Parting Thoughts

How to Size Propellers

By J. D. Neeson, President

Every engine has a maximum horsepower rating and a maximum Revolutions Per Minute (RPMs) that permits the engine to meet that horsepower rating. It is important that the outdrive ratio (or transmission ratio for inboards) and the propeller size allows the engine to reach this maximum.

So when a boat is going at full speed (the term often used is Wide Open Throttle or WOT) the RPMs shown on the tachometer should be the same as the maximum RPMs provided by the engine manufacturer.

1) If, when at WOT, the RPMs on the tach (always calibrate your tach to be sure it is correct) are higher than the manufacturer’s maximum RPMs, it means that either the outdrive ratio is too high or (more likely) the prop is too small. Most often the prop is incorrect. Manufacturers are very careful about matching engines with the proper drive (or transmission) ratios.

Using an undersized prop is bad for your engine. The engine will over-rev and operate over the design specification for the engine. A comparable metaphor would be driving a car at 60 miles an hour in second gear.

2) If, when at wide-open throttle, the RPMs on the tach are less than the manufacturer’s maximum RPMs, it means that either the outdrive ratio is too low or more likely, the prop is too large. Again it is almost always the prop size or pitch (or both) that is incorrect. It is like trying to drive a car up a steep hill in fifth gear.

Using an oversized prop is also bad for the engine because overloading the engine will eventually cause damage to the bearings and cylinder walls. In addition, when a prop is oversized, the boat will not go as fast as it is designed to go as the engine is not strong enough to turn the oversize prop.

The relationship between prop size and RPMs is clearly seen at WOT but it exists at all RPMs (speed). If the prop is too big, the engine has to work harder than it otherwise would to get to a certain speed. If the prop is too small then the engine has to run faster than it really should to attain a certain speed.

A rule of thumb for single props is: for every 1″ change in your prop size the RPMs change between 400-500 RPMS. Every 1″ in pitch your RPMs will change between 150-200 RPMs.

With dual props however pitch changes are measured as a function of both props. Therefore, the change is approximately 70% that of single props. So the change is 280-350 RPMs and 105-140 RPMs respectively.

However, these numbers are just approximations and should be used as benchmark guidelines and not specific recommendations. The length, weight and type of hull of the boat can change these ranges as well as the horsepower and ratio of your outdrive and transmission.

Categories: JD Neeson, The Express

Tags: jd neeson, propeller, sizing

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