A few months ago I mentioned the Navy was going with the all electric small
ship and I wondered what kind of power plant they planed to use to generate
the power. Now I think I know. It is called a combined-cycle engine.
It consist of a gas turbine engine with a steam boiler fed by the gas turbine
exhaust. The steam is used in a steam turbine. Both engines generate
electrical power with an over all efficiency of about 55%. These engines are 
also widely used in domestic power generating stations.

Most diesels are around 33% and gasoline four cycle piston engines are 
around 28% while the wankel may be %26 or %27.

The turbo compound wankel could be 20% to 25% better than that or about
31% or 32% overall. I am not exactly sure as no one has built one yet with 
modern computer control. 

One of the problems with the wankel turbo compound is
the high heat of the exhaust is hard on the turbine. A boiler would be 
more durable and the steam used to drive a small steam turbine
the output of which could be geared down and fed back into the e-shaft
and thence on to the prop. No telling what the overall efficiency would
be (or the weight) but it might be much better than that of the diesel.

If the boiler, steam turbine and condenser weighed less than about
150 pounds the power to weight ratio would still be competitive.
A serpentine SS tube boiler would contain the steam pressure at 
low weight and should be about muffler size. Interesting engineering
problem.
 
Paul Lamar
 
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