Propellers 101 Page 2 - Sail Magazine

Propellers 101 Page 2

A propeller converts the rotary output of a boat’s engine into an accelerated stream of water. This acceleration creates thrust that pushes the boat through the water. To work efficiently, propellers must be properly matched to their engines and transmissions. Other important factors include the boat’s displacement and waterline length and the clearance between the propeller shaft and the bottom
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Self feathering

Feathering props (Max-Prop, Autostream, Kiwiprop, and Variprop are a few examples) usually have relatively flat blades that pivot at their bases in a hollow hub. When the shaft is stationary the blades turn edge-on to the passing water. When driven in forward or reverse, the blades are held open by stops at pre-set pitch angles. In forward gear flat blades are not as efficient as those with more curved surfaces, but running astern the performance of feathering props is often superior to that of fixed and folding props, as the blades can pivot around to put their leading edge forward in either direction.

On most feathering props the blades are linked by gears or cranks within the hub so they open simultaneously. Most also have adjustable pitch stops, and that allows owners to experiment when matching the pitch of the prop to the boat and its engine. In some cases, pitch can be adjusted by a diver with the boat in the water. Owners can also transfer their expensive propeller to another boat or engine. The mechanisms must be lubricated periodically.

Self Pitching

The only self-pitching propeller is the Autoprop, which has independently pivoting blades, with the pivot offset by a dog-leg. When sailing the blades point nearly straight ahead, but under power, centrifugal force and water flow pivot them to an appropriate pitch angle for the shaft rpm and boatspeed. This can mean extra thrust and improved fuel economy, especially when a boat is motorsailing or powering against a head wind or sea. Because the prop’s diameter increases in reverse, extra space is needed to accommodate the blades.

Controllable pitch

These are propellers whose blade pitch angles can be controlled from within the boat. The ability to tweak the propeller’s pitch to match conditions over a wide range of boat and engine speeds improves engine loading, fuel efficiency, and thrust, particularly when motorsailing. Mechanical details vary considerably, but controllable-pitch props are very expensive, and require a special hollow propeller shaft. They’re seldom found on recreational boats.

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