Construction#

Sunwake SW-15 · SW-CON Rev 1 · August 2026

This study examines whether an owner can build a seaworthy, traditional-looking 15 m solar-electric catamaran in stitch-and-glue plywood and epoxy. Relevant precedents exist at and above this size. The estimate calls for about 250 sheets of plywood, roughly 5,500 hours, and careful fairing and weight control. This document covers the method, the structural scheme, the sequence, the effort and the risks. Quantities are in materials takeoff; the calendar and the money are in build timeline and cost; who cuts the kit is in cutting the kit. [ENG] marks items for the engineer of record.

1. Relevant precedents#

Electric Philosophy (Sam Devlin, 2021): a 12.7 m solar-electric catamaran in traditional Devlin stitch-and-glue wood and epoxy, 11.8 t, 9.5 kWp, twin 20 kW drives, with over 3,000 nm including Alaska behind it. Professionally built with heavy owner participation, it is the closest precedent for this method, boat type and size.
Dudley Dix's radius-chine plywood catamarans — Dix 430 at 13.1 m, Dix 470, DH550 at 16.8 m: amateur-completed ocean-going cats in plywood and epoxy. Proof that owner-builders finish boats this size and larger from panelised plywood, with a mature kit pipeline behind them.
Richard Woods' Skoota 36, an 11.0 m plywood power cat: the closest owner-built power catamaran precedent, with a published shell takeoff that anchors the scaling in the materials work.
Wharram Tiki 46: plywood and epoxy, larger again, routinely amateur-built.

At 15 m, "stitch and glue" in the literal wire-twist sense applies to the small parts. The hulls are better described as taped-seam plywood and epoxy composite over permanent bulkheads: panels pulled to the bulkheads and tack-welded with hot glue and screws rather than wire, every seam taped both sides with staggered biaxial, everything sheathed. Same family as Devlin's method; the name on the door still says stitch and glue.

At 15 m, Sunwake sits between the Dix 430/470 amateur builds and the DH550. It is near the upper end of established amateur plywood construction. A CNC kit is strongly recommended at this scale because hand-scarfing 250 sheets would add a large amount of work.

2. Method choice for the hulls#

Option	Decision
Multi-chine developable panels, four strakes per side	Baseline. At L/B 10.7 the panels are long and gently curved and the chines lie nearly along streamlines. The resistance penalty against round bilge is a few tenths of a percent — computed, not assumed, in the hull form study — and partly bought back by the accuracy and stiffness of plywood panels. Fastest build, kit-friendly, one jig.
Radius chine (tortured constant-radius plywood at the bilge)	Rejected. Flat four-strake panels match or beat it hydrodynamically without mould work or a new construction method.
Cylinder-moulded or cold-moulded rounds	Labour approaches strip-plank levels and conflicts with the owner-build plan. Rejected.
Strip plank	Produces a fairer topside but requires more specialist yard work. Rejected — see the hull form study §5.

The boat's traditional character comes from the sheer, stem, trunk coachroofs and windows above the waterline. These shapes are well suited to wood; see §6.

3. Structural scheme (sizing basis, ISO 12215-5/-7 Category B) [ENG]#

A monocoque of plywood panels on plywood bulkheads, with box beams integrated into the bulkheads. No separate metal or carbon primary structure is required at this displacement — carbon is optional only in the roof span if the solar field wants fewer posts.

The full schedule is in the design specification, section SW-100.2. The structural logic behind it, in four points:

The chines are stringers. Two taped, filleted longitudinal seams per side stiffen the panels where a flat panel is weakest. They help the preliminary scantlings work without exotic laminates and support the four-strake choice for structural as well as hydrodynamic reasons.
Berth platforms, tank flats and battery trays are engineered as structure, not as furniture. They are the midship stiffening.
The wet-deck panel is a shear web between the hulls, and the solar roof is a stressed panel between its posts. The whole bridge is one structure and has to be analysed as one. [ENG: FEA a whole-bridge model including the roof; ISO 12215-7 global load cases.]
Foam flotation in the bows and hull ends targets unsinkable-by-calculation. [ENG: verify.]

One important scantling choice affects both weight and cost: topsides at 12 mm against 9 mm plus stringers — about 94 m², about 135 kg, and a row shift in the plywood schedule. Rule it with the 12215-5 panel work.

4. Build sequence#

Strongback → bulkheads (all of them, for both hulls, in one batch while the lofting checks run) → hull 1 panels, tape, sheathe, roll upright → hull 2, which is expected to be about 30 % faster because the patterns and jigs already exist → join on the wet-deck jig → box beams and bridgedeck → house and trunk coachroofs → fair and paint before glazing → systems → interior → commission.

