I'd go somewhat old fashioned in my approach.
I'd go with an IOR influenced cruising sailboat which had lost its mast.
I'd replace the original masthead sloop rig with perhaps a pair of balanced-lug or gaff sails to keep the mast hight as short as possible. The masts then could be folded down.
A light-air sail could be set between them on nearly windless days.
I'd then go with a feathering prop.
I'd use Lithium-iron-phosphate batteries.
Solar cells would go on top of a permanent awning/dodger.
My speed/range expectations would be modest. Maybe 40 miles at an S/L of perhaps o.50. For a 25 ft WL that would mean about 2.5 kts in a dead calm, with plenty of sunshine.
I' want to keep the option to sail with no power, so the FR would be out of the question. High aspect ratio sails are good for upwind performance, but I suspect that they are not so hot for driving power. SPRAY replicas have been known to suffer poor performance with them, while performing reasonably well well with low aspect ratio sails. This is because of the apple-bow hull shape. I suppose a regenerating system would creat similar extra drag. Yes. Windward performance would suffer. But that is a trade-off I'd be willing to make.
