Russ;
You've put your finger on a very important difference between marine &
aircraft engines and the ones we are used to dealing with.
"...The project included several outboards, of various sizes. I had to set
them-up and run them on the dyno (no, Joe-- NOT a chassis dyno). I had a
HUGE problem getting the outboard drivetrains to stay together long enough
to finish the test series on each engine. Running props in water, I guess
they just don't need to design-in much torsional strength. At any
rate-- if I were going to do a lsr outboard, I wouldn't use any outboard
powertrain part past the crankshaft (on a bike or a car)-- and I'd be always
worried about breaking the nose (drive hub) off the crank..."
Marine & aircraft engines are designed to drive a "flexible load" and this
relieves the drivetrain of much of the strength requirements that would be
required in automotive applications. Even their crankshafts are light and
wimpy; since a prop isn't a very "solid" load, the drive train can flex
without creating high stresses like an engine that is designed to drive
wheeled vehicles.
At one time I had a few McCulloch (sp?) 2-stroke drone engines for a project
that was abandoned. These little jewels were air-cooled 4-cyl opposed
engines that put out 72BHP and weighed only 72 lbs complete! One of the
reasons it was so light-- other than using hard-chromed aluminum cylinders
and a magnesium case-- was that the crankshaft was the size of spaghetti.
Since it only drove a prop, it didn't require much torsional stiffness. I
think the same thing applies to outboards.
Regards, Neil Tucson, AZ
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