In article .com,
Peter Bennett wrote:

That's a good suggestion in some situations - but the 2GM15 is a two
cylinder 15 HP engine commonly used on sailboats around 30 ft, and is
probably raw-water cooled, so your suggestion won't work.

Why would the fact that you have Raw water cooling be a deterent?
All that is required is some High Temp Hose, and a Stainless
Liquid/Air Heat Exchanger. Install the heat exchanger between
the output of the engine block and the input to the Raw Water
Exhaust Elbow Input. If you want heat just turn of the fan
blowing thru the heat exchanger. If you don't want heat, just let the
Raw water flow overboard after going thru the heat echanger.
This isn't Rocket Scienc. Cogeneration of this type has been used
on many a small vessel for many, many years, Fresh water or Raw water
cooled.
Look, you already PAID for those BTU's when you purchased your
Dead Dinasour Guts. Better to use them, then heat up the ocean
with them. Remeber 33% of your BTU's go out the output shaft,
33% go out the cooling system, and 33% go up the stack, in diesel
engines.

Bruce in alaska who would like to sell you some more dinasour guts...
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add a 2 before @