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On Sat, 22 Aug 2009 09:32:43 -0400, "mmc" wrote:

"Andrew Erickson" wrote in message
...
In article ,
wrote:

On Fri, 21 Aug 2009 11:16:09 -0400, Andrew Erickson
wrote:

In article ,
wrote:

On Thu, 20 Aug 2009 20:21:25 -0700 (PDT), Robin
wrote:

I had a brainwave the other day about putting a small (300cc)
aircooled two stroke as an inboard in a small wooden boat. With
adequate air circulation can anyone see any potential issues here? I
would build the boat with this purpose in mind so adapting something
to fit would not be a problem.

Robin

How are you going to cool it adequately?

I would assume, for an air-cooled engine, that's where the "adequate air
circulation" comes in to play. One would probably have to be a bit
careful not to push the engine too hard too long, as well, since many
applications for these engines are not at steady high output. (There
are exceptions--ultralight aircraft engines for one example.)

I don't see any inherent insurmountable objections to doing this. A
speed reduction of some sort would be nearly essential, of course, or
else using a jet drive. Small two stroke engines in general tend to be
inefficient and noisy and smelly and a bit temperamental, although the
newer ones are vastly improved over older designs. On the other hand,
they are lightweight and generally unaffected by being off level
(limited mostly by the particular carburetor design), which can be
advantages for use in a small boat.

It's not as though there aren't a great many jet skis and boats with
older outboards zipping around just fine with two stroke engines every
day. Granted, most of these are water cooled, but still...

There is a world of difference between a water cooled two-stroke, and
an aircooled two-stroke, especially in a confined space. When I ask
about cooling, it's because there will be a need for BIG fans to make
this work. Air cooled motorcycles tend to overheat sitting in traffic.
They need to keep moving to maintain reasonable temps. "Police
Special" Harleys have a detuned and lower compression engine for this
reason. Just being outside in open air is not enough. Put an engine
like that as an inboard on a boat and you are in for trouble.

It really needs to be addressed. I'm not sure it can be done
practically.

Snowmobile engines traditionally were air cooled two stroke engines
(although more recently four stroke engines and liquid cooling have
become common), and generally inside cowlings, and they generally manage
to avoid overheating just fine. Likewise, chainsaw engines don't tend
to overheat all the time, despite often being run rather hard. It's a
question of the specific engine design (presumably based on the intended
application), not a problem common to all two strokes.

You are quite correct, though, that the original poster (should they
attempt the conversion) should give some careful consideration to
cooling. I don't think the end result would ever be an ideal boat
motor, but I also think it could be perfectly adequate and probably a
lot of fun if you're into that sort of thing.

--
Andrew Erickson

"He is no fool who gives what he cannot keep to gain that which he cannot
lose." -- Jim Elliot

Some small outboards were/are air cooled as well like the Sears Game
Fishers, I think up to 5hp.

Not even remotely the same thing as mounting an air cooled engine as
an inboard.