How to Find Your Shaft Length - ePropulsion Outboards

If you are looking for an ePropulsion electric outboard for your boat, choosing the correct shaft length is important. If you go too long it would still work but it may lack performance. You will also risk hitting bottom or submerged objects more frequently such as rocks. If you go too short, that’s the worst scenario as you‘re more than likely going to cavitate.

Please note that ePropulsion has a different shaft length measurement from gas-powered outboards. For electric outboards, the shaft length is measured from the top of the clamp bracket to the center of the propeller. For gas outboards, it’s measured from the top of the clamp bracket to the anti-ventilation plate.

As you can see, our XS is close to S from Mercury. Our S is close to L from Mercury. Our L is longer than L from Mercury. See below.

 

To find the suitable shaft length, the principle is to make sure the top of the propeller is matched to the lowest point of the transom with at least an extra 10 cm (4’’) space to ensure good water flow underneath the boat.

Hard to put it into practice? Well, in order to get the right outboard shaft length, follow three easy steps blow:

Step 1: Measure the transom height which is the length from the top of the boat transom to the bottom of the boat, as illustrated in the image below:

 

Step 2: After you get the transom height, add the radius of the propeller (check the user manual, and then technical drawing for the number) to calculate the minimum shaft length needed to make it work.

Step 3: Add another 10 cm (4’’) to the previous calculation for the final ideal shaft length.

Let’s take the Spirit 1.0 Evo Remote, radius of whose propeller is 5.5’’ as shown in the technical drawing, for an example. To match the 1964 16′ Starcraft aluminum boat with 21’’ transom height, the shaft length is supposed to be calculated as the following:

21’’+5.5’’+4’’=30.5’’

 

 

So the long shaft outboard motor should be the best choice according to the shaft options available:

Spirit Series Products

 Shaft Options Shaft Length
XS 52.5 cm / 20.7 inches
S 62.5 cm / 24.6 inches
L 75 cm / 29.5 inches

 

Navy Series Products

 Shaft Options Shaft Length
S 63.4 cm / 25 inches
L 75.9 cm / 29.9 inches

 

To make things easy for you, here is a chart to match the ePropulsion outboard shaft length with the transom height directly for your quick reference:

Shaft Length Recommendation

 Transom Height Shaft Length
30 cm / 11.8 inches – 40 cm / 15.7 inches Extra Short*
40 cm / 15.7 inches – 50 cm / 19.7 inches Short
Higher than 50 cm / 19.7 inches Long

 

* Extra short version is available for Spirit series only. The navy series doesn’t have an XS shaft.

* This shaft length chart is for your reference only. The calculation from steps above would be more reliable if you get conflicting results.

This chart gives you a general idea of how to choose the outboard shaft length according to the transom height of your boat. To be more specific:

  • For most small dinghies and aluminium boats under 15ft or 4.5m, extra short shaft or short will work depending on whether the transom height is over 40 cm / 15.7 inches or not.

  • For boats with the transom height higher than 50 cm / 19.7 inches, a long shaft version is more suitable.

There are also extra tips if you are using daysailers, one-design racing sailboats, sport boats, and other sailing boats:

  • For daysailers and short one-design racing sailboats, the ideal outboard shaft length depends on the position of the outboard mount. If you choose Spirit 1.0, we suggest you keep the outboard mount at a lower position to make the XS shaft length work. That’s because XS takes less space to store inside the cabin.
  • For daysailers, sports boats and other sailing boats it’s best to measure the distance from the water line to the top of your outboard mount.

You could download the technical drawing of ePropulsion outboard here.

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