re: Propeller Polarity

hlg@pacbell.net
Wed, 14 Jul 1999 18:51:14 -0700


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West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
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Why in the world would anyone want to put the prop on backwards? If you
could do so, the prop would still push the boat in the same direction but
would be less efficient because the curvature of the blades would be wrong.
That is the pitch (angle of attack) would be approximately the same and
still positive, but the camber would be inverted.

A friend told me of a trick he used on free-flight gas-powered model
airplanes. If the motor he had was too big and he wanted to reduce the
speed of the model, he would install the prop backward.

Harry Gordon
P14 #234, Manatee
Mountain View, CA

><< One
> concern of mine is the possibility of re-installing the propeller on
> backwards. >>
>
>On MOST outboard motors, it would be extrememly difficult to mount the
>propeller on backwards. Especially the smaller (3.5 and below) engines. The
>prop will usually be smaller on the aft end. Not only that, but most engines
>turn the prop 'right-hand', (or clockwise) if looking at it from behind.
>
><< Obviously the boat will run the opposite of expected, but what
>kind of damage is possible? >>
>
>If you are 'lucky' enough to get the prop to go on backwards, it may go
>opposite of what is expected. You may also immediately catch the prop on the
>skeg (the little fin on the bottom of the O/B) Most smaller engines use a
>composite prop, which would tear a blade off.
>
>If by chance you do get your prop on backwards, and the motor runs without
>err, you can still cause internal damage to your lower unit. All the guts in
>there are designed to have the forces and pressures on certain components.
>Even a motor with a reverse gear, wouldn't last all that long going in
>reverse all the time. Some motors use a 'thrust bearing', which alleviates
>the thrust from the internal parts of the gearcase, and transmits it to the
>stronger parts of the outer housing.
>
>If putting a prop on backwards is an issue, use a scratch awl, 'sharpie pen',
>or even a dab of white-out on both the prop and the lower unit to mark which
>way it goes BEFORE removing the prop....
>
>Cheers,
>Russ de Ment
>Borrowing Grandpa's P-19
>Silverdale, WA