How To Measure An Outboard Shaft Length [Correctly]

The measurement of an electric outboard motor shaft must include the transom height, the propellor’s radius, and an additional margin of between 4 to 10 inches to allow for different hull shapes.

🔑 Key Takeaways

1️⃣ To ensure optimum performance, the electric outboard motor must be set up correctly, and one of the critical measurements is the depth at which the propellor is placed in the water.

2️⃣ The measurements quoted in this article apply to conventional monohulls, leisure boats, yachts, or rubber ducks, all of which have a single keel below the waterline.

3️⃣ The available shaft lengths differ between manufacturers; however, the same principles and measurement methods apply in each case.

The correct shaft length will ensure that the propellor produces maximum thrust.

Electric outboard motor technology is advancing quickly, and performance is starting to match or surpass gas-powered outboard motors.

👍 The Correct Method To Measure An Outboard Shaft Length

Three factors will determine the correct length of the electric motor outboard shaft.

1️⃣ The height of the boat’s freeboard (the distance between the water surface and the deck – measured at the stern.)

2️⃣ The hull’s shape determines the depth of the keel. A flat bottom Jon boat or bass boat has a shallower draft (depth) than a boat with a deep V-shaped hull.

3️⃣ The type of mounting bracket used by the electric outboard motor.

If the electric outboard motor has an anti-cavitation plate, the plate should be in line with the keel (the lowest point on the hull.)

If the electric outboard motor does not have an anti-cavitation plate, the height is determined by the highest point on the propellor (when one blade is in the noon position) in relation to the keel.

It is essential to ensure that the propellor is no higher than this, but ideally, it should be 4 inches (10cm) below to provide a “clean” flow of water from under the boat.

Follow the steps below to calculate the optimal shaft length for your electric outboard motor.

Step 1 – Calculate The Height Of The Transom

The transom is the strengthened plate installed in the center of the hull at the stern of the boat.

It usually has reinforced holes through which the outboard motor’s mounting bolts can be secured.

The transom is designed to be strong enough to keep the electric outboard motor in the installed position and to resist the forces which accelerating, decelerating, and steering place on the unit.

The transom height is the distance from the top of the transom structure to the lowest point on the hull (keel.)

Step 2 – Measure The Propellors Radius

Once the height of the transom has been measured (step 1), ascertain the propellors radius.

This can be obtained in two ways.

If you have the electric outboard motors operations manual, the radius will be included in the specification’s data sheet (generally at the back of the manual).
You can measure the propellor from the unit’s center bolt to the outside edge of one blade.

Add the transom length from step 1 and the propellor diameter to calculate the nominal shaft length.

Step 3 – Add In A Margin

As discussed previously, it is important that when the propellor blade is in the noon position, a further 4 to 12-inch margin should be added.

There are two reasons for the margin.

It places the propellor blades in the direct water flow under the boat without the hull masking the flow, which optimizes its effectiveness.
It should also place the propellor below the disturbing current under the boat. A deep-V-shaped hull produces more disturbance, lower down in the water than a flat-bottomed boat, and the shaft should be longer to cater to this.

With these factors in mind, add another 4 to 10 inches (depending on the shape of the boat’s hull) onto the nominal shaft length calculated in steps 1 and 2.

The Final Measurement

An example of these measurements is shown below.

The measurements are for illustration purposes only and will differ between boats and motor models.

The assumptions used for this example are as follows:

Type of boat – flat-bottomed Jon Boat (therefore, 4-inch allowance.)
Length of transom – 21 inches.
Electric outboard motor specification (3hp, 1kw) with a 5.5-inch radius propellor.

The formula to calculate the shaft length is:

Length of transom + radius of propellor + margin for hull shape
This results in the following measurement – 21 inches + 5.5 inches + 4 inches = 30.5 inches.

The electric motor will have several optional shaft lengths available for purchase, and the closest (longest) length is the one that should be selected.

A 30.5-inch shaft may not be available on the chosen motor, so always choose the next longest shaft in preference.

Notes To Consider

When calculating the shaft length, always make an allowance for the method used to mount the electric outboard motor to the transom. Some motors use differently designed brackets that raise or lower the motor.

📏 Typical Shaft Length Measurements

On a conventional leisure boat, yacht, or rubber duck, the correct length of an electric outboard motor’s shaft will generally fall within the transom height bands listed below.

These measurements only apply if a standard attachment mechanism is used.

Transom Height	Shaft Description	Shaft Length
11.0 inches (28.0cm)	Extra Short Shaft	Approximately 15.0 inches (38.0 cm)
16.0 inches (40.0 cm)	Short Shaft	Approximately 20.0 inches (50.0 cm)
20.0 inches (50.0 cm)	Medium shaft	Approximately 25.0 inches (64.0 cm)
Higher than 20.0 inches (50.0 cm)	Long shaft	Approximately 30.0 inches (76.0 cm)

These measurements do not apply to kayaks, canoes, or multi-hull boats.

📐 What Is The Optimum Position Of The Propellor?

The propellor must be placed at the correct height below the keel to obtain optimal performance from the electric outboard motor.

The Effects Of The Propellor Being Positioned Too High

There are three possible consequences when the shaft is too short, and the propellor ends up being positioned too close to the water’s surface.

The propellor is too close to the water’s surface, which could cause it to cavitate, damaging the propellor over an extended period.
The propellor will be masked by the boat’s keel, preventing a sufficient flow of water over it.
When the electric outboard engine is rotated (to turn the boat), there is a risk that the propellor will strike the hull.

The Effects Of The Propellor Being Positioned Too Low

There are two possible consequences when the shaft is too long and the propellor ends up being positioned too low in the water.

It will create excessive drag, affecting the boat’s performance (particularly the top speed.)
There is an increased risk of a propellor strike in shallow water.

How Far Below The Transom Should The Prop Be?

The distance from the transom to the propellor should be precisely measured to ensure that it generates the maximum thrust.

The size of the transom will vary between boats. The measurement should also consider the distance from the bottom of the transom (or the keel) to the propellor.

The ideal propellor position is when the top of the propellor blade clears the keel by at least 4 to 10 inches (hull shape being the variable.)

🗣 Final Words

An electric outboard motor shaft length must be correctly positioned to ensure that it can provide maximum performance. The measurement must consider the height of the transom, the propellor radius, and the hull’s shape.

The key takeaway to remember is:

To ensure optimum performance, the electric outboard motor must be set up correctly, and one of the critical measurements is the depth at which the propellor is placed in the water.

If you are fairly new to electric propulsion, then I’d recommend taking a look at some of the articles below:

A [1st-time] buyer’s guide to electric outboard motors
Long shaft vs short shaft outboard | What’s The Difference?