The Gearbox is in many ways like the Engine insofar as we are only interested in its physical characteristics.

• Overall reduction ratio.

This is all we need to know, because for the purposes of the propeller caluclations all the gearbox does is alter the engine RPM by a fixed ratio and it is from this that we determine that maximum prop shaft RPM, and therfore the maximum propeller RPM

It is most important that you ascertain the correct ratio between the RPM of the propeller itself and the RPM of the engine crankshaft, so if you have any extra devices such as reduction boxes or vee drives in the driveline you must include the effects that these have on the overall ratio.

For the purposes of this propeller calculation the ahead / neutral / astern nature of the gearbox is irellevant, only the overall reduction ratio is important. If you are one of the very few owners who has a gearbox that has a different overall ratio in reverse than in forward, you need only concern yourself with the ratio in forward.

If you have a mechanical gearbox you can measure this directly by turning the propeller by hand ten complete revolutions (10 x 360 = 3,600 degrees) and measuring engine crankshaft rotations. Obviously this requires two people and some considerable amount of care, both for safety and accuracy, but it is possible to get a figure accurate enough to be useful, e.g 1.45:1 (“about 1.25:1 give or take a bit” is not good enough) You cannot use this method with a hydraulic gearbox.