Motor leg length, specifically for outboard motors, is measured from the shaft axis to the inside top of the mounting bracket. This standardized measurement, utilized by most outboard manufacturers, ensures compatibility when mounting the motor to a boat's transom.
Understanding Motor Leg Length Measurement
Measuring the motor leg length correctly is crucial for proper boat performance and safety. An incorrect length can lead to problems such as cavitation, reduced efficiency, or even damage to the motor and boat.
Key Measurement Points
The measurement is taken between these two specific points:
- Shaft Axis: This is the center of the motor's drive shaft.
- Inside Top of Mounting Bracket: This is the point where the motor connects to the boat's transom.
Why is this measurement important?
- Correct Boat Trim: A correctly sized motor leg ensures the motor's propeller operates at the appropriate depth, optimizing thrust and minimizing cavitation.
- Transom Compatibility: The leg length must match the transom height of the boat to avoid issues during installation and operation.
- Performance: Using a motor with a mismatched leg length can significantly reduce performance and fuel efficiency.
How to Measure
While the precise measurement point as described above is used for specification purposes by manufacturers, here's what you need to consider practically:
- If you are checking an existing motor: You should locate the lowest part of the motor that mounts to the boat. If the motor is mounted on the boat, find the inside top of the mounting bracket and measure to the middle of the motor's drive shaft.
- If you are checking a new motor or you are looking for a new motor: Refer to the specifications sheet for the motor you are considering.
| Measurement Point | Description |
|---|---|
| Start of Measurement | Shaft Axis (center of the motor's drive shaft) |
| End of Measurement | Inside Top of the Mounting Bracket |
In summary, outboard motor leg length is measured vertically from the center of the drive shaft to the top inside part of the mounting bracket.
