I have read that the general guideline is 2 hp per ton of displacement. We have a 16 hp on our 9,000 lb Ericson 30+, and it works well, but I wouldn't want to have any less power when dealing with wind, current and waves in some of the stormy conditions where we sail.
Also, keep in mind that the alternator uses some power, especially if you have a larger one than stock.
Frank