Two sequencing rules apply:

Start the photo build log at the first taped seam. Photograph every closure and date every stage. The record forms part of the evidence required by the underwriter. See electrical and insurability.

The weight ledger goes live the same week. Every 500 kg is worth about 0.11 knots at solar noon, and weight arrives one substitution at a time.

5. Effort#

Phase	Hours (estimate)
Hulls, 2 × about 775	1,550
Join, beams, bridgedeck	800
House, trunks, decks	650
Fairing and paint	950
Owner-side systems: low voltage, 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 2.5–3 years; at 20 hours a week, the estimate is 5–6 years. Shed at least 17 × 9.5 m with 4 m of clear height, dry and heatable for epoxy winters. The hulls can be built one at a time in a smaller bay and joined in rented space, which is a practical option if a full-size shed is unavailable.

Nine hundred and fifty hours of long-boarding can overwhelm a solo build. Allow for the full effort, keep long-board discipline from the first layup — tape placement and panel accuracy are fairing decisions made months early — and consider hiring a crew for two or three weeks at the worst of it.

6. Traditional styling in plywood#

What production tooling makes expensive, wood makes nearly free, and the reverse is just as true.

Well suited to plywood:

A full sprung sheer. It follows the panel edge curve, costs nothing extra and has the greatest visual effect.
A spoon stem, via a laminated stem cap.
Trunk coachroofs over the hull queens.
Arched-top lights in laminated plywood frames — bandsaw and glue.
Windshield brow, crowned fascia, painted cove and boot stripes.

Poorly suited to plywood:

Compound tumblehome.
A true fantail stern. The integral platforms give the stern its grace instead.
Round-bilge topsides.
Flush-curved glazing.

Finish philosophy: paint everything, varnish almost nothing. Brightwork is capped by specification at 6 m², which is about a weekend a year. A dead-fair navy topside is the costliest cosmetic ambition on the boat — see the fairing line above — and has the greatest effect on the finish.

7. Division of work#

The contract should state this division clearly.

Owner: all wood, fairing, paint, deck hardware, 24 V systems, plumbing, interior.

Professional: lines and engineering package; the complete high-voltage system — battery, drives, generators, MPPTs, chargers, CCS, protection — installed and commissioned in defined visits to the owner's shed, with survey checkpoints photographed for the underwriter.

There is a middle variant, costed in build timeline and cost, in which the owner runs the high-voltage cable, conduit, trays and mounts dead, buys the battery as certified sealed packs that never exceed 60 V until the integrator's visit, and the integrator performs first energisation and commissioning. That variant saves install labour, spends it on certified packs and oversight, and is the version the 26-month schedule is built around.

8. Risks and mitigations#

Risk	Mitigation
Weight creep	Live weight ledger from day one; 6 % margin held; every vendor substitution above 20 kg logged with its speed consequence attached.
Fairing burnout	Allow the full 950 hours; use long-board discipline from the first layup; consider a hired fairing crew; this is the phase the crewed scenario is intended to protect.
Plywood quality	BS 1088 with real certification, one vetted supplier, a sample sheet destructively checked, stored flat and dry. Much of what is sold online as BS 1088 is not.
Insurance and survey of an owner build	Photo build log at every closure; staged surveys; conformity documentation; the high-voltage system professionally engineered and commissioned. Insurability is a design deliverable, not an afterthought.
Osmosis and rot at penetrations	Every hole drilled, filled and re-drilled; no fasteners through the bottom skin. Okoume has no natural rot resistance and survives only as a fully encapsulated composite — the epoxy envelope is structural in a sense the scantling book does not capture.
Resale	Allow for the normal discount on custom owner-built boats; thorough documentation and a proven construction method may help.
Project stall	Sequence for visible milestones — a complete hull inside year one, both hulls rolled by month 11. Pre-buy long-lead systems only after the join.
Kit and file errors	Pilot cut — one bulkhead and one full-length strake, dry-fitted — before the full sheet order releases. An afternoon of protocol against a five-figure mistake.

9. Recommendation#

Proceed with the taped-seam plywood method. It has relevant precedent at this size, requires no exotic materials, and suits the long developable panels and simple interiors in the arrangement. The four-strake hull adds little hydrodynamic penalty.

The next actions are in open questions. The two that gate this document specifically are the scantling book, which only the engineer of record can write, and the panel development and unrolling check, which is a critical competence to hire for.