Effect of Propeller Pitch on Boat Speed

Optimizing the performance of Boston Whaler boats
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Effect of Propeller Pitch on Boat Speed

Postby jimh » Sun Jun 05, 2022 4:29 pm

It is quite common that a propeller manufacturer makes a series of propellers of the same basic design with only a change in pitch, and in many instances the change in pitch is in increments of 2-inches pitch change.

As a general rule a propeller is designed to operate most efficiently at a particular speed through the water.

As a general rule, engine gear ratios seem to change depending on the engine power, with lower-power engines having greater gear reductions and higher-power engines having lower gear reductions. Here the terms higher and lower refer to the numerical figure which indicates how many revolution of the engine crankshaft are needed to produce one revolution of the propeller shaft.

In my Propeller Calculator, the GEAR RATIO is a variable that can be entered by the user. If a gear ratio of 1:1 is entered, then the value of entered for RPM will be the propeller shaft speed of revolution.

It occurred to me that many engines have an engine speed limit of about 6,000-RPM and a gear ratio of around 2:1. This means the propeller shaft speed would be 3,000-RPM at maximum.

I was curious to see what boat speeds would occur if I kept the propeller shaft speed constant at 3,000-RPM, set the SLIP to 10, and just changed the pitch by 2-inches. Here is the outcome of that inquiry, beginning with a pitch of 15, and progressing in 2-inch increments. Again, the RPM is fixed to 3,000, the GEAR RATIO is 1, and the SLIP is 10:

    15 38.5
    17 43.5
    19 48.6
    21 53.7
    23 58.8
    25 63.9
    27 69.0

The outcome is interesting: a two-inch pitch change produces about a 5-MPH increase in speed. This suggests to me that for the propeller pitches listed above, the expected top speed for that propeller is as indicated for a 3,000-RPM shaft speed. Or stated a bit differently, expect that at the boat speeds listed above the propeller of the associated pitch will be operating at a speed through the water that will produce efficient operation.

Also, by interpolation, you can infer that a propeller pitch change of only 1-inch would likely only cause a boat speed change of about 2.5-MPH. I believe that this explains why most propeller lines are made only in pitch increments of 2-inches. A 1-inch pitch change in propeller lines is unusual and is generally available only in rather specialized propellers. I believe that the thinking behind having 1-inch pitch change available is probably related to the engine having a very narrow power band. Such an engine would need to be able to accelerate into a rather limited RPM range to achieve its optimum performance. This characteristic might be seen more often in engines of four-stroke-power-cycle design.