More than that, they're on a sailboat! Electric motors offer regen when under sailpower, which is absolutely huge, or you can get a towable torpedo generator.

More than that, they're on a sailboat! Electric motors offer regen when under sailpower, which is absolutely huge, or you can get a towable torpedo generator. Otherwise, get a small wind turbine. You can subsist on PV alone but there are far better options, especially when available PV area is so limited.