The higher the gear ratio
the slower the propeller rotates and propeller diameters are needed. One popular motor supplier offers three transmission
ratios for their “20 HP” model. This results in 6 potential propellers:
2 blade prop
3 blade prop
Gear Ratio Shaft RPM Dia. Pitch
2.2 1455 14 9
2.6 1231 16 10
Obviously gear ratio selection is important to selection of the propeller.
Many boats were designed for a direct drive gasoline
engine operating a 12 inch propeller at 3000 rpm. To save the expense of new molds and designs many of today’s boats
still are produced using the same molds and propeller space. If the propeller is not considered when selecting the motor and
gear ratio your new motor could need a 17 inch propeller but your boat will only allow a 12 or 13 inch propeller.
Include the propeller in the motor selection
process. Here are a few tips to get started in the sizing of motor gearing and propeller.
Go to Motor and gear selection notes
Go to Determining maximum propeller size