Lines plan SW-LP01 Rev 0a#

Sunwake SW-15 · SW-LP01 Rev 0a · August 2026

Status: preliminary, for design development — not for construction. This drawing shows that the specification can be represented by a fair chined hull and serves as the geometry brief for the naval architect of record. Final fairing, CFD and contract lines remain within the architect's scope. Drawing sheets: drawings/sheet1-lines-plan.svg (lines and offsets), drawings/sheet2-profile.svg (character), drawings/sheet3-sections-and-facets.svg (midship section and the facet study). All three are generated from the following table of offsets by calc/scripts/make_drawings.py; they cannot disagree with it.

Revision history#

Rev 0 — single chine below the design waterline, two strakes per side. Solved to the target hydrostatics and retained as the hydrostatic basis.

Rev 0a — four strakes per side. The hull form study was used to test whether more facets would improve the early two-strake hull. It shows that Rev 0's single deep chine cost about a percent by its placement alone; two working chines plus a boot seam cut the total penalty against round-bilge strip plank to a few tenths of a percent, which is better than tortured-plywood radius chine, for about 150–250 build hours. Rev 0a sections are re-derived holding station areas, the design waterline curve, draft and sheer identical to Rev 0. The chine lines are faired with monotone piecewise cubics; re-integrating the published offsets closes displacement to within 0.2% of target. Hydrostatics, the berth check, machinery and styling are unchanged.

What Rev 0a is#

The demihull is four developable strakes per side — bottom, bilge, upper bilge, topside — so eight panels go round the whole canoe body, plus decks. Two working chines turn about 39° and 18° amidships. A third seam, the boot seam at z = +130 mm, turns only about 4° through the midbody: it is the longitudinal plywood joint the 1.9 m topside needs anyway, taped rather than butt-blocked, and it is not a working chine.

The bottom panel carries 11° of deadrise amidships and warps to a steep 72° V at station 1. Above that: a hump-free sectional area curve, a fine hollow entry, a straight 6.5° run, a lightly immersed transom running out into integral stern platforms, a stem plumb at the waterline with a 0.30 m spoon above it, and a sprung sheer — 1.97 m at the bow, 1.32 m at its low point 11.8 m aft, 1.40 m at the transom.

The demihulls are identical and symmetric about their own centerlines. One jig can be used for both.

Achieved hydrostatics#

Computed by integrating the following table of offsets (calc/sunwake/hydrostatics.py), at the design waterline, half load, seawater 1.025. The right-hand column is what the design brief asked for.

Two independent routes to displacement — integrating the lines, and summing the weight ledger — agree within 0.2%. The test suite checks this agreement during regeneration.

Flagged changes to the specification#

These are the places where the drawing moved a specification number. They are carried here and applied at the specification's Rev 1, per the project's change control.

1. LOA 15.3 → 15.60 m — the 0.30 m spoon stem plus 0.30 m of platform beyond station 10.
2. Effective LWL 15.0 → 15.30 m. The performance tables stay quoted on 15.00 m, which is the conservative reading and the integration basis.
3. Draft, canoe body 0.62 → 0.675 m; to prop tips 0.92 → 0.95 m. This is the one hard rub: the Bahamas banks margin goes to zero. Levers in finding 2.
4. Beam overall 7.1 → 7.29 m at the deck flare. Trimming the after flare to about 15° holds 7.1 m if the figure matters; it costs nothing hydrodynamic.
5. Construction: four strakes per side. The brief's carried "chine tax about 2–3%, accepted" improves to roughly half a percent.

Findings for the next phase#

1. Four strakes provide most of the available hydrodynamic benefit. The full ladder is in the hull form study. Four strakes is the knee of the curve and outperforms the radius-chine case without molds. The as-drawn hull sits slightly above the ideal four-strake point because the closure solve sets chine height to carry sectional area; it does not optimize chine placement. Refairing may recover another few tenths of a percent.

2. Draft, not power, is the binding constraint at this loading. Cp solved at 0.592 with LCB at 53.9% and the sectional area curve stayed hump-free without being forced. Every route to shallower water goes through beam or fullness, in this order:

• Refair at BWL 1.45–1.50 m. Solved at 1.50 m this lands the canoe draft near 0.637 m and the props near 0.91 m, at no measurable power cost — the model moves by a few tenths of a percent either way — and with friendlier berths. This is the standing refair candidate. [ENG]
• Arrive light on the banks. 0.675 m is a half-load figure.
• Accept 0.95 m at the line, and plan the Bahamas legs around tide.

