Propellers 101 Page 3

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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Prop problems

If a propeller is not properly matched to its boat or engine it may overload the engine, prevent it from reaching its designed rpm, or maybe underload it, in which case the engine revs high but the propeller doesn’t produce enough thrust. Or the propeller may cavitate—which involves the formation of tiny bubbles that separate blades from the water-reducing thrust, creating noise, and eventually pitting the blades. Pitting may also be caused by galvanic corrosion between dissimilar metals or by electrolysis caused by stray electrical currents. Zinc shaft anodes counteract this. Bent shafts, worn stern bearings, or damaged blades may result in vibration.

Latest trends

Most sailing propellers are made of cast bronze, but some are stainless steel. And lighter weight reinforced composites (such as the Kiwiprop) are likely to become more common.

Resources

Autoprop

Flexofold

Gori

J Prop

Kiwiprop

Luke

Martec/Autostream/Slipstream

Max-Prop

Varifold/Variprop

Bluewater cruiser Aussie Bray has circumnavigated aboard his 44-foot cutter, Starship.

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