eliminating prop walk and listing...

[saleens7]

i saw this on another forum and i though it might be useful information....EDIT:::(title should read, "eliminating prop TOURQUE, not prop walk...dont want to make this thread misleading)


For the right length rudder the torque from the prop on the hull can be brought to zero. The rudder length under the prop shaft level needed to accomplish this is approximately the same as the propeller diameter plus a little more.

For a boat that runs with half the propeller over the surface and the “rudder water line” level with the prop shaft the prop torque is eliminated if wet rudder is the same as prop dia. For boats with wetter prop and/or rudder water line above the prop level a little more length is needed.

The reason this is possible is that a surface-piercing prop exerts a lateral force as well as a torque. The lateral force is countered by the rudder (or there is prop walk), The rudders counter force is acting below the prop and is thus exerting a counter torque. If these torques are equal there will be no list from prop torque.

Calculations are approximate and some details are unspoken.

heres a link to the thread for more info

http://www.rcuniverse.com/forum/m_5769701/tm.htm

[Fluid]

I'm sorry, but I can't agree with that modeler's opinion. It doesn't work that way on any of my models, even those with a longer rudder than he suggests. The physics doesn't work either, if he really means torque. To me, torque is the rotation of the hull in the opposite direction of prop rotation - it is the "equal and opposite" reaction. Hit the throttle hard from a stop and most boats will roll to the right. Lengthening the rudder doesn't effect that materially, particularly after the boat is in motion.

Now if he meant prop walk, then he is correct. A surface drive prop acts like a paddle wheel - as each blade hits the water it tries to drag the transom to the left, pointing the bow to the right - that is prop walk. A long enough/wide enough rudder really reduces the prop walk (by resisting the drag to the left). So does a thinned, very sharp prop.

Perhaps I just don't understand what he means, his native language is not English.


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