Your Propeller

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  • Pub No.: 7714

by C.H. Van Dusen

An article which describes in the simplest terms the underlying principles of propellers, why they work, slip, and they way they are chosen.

In reality it is very simple to understand. If you screw a nut on a bolt, the distance that the nut travels in each complete revolution is called the pitch. Get this clearly. Now, in the propeller, we have exactly the same thing and the pitch of the propeller (usually given in inches) is the distance that the propeller would travel through the water, in the ahead direction, if it could be screwed through the fluid as the nut is screwed onto the bolt. Since the propeller is turning in water though, it will be at once apparent that it is not possible for the propeller to travel forward or backward without some loss due to the slippage of the water off the blades. The water is not solid enough, in other words. Therefore, we come to a rule which, for the sake of argument, we will term Rule 1: The difference between the actual speed of the boat and the actual pitch speed of the propeller is called “slip.” It is usually expressed in percentages

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