On Wed, 21 Apr 2010 20:07:54 -0700 (PDT), Frogwatch
wrote:

My 28' S2 weighs 7700 lbs. If the required power is directly
proportional to weight then my 13 hp diesel is enough.

It's certainly enough most of the time. We had a Westerly 28 many
years ago that I repowered with a 13 hp Volvo diesel. It was about
the same weight as yours and the engine was fine for all normal
conditions. We never got hit with any really ugly weather with that
engine however.

Our next boat was a Cal-34 that weighed about 10,000 lbs. It had a 30
hp Atomic 4 gas engine that was always more than adequate. I was once
hit with a 60 kt white squall with that boat on open ocean north of
Cape Cod. There was plenty of advance warning and we rode it out with
a triple reefed main, slowly motoring about 40 or 50 degrees off the
wind. There were no problems with control or power in those
conditions but it was over in less than 30 minutes. The biggest issue
was visibility. You really need something like ski goggles or a pilot
house when the wind is blowing like that.