Mount them flat. A boat on a mooring or at anchor turns through the whole compass as the wind and tide shift, so a tilted panel spends part of every day pointed away from the sun. Averaged over a full swing, a tilted panel makes only cos(tilt) of what a flat one makes: a 40° tilt gives up 23% and a 60° tilt gives up half. Tilt only when the boat is held on a heading.
Flat, on anything that swings. The reason is not the tilt, it is the direction. On land a tilted panel keeps pointing true south all day; on a swinging mooring the heading is whatever the tide last decided. Average the panel's output over every heading it takes and the direction term cancels out, which leaves a plain cosine of the tilt angle.
| Tilt | Share of a flat panel | Given up |
|---|---|---|
| 10° | 98% | 2% |
| 20° | 94% | 6% |
| 30° | 87% | 13% |
| 40° | 77% | 23% |
| 50° | 64% | 36% |
| 60° | 50% | 50% |
That is why a panel following the curve of a bimini or sprayhood costs so little. Shallow angles are nearly free: 10° costs about 2% and 20° about 6%. Fighting the shape of the boat for a few percent is not worth the mounting problem it creates.
Only when the boat holds a heading: alongside a pontoon, in a berth, or anchored with a stern line ashore. Then the ordinary land numbers apply, and they are not small in winter.
| Latitude | Midsummer | Midwinter |
|---|---|---|
| 25° (Florida Keys, Bahamas) | +0% | +51% |
| 35° (Gibraltar, Chesapeake) | +2% | +91% |
| 45° (Nova Scotia, Biscay) | +8% | +172% |
| 50° (English Channel, Vancouver Island) | +12% | +250% |
| 55° (west Scotland, the Baltic) | +17% | +397% |
These are midday, clear-sky figures for direct sun, computed from the sun's noon elevation. Clouds and diffuse light pull the real daily gain below them. The shape holds either way: at cruising latitudes in the sailing season a tilt is worth very little, and the swing penalty above wipes it out.
Follow the fabric. A panel that sits at whatever angle the canvas already has is within a few percent of flat, and it stays out of the way of the boom, the mainsheet, and your head. Semi-flexible panels exist for exactly this, and the cosine table above is the reason they cost you almost nothing.
If the boat is laid up on a swinging mooring through the winter, more panel beats better angle. A tilt cannot help you when the heading is random, so the only lever left is area. Size the array to hold the batteries up through the shortest days rather than to charge fast in June.