http://www.homepower.com/files/mark8.pdf

Any opinon on this being used as a genset on a sailboat?

An excellent PDF file on building a genset from a 5HP motor and car
alternator.

Including images and schematics of resistive field controller as well
as an electronic field controller version.

Most interesting is:
"Type and Size of Engine
We’ve since tried many different combinations of
engines and alternators. Small gas engines between 3
and 8 horsepower work well. We found that the Honda
small engines will run more than 5,000 hours without
major work, Tecumseh engines about 800 hours, and
Briggs & Stratton engines about 600 hours. The
particular Honda G40 (170 cc, Å4.5 hp) engine pictured
here has run for over 13,000 hours with the same
rings, bearings and valves. The Honda also has the
advantage of a 100 hour oil change interval, compared
with 25 hours for both the Tecumseh and the Briggs &
Stratton. If you consider the operating life and
operating cost of small engines, then the higher quality
engines are much less expensive despite their higher
initial cost. "

Mic