Bob,

Your original post no longer shows up but I think from some responses I may
have missed the point that your engine is salt water cooled. If that is the
case, I would not rebuild it. My engine was run in fresh water till I
bought the boat so is probably worth an overhaul.

I was told at a boat show that the new Yanmars drop right into the same
space as the old ones. These are great engines and a new 3GM is probably
your best bet.

BTW more often than not when I'm advising someone on repowering a sailboat,
I'll find that they have a 3 cylinder engine but a prop that can only use
the horsepower of a 2 cylinder. You very well might be able to put in a
lighter and cheaper 2GM that would be happier running closer to its design
point and give you the same performance you have now. It wouldn't be quite
as smooth but the 2QM running a 2 blade prop in my boat is as smooth as I
could ask for. Fuel economy will also be slightly better taking the same
horsepower out of one less cylinder. The 2GM should fit easily on the same
beds as the 3GM with more room at one end.

I can give you a rough answer on this if you tell me:

Displacement
Length on Waterline

I can give you a better answer if you an also tell me:

Current HP and reduction gear ratio
Prop size, pitch' and number of blades
Propeller aperture size
C
Current max RPM and speed at that RPM

--
Roger Long