineered sole; navy and flag-red soft goods; bronze and brass at the classic touchpoints.
• Joinery: modular furniture built off the boat, foam-cored or plywood panels with hardwood edges, every module 30 kg or less or split. Building the interior while the hulls are still open offers the largest schedule saving.
• Insulation: hull sides in accommodation 25 mm PET foam with lining; overheads 40 mm; battery bays 50 mm; machinery bays acoustic package. The bridgedeck underside is the saloon floor — 30 mm of acoustic foam there, because tunnel water noise is the saloon's carpet.
• Headroom: saloon 1.98 m; standing 2.05 m in the hull dressing well; sitting 0.76–0.84 m over the mattress with the trunk coachroof, 0.62 m without. The hull coachroofs do not rise for full standing headroom; preserving the low profile takes priority.
• Berths: 1.52 × 2.03 m queens outboard on platforms 0.70 m above the waterline datum, head forward, with a walking aisle inboard; 200 mm hybrid foam mattresses on ventilated slat bases.
• Head: dry-deck shower stall with a seat and a teak grate; separate compartment with a macerating WC; both with opening ports and an extractor.
• Exterior brightwork limited to cap rails, cockpit handrails, the transom badge and the mast — total varnished area 6 m² or less. The limit is intended to keep annual brightwork maintenance to about one weekend.
• Paint: epoxy barrier below the waterline with a hard antifoul compatible with Maine haul-out; topsides two-part polyurethane in deep navy; house cream; gold cove and boot stripes; roof fascia cream.

SW-800 · Performance, noise and acceptance trials#

SW-900 · Safety and compliance equipment#

Six-person valise liferaft in the cockpit locker · EPIRB and two PLBs · AIS-B+ with MOB integration · 24″ doppler radar with a thermal camera for the Maine fog package · LTE and satellite · two fixed DSC VHF plus two handhelds · lifejackets for six, four with auto-inflating harnesses, plus jacklines to the foredeck · MOB gear: Lifesling, throwline and the in-water-deployable transom ladders · foghorn with an automatic signal mode · offshore first-aid kit · lightning: air terminal on the mast, 70 mm² down-conductor to a submerged ground plate in each hull, surge protection at the high-voltage bus, at the low-voltage panels and at the masthead electronics (ABYC TE-4) · bonding and corrosion: full DC bond, isolation transformers so no galvanic isolator is needed, reference-electrode monitoring, anode plan per 100.4.

End SW-01 Rev 0. The open engineering items are compiled in open questions, which is also the statement-of-work attachment for the naval architect.