Selecting the correct outboard shaft length is crucial for maximizing performance and avoiding problems like cavitation or damage from contact with submerged objects. In this guide, we’ll walk you through how to accurately measure outboard shaft length and offer helpful tips for electric motors. Whether you're upgrading or starting from scratch, pairing the right shaft length with quality boat build parts ensures a smoother and safer ride.

1. Measure the Transom Height

Measuring the transom height is the first step in figuring out the appropriate outboard shaft length. This is the distance between the top of the boat's transom, where the motor is installed, and the bottom of the hull. The baseline for choosing the appropriate shaft length will be provided by an accurate measurement of this.

2. Calculate the Shaft Length

After you have determined the transom height, add the propeller radius (found in the user handbook or technical drawing) for proper clearance. This keeps the top of the propeller aligned with the transom's lowest point, offering smooth water flow.**

For illustration, if the transom height is 21 inches and the propeller radius is 5.5 inches, the formula would appear like this: Transom height + propeller radius + 4 inches (safety clearance) equals shaft length.

Therefore, 21 + 5.5 + 4 = 30.5 inches. In this example, a lengthy shaft would be great and ideal.

3. Account for Different Shaft Lengths

Shaft length differs depending on the type of outboard; therefore, it is important to understand your options. The length of electric outboards, such as ePropulsion motors, is measured from the clamp bracket's top to the propeller's center. In comparison, gas-powered outboards are measured to the anti-ventilation plate

Extra Short (XS): 20.7 inchesShort (S): 24.6 inchesLong (L): 29.5 inches

• 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, such as dinghies or aluminum boats under 15 feet, an extra short or short shaft is usually preferred, depending on whether the transom height exceeds 40 cm (15.7 inches). A long shaft is typically preferred for larger boats or those with a transom height greater than 50 cm (19.7 inches).

5. Special Considerations for Sailboats

The best way to measure the length of a sailboat, especially a daysailer or sport boat, is starting from the waterline to the engine mount. In some circumstances, an exceptionally short shaft may be desirable for convenient storage, although short or long shafts are best for deeper mounts.

By following these steps and making accurate calculations, you can confidently choose the ideal shaft length for your electric outboard motor. This attention to detail not only enhances performance but also helps avoid common setup issues—especially when outfitting compact vessels like those featured in our Tiny Boats For Sale collection.