On Mon, 07 Feb 2005 20:41:30 GMT, ewan vaguely
proposed a theory
.......and in reply I say!:

remove ns from my header address to reply via email

IMO, unless the boat is small and fast (rapidly up on plane), and the
engine is exposed to the air (noise), you will have troubles. Boat
engines are made to either slog, or have plenty of cooling to allow
for the hard yakka. Bike engines are made to flow freely, and usually
the harder they really work (that's grunt, not revs) the more air is
flowing over them. This would apply even to wayter-cooled ones, where
fins are simply replaced by the radiator unit, which needs its own air
flow.

I suppose you could use a huge belt-drive fan, a la cars/trucks. More
power gone. More noise.

You can get gearing to allow for the rev problems. As somebody said, a
bike has a gearbox. You could proably get something to work, or even
"change gears". You can put in any number of bearings to allow for
thrust problems. You can put in fans to remove fuel vapours (although
an engine stuck up in the air is not going to be so bad here) But how
far do you go.

Basically, bike and VW engines are used for _planes_, not boats. They
are lightweight for power, and thrive on lighter loads and/or good
airflow.

this is more of a theoretical question more than anything else but a
couple of mates and i were chatting about boat building and engines in
general.

basicly ive heard a lot of engines are based upon marinised car
engine based but i got to thinking how simple it would be to use a shaft