Looking around for old mariners' advice...
Lots of theories.
The one constant is that what was considered enough for a sail boat back in the 60's, nowadays would be a lot less than most boat owners want. Everyone is in a hurry, even me way too often.
With some quick Goolgling I find one guy's opinion that 4 hp per ton is good. Let's see... for my boat at a bit over 10000# disp, that would be only 20 hp.
Hmmm. While my stock 23 hp is less than I want for nasty days with big waves on the nose and adverse wind and/or current, I know that my hull would not benefit from anything over about 30 hp. (FWIW if our faithful Universal ever dies, a new three-cylinder Beta or Universal with about 27 hp will replace it.)
So, for an auxiliary for what we modern mariners call a "full-powered aux. sailing boat" I guess that I would vote for about 5 or 6 hp per ton.
How all that relates to measured sail area (or whether it even really should do so), I just do not know. Maybe one of the sailmakers on this site could give me a clue. Going with the "same average speed under sail" divider, theoretically I should have about 21.3 hp, with my 575 sq ft. Interesting.
Neither of these methods speaks to hull shapes. Now that most production sloops have the same beam-length ratio as "fat" cat boats did 30 years ago, they also need proportionately more power to force all that beam thru wave trains and steep chop.
Sidebar: Having endured one slammin' delivery in 7 foot seas in a boat a 14 foot beam, I rather hope never to do another! Smacking into liquid walls every 7 seconds all day long was not very much fun.
Interesting topic, and there may not be any perfect answer.
Regards,
Loren
