Propeller Handedness and Propeller Shaft Rotation

Optimizing the performance of Boston Whaler boats
lgdpt
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Joined: Sat Mar 12, 2016 2:14 pm

Propeller Handedness and Propeller Shaft Rotation

Postby lgdpt » Thu Mar 18, 2021 12:27 pm

For a Yamaha 70-HP outboard engine, what propeller should be used: Right or Left rotation? Thank you

jimh
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Propeller Handedness and Propeller Shaft Rotation

Postby jimh » Thu Mar 18, 2021 1:20 pm

Your question is in regard to the propeller "handedness" and not to a rotational direction. Rotational motion is generally described as clockwise or counter-clockwise.

Regarding the left or right handedness of propellers, the general rule is taken from a helix, since the propeller is essentially a screw and its rotation inscribes a helix. Using your right hand, wrap your hand around the propeller shaft in the direction of circular motion. The thumb of the right hand will point in the direction of advance.

In outboard engines, when viewed from astern a propeller shaft appears to rotate in a clockwise rotation. This is called a "standard rotation" engine. According to the right-hand rule for a helix, a "right-hand" propeller will produce forward propulsion when the engine is shifted to "FWD."

Left-hand propellers are generally only used in twin-engine situations, and require one of the twin engines to be built with a different gear case, one that turns opposite to the standard. This engine configuration is called a "counter rotation" engine from the fact that the propeller shaft will rotate counter-clockwise in FWD gear. Note that the engine itself does not change rotation. The rotation change is implemented in the gear case.

A counter-rotation gear case is actually different from the standard rotation gear case, that is, the counter rotation gear case is not the same as a standard rotation gear case and it is not that just the shift linkage was modified to switch the forward and reverse rotation sense. The actual design of the gear case is different. The difference generally involves how the gear case bearings will absorb the force of the thrust produced in each direction. Usually the bearing to absorb the forward thrust is larger and stronger than the bearing that absorbs the reverse thrust.

It is very unusual for any outboard engine below about 150-HP to be available as a counter-rotation model.