Four Hundred Watts Over the Cockpit

This is the project everything else on the boat is waiting on. Refrigeration, the Raspberry Pi nav stack, the autopilot, and every future idea that starts with "what if we ran…" all come out of the same battery bank. Until there's a real charging source that doesn't involve running the diesel, the boat's ambitions are capped.

How we got here — three designs deep

Build logs that only show the finished thing aren't much use to anyone, so here's the actual path. We're on the third idea, and the first two both taught us something we needed.

Design one: flexible panels on the bimini

We started with flexible panels — the obvious choice for a curved bimini, in theory. We sent them back for two reasons.

First, we'd have had to frame them anyway. A flexible panel that isn't supported across its whole area is one that works loose, flogs in a blow, and eventually cracks cells you can't see. Once you're building a frame, the main advantage of "flexible" has evaporated.

Second, flat is kinder. Rigid panels mounted flat sit in clean air instead of sandwiched against canvas cooking in their own heat, and flat cells don't fight their own geometry. So: rigid panels, mounted flat.

Design two: build our own arch out of pipe and connectors

The next plan was a whole second structure — a custom bimini frame built from stainless pipe and fittings, no tube bender and no TIG welder required. We still like that approach on its merits, and if this boat didn't already have an arch we'd be building it.

But it does. There's a perfectly good stainless arch aft with the mainsheet traveller riding on a plate across the top of it, and every hour spent building a second structure next to the first one was an hour of duplicated work, duplicated weight, and a second set of deck penetrations. The honest read was that we were building an arch because we'd already decided to build an arch.

Design three: a rack that clamps to the arch we own

So the current design is a bolt-on stainless rack that picks up on the existing arch — two clamps out in the bends up forward, two diagonal struts down to the legs aft — and carries the two panels flat, just below the traveller plate. The traveller stays entirely out of it. No bolts swapped, no new holes, no re-bedding, and the whole thing comes off with hand tools.

The canvas idea survives, just decoupled from the panels. Once the rack is up we'll add canvas to fill in whatever shade gaps are left over the cockpit. Shade is not a side benefit of this project — on a hot afternoon it's arguably the main one.

Why a wider slot fixes the structure

This is the part of the design that surprised us, and it's the reason there's a Rev F at all.

The arch will only let the frame clamp at about X = ±42 in from centreline, out past the traveller plate where the tubes start bending down. That leaves a long lever between the clamps and the fore-and-aft arms, and it was pushing the front crossbar down to a safety factor of 1.9. There are only two ways to shorten that lever: move the clamps inboard, which the arch forbids, or move the arms outboard — and the arms can only go where the panels are.

So the gap between the two panels turns out to be a structural control, not a styling choice. Widening it slides both panels outboard, the arms follow, and the lever shrinks. The load is identical; the geometry just gets better. And the same change hands you a genuinely usable window to look up through at the masthead.

The front pickup, in the bend

No clamp can wrap anything inside X = ±39.5 in — the traveller plate is welded along the tubes for that whole span. Bare tube begins only where the plate ends, 2 to 4 in below the plate's top face, and by then it's already turning down.

• The clamp goes on the first genuinely straight bare section, just past the tangent of the bend — not in the curve. A rail clamp bearing on a curve rocks on its edges and never holds. A few more inches down the leg costs only riser length.
• A riser strap makes up the height. The clamp lands 3 to 6 in below where the front crossbar needs to be. The load through it is almost pure vertical, so it carries no meaningful bending moment.

The vertical stack

Datum P is the top face of the traveller plate. Everything on the boat gets measured from there, because it's the one surface you can lay a straightedge on. The boom sets a hard ceiling 2 in above it; the top of the panel glass finishes at P + 0.81, so there's 1.19 in in hand.

The frame

• H1 clamps on the first straight bare tube past the bend, roughly X = ±42, with riser straps up to C1.
• C1 at Y = +2.0, running out to the clamps.
• C2 and C3 ride on top of the arms at 26 in and 48 in aft.
• A1 arms at X = ±33, hanging under all three crossbars — under the panels' outer thirds, and only 9 in inboard of the clamps.
• S1 struts from the arms at C3, running down, forward and outboard to the arch legs, well below the panel plane.

The sight slot, now genuinely usable

Here's the part that matters on a sailboat, and the reason we didn't just fill the whole arch with panel. At 6 in the slot was a keyhole you had to line yourself up with. At 18 in it's a window you can stand in front of — you can see the masthead fly, the top of the main, and the burgee from any normal helming stance.

Tube — sizes, walls and cut lengths

All 316/316L, polished ornamental or seamless. One wall thickness for the whole frame; the struts are the only lighter tube. Y is measured aft from the traveller plate's aft edge, X from centreline.

Mounting hardware

Everything wetted is 316 / A4-70. The traveller stays entirely outside this build — no bolts swapped, no holes, no re-bedding.

What it has to survive

At 70 mph — 61 knots — dynamic pressure is 12.5 psf. A flat panel with airflow on both faces carries a net coefficient near ±1.5, so that's 18.8 psf across 20.8 sq ft: 391 lb trying to lift the rack off the boat. Widening the slot doesn't change that number at all, since the panel area is the same. It just gives the structure better leverage against it.

