Understanding Propeller Pitch

Image Source: Logan Marshall International Self-Pronouncing Reference Library (Philadelphia: International Press, 1911) 4:395

When it comes to choosing a propeller, both pitch and diameter are extremely important. This post is about understanding how propeller pitch works and how it can affect boating performance.

A propeller is a mechanical device consisting of blades that spins 360 degrees around a base shaft that forces an object, such as a boat, to move forward. It does this in much the same way that a screw is used, leading to propellers also being known as “screw propellers.” Brett Becker of Boating Magazine defines pitch as “the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw through wood.” So a propeller with a 30 inch pitch will move a boat forward 30 inches for every complete 360 degree spin if it was in a solid surface. However in water, the propeller will be slightly less effective.

A propeller can be compared to a car’s axle ratio. For example, the smaller the ratio, the more pulling power there is. A propeller works the same way, the lower the pitch, the more efficient it is. However, a lower pitch makes the engine reach maximum rpm at slower speeds and the engine’s power is not optimized.

The opposite is also true. A higher pitch will get higher speeds, but it will have a lower acceleration. Engines with lower horsepower can also be worn out if the pitch on the propeller is too high since the propeller will be much heavier and require more power.

For a recreational boat, each inch of pitch taken away is worth an additional 200 rpm. For example, when going from a 40 inch pitch to a 38 inch pitch will increase rpm by about 400 revolutions. By lowering the pitch, this will increase rpm and vice versa.

There is no one-size-fits-all solution, as the best pitch is determined by the performance goals of the boat. For example, a higher pitch is better for recreational boats requiring more speed, while fishing boats would work well with a lower pitch.

In addition to the pitch, there are many other factors that can affect a boat’s performance. Contact Eris Propellers to select the propeller with the appropriate pitch for your boat.

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