Roger Long wrote:

(It takes motoring half a day to do it
with the waste heat alone from my 20 hp diesel).

Something may be wrong with your setup, Roger. IIRC, you have a very
similar engine to my Yanmar 2GM20FC. I run the coolant through an 11
gal heater, and it get quites hot in well under an hour, maybe even 30
minutes. Actually for a few years the engine was running a tad hot
(180 degrees) but after cleaning out the heat exchanger its normally
at 165-170, which probably slows the heating a bit. Even if all I do
is run the engine for a little while to set an engine, it usually
heats enough for showers for the day. When I run for charging, the
engine never gets above 160, so in that case after an hour run its
still not quite as hot as when we're pushing hard.

If you had a large alternator (like my 110 Amp) then by putting a
large load on it you might be able to raise the engine temp enough to
heat the water faster!