Hold 1.404 m for the general arrangement; carry the refair as the live option.

3. The queen berth fits comfortably. There is 1.65 m clear at the foot of a 2.03 × 1.52 m queen lying between 10.40 and 12.43 m with the head forward, and 1.74 m at the head — 131 mm of margin on the mattress. The under-berth void, at least 1.0 m deep and 1.1 m wide, takes the 64 kWh 352 V island with room for the 96 kWh growth. Standing 2.05 m in the dressing well; sitting 0.76–0.84 m over the mattress with the optional trunk coachroof, 0.62 m without, so trunk lights stay an aesthetic choice rather than a necessity. See accommodation.

4. Machinery and accommodation fit within the same hull envelope. Motor bay 13.35–14.55 m with a working width of 1.13–1.18 m; one 20 kW variable-speed generator per hull; about 600 mm propellers behind partial skegs with line cutters; a 300 L integral keel tank per hull at the LCG, with about 635 L of volume available below z = −0.25 m. This allows total tankage to grow to 800–1,000 L. Water and holding tanks forward of the battery bay. Crash bulkheads both ends; watertight bulkheads at 1.50, 6.00, 9.90, 13.35 and 14.55 m.

5. Twin-hull interference at s/L 0.33 is quantified, and it is the first CFD check. The Michell model puts a favorable dip at 7–7.5 knots, where the two wave systems partly cancel and the pair makes about 76–80% of the wave resistance of two independent hulls, and an interference hump at 8.5–9 knots, where they reinforce and the pair makes about 145–148%. That is directly on top of the boat's silent-cruise and single-generator passage speeds, making this result a priority for CFD validation. Chart: drawings/chart-interference.svg.

6. The aft run-out of the upper strakes is a detailing item. [ENG] Near the transom the boot seam stops being soft: its turn grows from about 4° amidships to roughly 20° at station 9 and 28° at station 10, and chine 2's turn grows the same way. Whether chine 2 instead sweeps up through the waterline to the transom corner — standard chined-stern practice — is a question for the refair, taken together with the unrolling check.

7. The maximum section sits at 60% of LWL, slightly further aft than the 57% the original solve reported, because the faired interpolant peaks between stations 5 and 6 rather than at a tabulated station. The sectional area curve is flat enough there that this is a fairing question, not a form question. The naval architect must hold LCB in the 53.5–54.5% band while settling it to preserve the target longitudinal balance.

Table of offsets — Rev 0a#

Millimeters. Heights z are measured from the design waterline, positive up. Half-breadths y are from the demihull's own centerline. Stations at 1.500 m. β₁ is bottom-panel deadrise.

At station 0 the keel has risen to the waterline and all four strakes have converged into the stem, so their offsets there are generated from the stem taper rather than tabulated; the code that does it is in calc/sunwake/offsets.py and says so.

Boot seam at z = +130 mm throughout.

Strake widths amidships, measured along the girth: bottom 0.351 m, bilge 0.357 m, upper bilge 0.494 m, topside 1.314 m. The three submerged strakes total 1.20 m, so three of them rip from one 1.22 m sheet — which is why four strakes nests better than the arithmetic suggests.

Stern platform beyond station 10: the keel runs out at 6.5° to a trailing edge at (15.30, −75 mm); platform sole at +350 mm between x 15.04 and 15.28; a reverse transom face at 12° from the sheer at x = 14.82, with molded side steps in the face.

Stem tip at (−0.30, +1.97).

Next steps#

1. Owner review of the character drawing — the sprung sheer, the spoon stem, the platforms — and the trunk-lights-over-the-queens question. Those are the two aesthetic gates before the general arrangement.
2. Naval architect refair on the Rev 0a basis: chine-line fairing against the unrolling check; the chine 2 transom run-out (finding 6); BWL 1.45–1.50 m against the Bahamas draft (finding 2); LCB held at 53.5–54.5%; forward flare concavity; the transom immersion trade in the 6–10 kt band.
3. Panel development and unrolling of the four strakes per side, with a strain report per strake showing the flat panel lies within what the specified plywood thickness tolerates — then CNC nesting. See cutting the kit.
4. Variant CFD: interference at s/L 0.33 (finding 5 is the headline question), four-strake against round bilge at matched hydrostatics, a platform length sweep, and the skeg wake into the propeller plane.
5. Midship section drawing fixing the vertical datums — wet deck 0.90 m, saloon sole 1.12 m, berth platform 0.70 m — once the trunk decision lands, then the general arrangement.