Choosing the correct outboard shaft length is critical to ensure optimal performance and avoid issues like cavitation or damaging submerged objects. Here’s a guide on how to measure outboard shaft length, with key tips for electric motors.

1. Measure the Transom Height

The first step in determining the proper outboard shaft length is measuring the transom height. This is the distance from the top of the boat’s transom (where the motor is mounted) to the bottom of the hull. Accurately measuring this will provide the baseline for selecting the right shaft length.

2. Calculate the Shaft Length

Once you have the transom height, add the propeller’s radius (which can be found in the user manual or technical drawing) to ensure adequate clearance. This ensures that the top of the propeller aligns with the lowest point of the transom, allowing for smooth water flow.

For example, if the transom height is 21 inches and the propeller radius is 5.5 inches, the formula would look like: Transom height + propeller radius + 4 inches (safety clearance) = shaft length.

So, 21 + 5.5 + 4 = 30.5 inches. In this case, a long shaft would be ideal.

3. Account for Different Shaft Lengths

Shaft length varies by outboard type, so it’s important to know your options. For electric outboards like ePropulsion motors, the length is measured from the top of the clamp bracket to the center of the propeller. Compare this to gas-powered outboards, which measure to the anti-ventilation plate.

Here’s a quick reference guide for ePropulsion outboards:

• Extra Short (XS): 20.7 inches
• Short (S): 24.6 inches
• Long (L): 29.5 inches

4. Select Based on Boat Type

For small boats like dinghies or aluminum boats under 15 feet, an extra short or short shaft is typically best, depending on whether the transom height exceeds 40 cm (15.7 inches). For larger boats or those with a transom height over 50 cm (19.7 inches), a long shaft is usually more appropriate.

5. Special Considerations for Sailboats

For sailboats, particularly daysailers and sport boats, it’s best to measure the distance from the waterline to the outboard mount. In some cases, the extra short shaft may be preferable for easy storage, while short or long shafts work best for deeper mounts.

By following these steps and calculating carefully, you can ensure that you select the ideal shaft length for your electric outboard motor.