A couple years ago, I looked into Sterling engines which are EXTERNAL
combustion engines, meaning they use the heat from some source to heat
a working fluid. Sterlings are the most thermodynamically efficient of
all heat engines but have never been popular due to poor power to
weight ratio. There are modern Sterlings that are incredibly efficient
and burn diesel, natural gas, or just about anything, even waste motor
oil. Sterlings require a good heat exchanger to cool the working fluid
and boats have this in excess.........seawater. Sterlings are constant
RPM engines meaning they are best for generating electricity while an
electric motor produces propulsion.
The commercially produced Sterling I saw used Helium as the working
fluid and the system was sealed. In spite of the best sealing, helium
has the habit of leaking out of anything (which is why it is used in
vacuum leak detectors). This means a Sterling might need to be pumped
out and refilled with Helium every so often.
It would be interesting to see a sailboat use a large battery bank as
ballast and have a Sterling to generate electricity. ANother good
thing about Sterlings is that they are QUIET.