MOBILE OUTBOARD MECHANIC

What pitch propeller should I have on my outboard?

There is no standard correct pitch for a particular outboard as it depends on the boat and outboard set up. The correct pitch for a particular boat and outboard depends on a variety of things including the type of boat, the weight of the boat, the weight of the load in the boat, the height of the outboard on the transom of the boat and the type of material the propeller is made of.

Propeller pitch is the theoretical distance the propeller will move forward in one revolution. For example, a 15 inch pitch propeller will theoretically travel 15 inches forward for every 360 degrees that it turns. The pitch is achieved by the angle of the blades of the propeller. The stated pitch amount is only a theoretical amount as the propeller “slips” through the water and 100% efficiency is not normally achievable. Prop slip is not an entirely bad thing as it actually helps the outboard get up revs quickly.

Your outboard will only produce its maximum horsepower if the engine is allowed to rev out to the recommended maximum rev range. The maximum rev range is usually between 5000rpm and 6000 rpm or between 4500rpm and 5500rpm but the owners’ manual or workshop manual should be checked for your outboard.

Having a propeller with too large a pitch will cause the engine to not get to the recommended revs and be loaded all the time, sort of like riding a bicycle in too high gear. This is important even if you are not running the outboard at the maximum revs as the load on the outboard throughout the rev range may cause serious engine damage including piston detonation and flogging out of the gudgeon pin boss of the pistons.

Having a propeller with too small a pitch will cause the engine to rev over the recommended maximum rev range. Two stroke outboards that rev past their recommended rev range can break reed valves which can lead to major engine damage. Over revving a four stroke outboard can cause stress to the internal components which can reduce engine life.

Choosing the correct pitch propeller for your particular outboard and boat set up is very important. Your outboard mechanic can suggest a pitch based on past experience but the only way to be sure is to run the vessel in the water under normal operating conditions and loads, and check what the revs are at wide open throttle. For smaller outboards without a tachometer on the dash, the vessel will have to be tested with a workshop tachometer fitted.

If the weight in the boat changes substantially you may have to test the vessel again and put on a prop that has a different pitch.

Pitch is only one part of the consideration. The number of blades, the rake of the blades, the way the blades are cupped, the construction material, the way the exhaust is extracted, the outboard height on the transom and the type of prop bush are all important considerations.

- Contact Cairns Outboard Service for your outboard service requirements.

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