Propeller torque is an important piece of data in determining what a boat can do. While you may know the amount of torque on the engine's propeller shaft, the actual amount of force applied to the water depends on a number of other aspects as well. Depending on the propeller's size and demensions, the engine's power may be maximised for speed or for power; which one you actually get is determined by the propeller torque.
Look up the engine's maximum horsepower and RPM in the user manual or specification sheet.
Calculate the engine torque in foot-pounds by multiplying the horsepower by 5,252 and dividing the result by the RPM. So if a 30-HP engine has a maximum RPM of 5,000, then 30 x 5,252 = 157,560, and 157,560/5000 = 31.512. Thus the engine torque is 31.512 foot-pounds, or simply 32 foot-pounds.
Calculate the shaft horsepower at the propeller (SHP) by multiplying the engine horsepower by 0.97. Thus, a 30-HP engine has an SHP of 29 (30 x 0.97 = 29.1).
Look up the number of bearings between the output of the gearbox and the propeller in the engine manual or spec sheet. Look up the gear reduction ratio from the same source; the figure should look something like “2.15:1.”
Calculate the percentage of power loss due to bearings by multiplying the number by 0.015. For instance, the loss of power for an engine with three bearings between the engine output and the propeller is 4.5 per cent (3 x 0.015 = 0.045 = 4.5%).
Calculate the propeller RPM by dividing the engine RPM by the gear box reduction ratio. For an engine with 5,000 RPM and a 2.15:1 reduction ratio, the propeller RPM is 2,326 (5,000/2.15 = 2,325.58 = 2,326.)
Calculate propeller torque by multiplying the SHP by 5,252 and dividing the result by the propeller RPM. For the engine in our example, the propeller torque is 66 foot-pounds (29 x 5,252 = 152,308, and 152,308/2,326 = 65.49244 = 66.)
There are several torque calculators online that can help you make these calculations; all you need to do is enter the data from the engine specifications. If your engine came with a standard propeller already installed, you can frequently find the propeller torque in the manufacturer's specifications.
Unless you are an experienced engine builder, do not use non-standard propellers on marine engines. Use factory replacement parts or follow the manufacturer's recommendations for propeller types. Use of non-standard propeller designs can damage the engine.