Boat Propeller Terms

The terms Ventilation and Cavitation are normally confused.

Ventilation is when the propeller sucks air down from the surface or gets caught in an air pocket in the water. This can come from a disturbance on the bottom of the hull, or the motor may be mounted to high on the transom causing it to suck air. It can also be from the exhaust gasses that get expelled through the centre of the propeller getting sucked back into the prop. The propeller spins in this air pocket instead of pushing its way through the water. Normally you will need to reduce the rpm's right back to let the air go and then take off again. It feels a lot like the clutch slipping in your car.

The cause of the pitting on the blades is from quite a violent reaction under the water. Cavitation is caused from low and high pressure areas in the water.

When water is pressurised it raises the boiling point of the water. This concept is used in household pressure cookers, when water is pressurised it can be heated above 100° C without boiling and turning to steam. Water also works the other way - if you lower the pressure below atmospheric pressure it lowers the boiling point.

As the boat moves through the water, if the bottom of the hull has imperfections, it creates low pressure areas. These low pressure areas cause the water in that area to boil and turn to steam. These pockets of air travel backwards through the water. When they hit the propeller which is creating very high pressure areas on the blades the steam implodes back into a liquid form with a very violent reaction literally eating away the material of the propeller.

Pitch and Diameter

The diameter of the propeller is the outer measurement of the blades, measured from tip to tip. The pitch is the angle of the blades, how far forward the propeller moves through the water per single revolution. It is normally measured in inches. The rating on the propeller does not take into account the slip of the prop. If you can imagine placing the propeller in a big block of cheese and turning it one revolution, it would move forward through the cheese. When you measured how far through the cheese the propeller moved, this would be the pitch of the propeller. When the propeller is in the water it will slip, depending on your boat set up and which propeller you are using, some rigs will have more slip than others.

Choosing the right propeller for your boat is an art form in itself. Before even attempting to prop your boat you have to make sure that you have the right engine set up. Engine height is critical to get the best performance, handling, top end speed and fuel economy.

Check out http://www.outboard-boat.com for similar articles and advice.

Written by Geoff Morrish from http://www.outboard-boat.com

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