How to measure outboard shaft length

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. Hard to put it into practice? Well, in order to get the right shaft length, first, measure the transom height, the length from the top of the boat transom to the bottom of the boat.

After that, you could choose the recommended shaft length based according to the transom height.

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

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

For daysailers and short one-design racing sailboats, it 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 so XS will 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.

It’s always worth checking with your local dealer on which shaft length they recommend for your boat and local conditions. If you still have any questions regarding the shaft length, don’t hesitate to contact us. You can also download the technical drawings of ePropulsion outboards via our download centre.

One of the most common questions we get asked is how to know the shaft length of an engine? This is easier to tell than it sometimes seems.

Overview

When an engine is referred to as a short shaft, it's considered to be a 15" motor. Long shaft is considered 20”. An XL shaft is 25". In rare cases, mostly in big saltwater models, engines were made in a 30" shaft, or an XXL.

In none of these cases is the shaft length exactly 15, 20, or 25 inches. It's generally a measurement that falls closest to the number. To be clear, if I take a lower unit off and measure the driveshaft at 17", that would be considered a short shaft, being closest to 15 inches.

If you are measuring for a lower unit replacement and your original lower unit is off of the engine, simply measure from the top of the waterpump to the top of your driveshaft. A shaft length that falls closest to 15 inches is short, closest to 20 inches is long, and so on.

If your engine is mounted on a boat or a stand, locate the transom bracket of your engine and use the guide below to measure.

Measuring the outboard motor shaft length is just as important as choosing the motor itself, but luckily the process is relatively quick and easy. On the other hand, the boat’s efficiency and safety depend on your choice.

In this article, you will find a detailed illustration and step-by-step instructions for measuring the outboard shaft length.

Measure Shaft Length

To perform the necessary calculations, you need to prepare a tape measure or another tool convenient for you to take measurements. Prepare a pen and a piece of paper, or you can write everything down on your phone. You need to measure three lengths; the whole process includes three simple steps.

Instructions

1. Locate the transom top and the lowest point of the boat hull and measure the distance between them as shown below.
   
2. Then, it would be best if you found the radius of the propeller. You can find the radius of the propeller in the owner’s manual, or you can find a diagram on the internet using the name, model, and year. You can take measurements yourself, but it is always helpful to double-check the data. When you find the propeller radius, see the examples below, add the transom height you calculated earlier to it, and the get number is the minimum shaft length.
   
3. Whenever you are ready, add 10 cm to the resulting value and get the ideal shaft length for your boat.

Examples

So, let’s keep in mind the calculations that are written above and given below for the ideal shaft length recipe:

• Transom height + Propeller Radius + 4 in (10,16cm) = ideal shaft height

For example, let’s take a boat with a transom high of 20 in (50,8 cm), and the engine’s propeller will be 5 in (12,7 cm). Then, to find the shaft that suits the boat, we need to complete the following calculations:

20 in + 5 in = 25 (minimum shaft length to make it work)

20 in + 5 in + 4 in = 29 in (perfect shaft length)

When the most straightforward part is done, you can continue with your purchase decision and install everything together for future recreations.

In case you have any additional questions feel free to ask! We hope that the article helped and would like to invite you to go through our selection of motors and shafts with all the needed dimensions and characteristics. Our expert will guide you through an extensive choice of outboard motors and shafts from ePropulsion, ThrustMe, RIM, and TEMO.

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