Chasing the Last MPH in Top Speed

Optimizing the performance of Boston Whaler boats
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Chasing the Last MPH in Top Speed

Postby jimh » Mon Apr 12, 2021 2:01 pm

Many boat owners are seeking the most performance possible from their engine and propeller by looking at the maximum boat speed possible, often in increments of speed as small as 1-MPH or less.

For boats running in the 40-MPH range, a propeller of 17 to 19-pitch is typically used.

With a propeller of 17-pitch, to create a 1-MPH change in speed only requires the propeller shaft revolution speed change by about 63-RPM. The propeller shaft speed is generally a reduction from the engine shaft speed, but even with a 2:1 reduction, that last 1-MPH of boat speed represents about 125-RPM of engine speed.

With 19-pitch, adding 1-MPH just needs 56-RPM more propeller shaft speed. With 1.85 gear reduction that means 103-RPM engine speed increase.

With higher pitch, the increase in speed with increased propeller shaft speed will be greater. For really high-speed boats using, say, a 27-pitch propeller a 1-MPH change occurs with just a 37-RPM change at the propeller shaft. With 1:85:1 gearing, that is 69-RPM at the engine shaft.

In may propeller test situations, the engine speed is probably not know to an accuracy better than about 100-RPM with any analog gauge and perhaps to about 50-RPM with digital gauges. The boat speed is usually deduced from GNSS speed-over-ground data using the time-and-position method, whose accuracy is probably about 0.5-MPH, unless very special GNSS receivers are used.

The propeller pitch dimension is usually an effective-pitch metric that represents the average blade pitch of propeller blades with progressively changing pitch design.

The results of the measurement inaccuracies is that chasing boat speed variations of about 1-MPH is not likely to be highly repeatable. There are just too many variables involved. Further in actual testing, the engine power available can change based on environmental influences such as air temperature, air pressure, and humidity. These factors can easily introduce 1-MPH changes in a boat's maximum speed. Water salinity and water temperature are also variables.

In summary, testing propellers for best performance and getting results that vary by only 1-MPH cannot be attributed exclusively to the propeller. Errors in measurement of engine speed and boat speed could be influences affecting the propeller evaluation, as could weather. And, of course, sea condition such as winds and currents are also variables that should be considered in propeller evaluations, especially when the tests are not run just minutes apart.