On Apr 21, 7:17*pm, Wayne.B wrote:
On Wed, 21 Apr 2010 18:09:16 -0700 (PDT), Tim
wrote:

There's a lot of sail boats that have 125 hp on up. The Ford Lehman
diesels are or at least were a popular marine engine for large sailers
as well as trawlers.

I know this is a 72 ft'r but it has twin Leymans at 120 hp each.

It turns out that the horsepower required for a boat to reach so
called hull speed is mostly a function of weight, and it is a
surprisingly small number for boats less than 100 tons or so.
Then you have to add in a fudge factor however for adverse conditions,
plus parasitic losses for things like alternators, refrigeration
compressors, hydraulic pumps, etc. * There are also losses in the
transmission and cutlass bearings. * A 70,000 pound trawler in theory
needs less than 90 hp to reach hull speed *but to have reasonable
margins of safety you need 3 or 4 times that much.

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