Others have already posted some negatives about aircooled engines in boats,
but many of those negatives can be overcome.

Exhaust smoke/smell/ noise can be overcome by running the exhaust through
a water lift. The lift won't actually spit out any water, but if you fill
it up and clean it out occaisionally, it will filter the soot and cut down
on the noise

Modern injected light weight aircooled diesels seem to run much cooler
than some of the earlier models from 40 years ago. It should be possible
to run a bilge fan off the power output of a flywheel style alternator
which would exchange enough air for at least an hour's worth of operation
in even a small enclosed engine compartment.

My assumption is that aircooled is attractive to you on the basis of price
and that you have a limited volume in which to fit an engine. I also
assume that you intend to use it as a true auxillary in a sailboat, i.e.
you do not intend to do any extended motoring, you just want to maneuver
out of congested harbors. That said, you would be ill advised to use
anything from Ducati based upon initial cost and the price of repair
parts. I would suggest the following engine:

http://launtop.com/ebusiness/en/prod...5&productid=40

The LA186 is 10 hp and the last time I priced one out (before the dollar
tanked) the price was USD$250 FOB China. These guys actually make all the
aircooled diesels for Yanmar. These are the same engines that you see on
construction equipment like whacker packers (tampers for trench work).
They are very reliable and you can get parts from any Yanmar dealer
worldide. The price you pay for a whole engine is less than you would pays
for a baggie of bolts from a Ducati dealer. These engines also have a lot
of output shaft options allowing you to choose one that can easily mate to
whatever trani you have been able to find.

Let us know how it works out for you


David
S/V Nausicaa