Every steel member now sits above 3×, and the arm — which has been the weakest link since the first sketch — finally has real air in it. As always, the tube isn't the limit; the two front clamps are. Buy the heaviest pattern that fits and re-torque them after the first hard sail.

Wiring the two panels

With the panels now 18 in apart you'll need a longer link between them — allow an extra 2 ft of MC4 extension over what a tight array would use. The B&R rig helps otherwise: no backstay to lay a shadow down the middle. Run the cable down an arch leg in cushioned clamps — the opposite leg from the traveller line — through a bedded gland into the aft locker, then to the controller and a 15 A disconnect.

What 400 watts should buy us

On paper, a 400-watt array in decent summer sun should return something in the neighbourhood of 100 or more amp-hours a day into a 12-volt bank. Real numbers depend on latitude, season, shading from the rig, panel temperature, and how honest your charge controller is — we'll publish what we actually measure once it's running, rather than what the spec sheet promises.

What matters is the shape of it. A hundred-odd amp-hours a day is the difference between a boat that has to run its engine to stay alive and a boat that doesn't. It's what makes refrigeration at anchor reasonable, and it's what would make something like a 12-volt watermaker even worth discussing — that project needs about 36 amp-hours per session, which is absurd off a bank alone and unremarkable off a working solar array.

Measure before you order

Ten measurements stand between this design and a purchase order. If you're copying this for your own boat, these are the numbers that will differ.

1. Where bare, straight tube actually begins — as an X from centreline and a drop below the plate's top face, both sides. This positions the H1 clamps and sets how much of C1 you trim.
2. Tube OD at that station. Sets the H1 clamp bore; it may differ from the OD under the plate if the bend was formed.
3. Is that section genuinely straight? Lay a short straightedge along it. If nothing straight exists within 8 in of the plate's end, the clamp needs re-speccing rather than fighting a rocking fitting.
4. Which face of the leg the traveller control line runs down, at both the H1 and S1 clamp stations.
5. Traveller plate thickness, and confirm its aft edge sits on the aft tube's centreline.
6. Plate aft edge to the transom edge, horizontally. The array reaches 50.7 in aft — this is the check most likely to force a change.
7. Traveller car fully eased, both ways. Nothing within an inch of the panel's leading edge; control line clear of C1, the risers and the struts.
8. Boom and vang clearance sheeted hard in, and with the boom right out on a run.
9. Plate top above the cockpit sole, and standing eye height at the wheel. Confirms the sight slot above.
10. The arch's deck bases — bolts, backing plates, gelcoat crazing. Everything the array does in a blow ends here.

Build sequence

1. Dry-fit on the dock, not on the water, and leave everything finger-tight until step 9.
2. Find and mark the clamp stations first — the first straight bare tube past the bend, both sides. Check they're symmetric; if they aren't, C1 still works but the risers will differ.
3. Fit the H1 clamps with EPDM under them. Confirm each beds evenly and doesn't rock on its edges. If a clamp rocks, move it further down the leg rather than tightening harder.
4. Hold C1 up against the risers and set its height by the rule: C1's top surface 5/8 in below the top face of the traveller plate. Mark and cut the risers.
5. Trim C1's ends to the clamp stations plus a little, U-bolt it to the risers, and check it's level athwartships and square to centreline.
6. Hang the two arms under C1 at X = ±33 in cross clamps, running aft. Level them fore-and-aft with a torpedo level.
7. Add C2 and C3 on top of the arms at 24.0 and 22.0 in. Square the frame by measuring the two diagonals — match within 1/8 in.
8. Fit one strut. Before marking it, walk the traveller car fully to each end and confirm the control line stays clear of the strut, its foot clamp, and the H1 clamps. Then mark, cut both to match, deburr and polish.
9. Strip it back down. EPDM under every clamp on the arch. Tef-Gel every 316-on-316 thread — every one.
10. Reassemble and torque: 1/4-20 to ~75 in·lb, 5/16-18 to ~130 in·lb (lubricated values — never dry-thread figures on stainless). Cap C1's ends.
11. Panels on last, aft one first, one person each side. Check the 18 in slot and the 1 in leading gap with a rule before tightening the edge clamps.
12. Stand at the wheel and look up through the slot. At 18 in you should find the masthead from any normal stance — if you can't, something is off in the height assumptions and it's worth stopping before you torque anything.
13. Wire, gland, breaker, controller. Check polarity with the panels covered before connecting anything.
14. Re-torque everything after the first day sail and again after a month — the H1 clamps especially, since they hold the front of the array to a curve.

Still to sort out

Being honest about what isn't decided yet, because build logs that only show finished work aren't much use to anyone:

• Final clamp pattern — we can't pick it until we've measured where straight bare tube actually starts
• Charge controller selection and exactly where it mounts in the aft locker
• Whether the existing AGM house bank stays or gets rethought once we can actually keep it topped up
• Real-world output measurement, panel temperature, and how much the rig shades the array on each tack
• Where the canvas goes back in to fill the shade gaps once the rack is up

All of it goes on YouTube as we work through it — the mistakes included, since those are the useful part — and the longer write-ups land on Substack.

Panel data: Renogy ShadowFlux 200 W · ENF datasheet. Hull data: Sailboatdata Hunter 290 · Hunter 290.