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.

A couple years ago, I looked into Sterling engines

You mean STIRLING?

Here's one run off a solar collector: (scroll to bottom)
http://www.stirlingsouth.com/Roy/others/alphonse.htm

http://stirlingenergy.com/

wrote:
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.

But they're not efficient, the mechanical conversion is no better or
worse that an internal combustion engine (most of the energy goes out
the exhaust pipe, in the stirling the engine itself doesn't have exhaust
but whatever does the heating & cooling has a similar heat cycle.

The same old problems arise in that lots of seemingly free or cheap(er)
power alternatives just can't produce large amounts of power. It seems
the modern internal combustion engine is as good as we can currently do.

Thus far anyway things like the Prius, Ficht, E-Tec & others have only
reinforced just what a big ask it's going to be to get environmentally
acceptable new power systems into general use.

As a side observation, here because we're much more motherland (UK)
based than you, what you call rod ratio, we've always known as stirling
angles. You can see the relevance of rod length in a stirling engine.

K