Measure outboard motor shaft length by checking the distance from the top of the transom to the bottom of the cavitation plate.
The shaft length of an outboard motor is the distance from the top of the mounting bracket to the anti-ventilation plate above the propeller. To measure it, place the motor in the fully down position and measure from the top of the transom to the bottom of the cavitation plate. This ensures the propeller operates at the correct depth for optimal performance and efficiency. Common shaft lengths are 15", 20", 25", and 30", with 20" being standard for most boats.
Common Outboard Shaft Lengths and Applications
| Shaft Length | Transom Height | Typical Use |
|---|---|---|
| 15" (short) | 15" | Inflatable boats, small tenders |
| 20" (standard) | 20" | Most recreational boats |
| 25" (long) | 25" | Offshore, deep-V hulls |
| 30" (extra-long) | 30" | Large offshore vessels |
Steps to Measure Shaft Length
- Place the outboard in the full down position on a level surface.
- Measure from the top of the transom to the bottom of the anti-ventilation plate.
- Compare the measurement to standard shaft lengths to select the correct motor.
Importance of Correct Shaft Length
- Ensures propeller depth for optimal thrust and fuel efficiency.
- Prevents propeller cavitation and ventilation.
- Avoids transom interference or excessive propeller exposure.