Why do outboard motors of the same power sometimes take different prop sizes?
This is due to differences in lower unit gear ratios. Stock outboards are geared so that the propeller shaft turns at a slower speed than the rpm at the powerhead. This is usually expressed as a ratio such as 12:21 or 14:28, referring to the number of teeth in the drive gears. In the first example, the crankshaft gear has 12 and the propeller shaft gear has 21. This means the propeller shaft turns only 57% as fast as the indicated rpm at the powerhead. The lower the gear ratio, the larger the propeller that can be used and vice versa.
Related Questions
- Why are the operating times and operating currents different when driving motors using a constant voltage power supply vs. pulse control from a motor driver IC?
- Why do outboard motors of the same power sometimes take different propeller sizes?
- Why do outboard motors of the same power sometimes take different prop sizes?
