How to Prop a Cummins Marine Engine
As with any boat, if you can't reach the rated engine speed at full throttle, or "wide-open throttle," you need to reduce your propeller's pitch. If, with a diesel inboard engine, like the Cummins, you can't reach the rated engine speed at full throttle, you need to pursue a different course first and, when that work is done, address the problem of the propeller's pitch. Once the problem is recognized, all it takes is some simple arithmetic.
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Instructions
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Find the rated speed of your engine, located on the "Marine Performance Curve" and "Marine Engine Performance Data" sheets in the "Engine Operator's Manual." These sheets are provided by Cummins Marine.
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Take your boat out and push the throttle forward to the lesser of wide-open throttle or the rated engine speed. Whether your engine speed is less than the rated engine speed at full throttle, which merits a reduction in the propeller's pitch, or you are unable to go to wide-open throttle without exceeding the rated engine speed, necessitating an increase in pitch, note the engine speed.
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Base your calculations on 105 percent of the engine speed at the time of observation. If, for example, you are powered by a Cummins NT855-M, with a rated engine speed of 1,800 RPM and the engine was operating at 1,450 at-wide open throttle, multiply the engine speed noted--not the rated engine speed--by 1.05. Subtract this from the rated engine speed, 1,800 RPM in the case of the NT855-M. The result for this engine is 1,800 - (1.05)(1,450) = 1,800 - 1,522.5 = 277.5.
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Divide the result by 100: 277.5/100 = 2.78. Round to the nearest whole number, which in this case is 3. Since each inch of reduction of your propeller's pitch results in a reduction of approximately 100 RPM, If your propeller's current pitch is 21, this means you reduce it by the result of your calculations, 3 inches, meaning that, where you were using a propeller with a 21-inch pitch, you should be using a propeller with a 17 inch pitch.
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If your engine is reaching the rated engine speed before you pushed the throttle wide open, have your boat dealer either adjust the throttle or make arrangements for a throttle adjustment, so that the engine is operating at the rated engine speed at wide-open throttle. Then, and only then, should you adjust your propeller's pitch.
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Tips & Warnings
Always consult with your Cummins dealer before changing the propeller's pitch.
Have the propeller changes done by competent marine personnel.
