Thanks for sharing your experience. That was about what I'd guessed but it's good to hear it from someone who actually tried it.
I spent a lot of hours experimenting with an air cooled generator set
a while back. The exhaust sound was rather quickly shut down with the
third generation muffler I built. The greatest source of noise is the
cooling fins. Each one acts like a speaker cone on every cycle of the
piston. It can be tamed with insulation and ducting but only at the
expense of a large amount of space consumed. The problem is you have
to ensure adequate air flow for cooling. This means long baffled
inlet and outlet chambers. Buried in the bilge of an otherwise large
boat you might stand a chance of success. Probably not at all
appropriate to an open launch.
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