Yanmar Pleasure Boat Marine Engine Help

The Propeller

The propeller is where you should start when selecting a transmission. See what diameter prop can be swung and select the gear ratio accordingly.
Follow the links on the left for the optimium propeller diameter and necessary direction of rotation.

Propeller sizing is only an estimate and must finally be determined by seatrials.
Propeller pitch is determined by boat speed and can be calculated for individual boats.
There is a minimum propeller diameter for a particular hull shape, check the link on the left for the formula.

Propeller types:
Fixed pitch, normally used on everything that is solely motor powered.
Pro's: Cheapest type, can be more accurately manufactured. Con's: High drag for sailboats.

Folding, the blades fold back with the water pressure when the shaft stops rotating, can be geared or non-geared blades.
Pro's: Low drag, cheapest low drag type although geared are more expensive than non-geared types. Con's: Blades should be aligned in the 9-3 o'clock position to minimise drag when seriously racing. (A 'window' in the hull can facilitate this.) Higher maintenance requirement.

Feathering, the blades turn to present the thinnest cross-section to the water flow when the shaft stops rotating.
Pro's: Best drag reduction. Con's: Most expensive, highest maintenance.

Caution: Do not install a 'Maxprop' on a sailing vessel that can exceed 18 knots under sail. The prop may auto-unfeather and damage the engine. For any vessel slower than this the 'Maxprop' is an excellent choice.

CPP Controllable pitch propellers. Are used mainly in large motor sailers so that the propeller blade angle can be adjusted for optimum engine performance or economy. Pro's: Can adapt the engine load to the prevailing conditions. Con's: Big, heavy and expensive.

NOTE: Cupped blades on a propeller serve no purpose on a vessel slower than 30 knots!