The main difference between a long-shaft outboard motor and a short-shaft motor outboard is the length of the shaft.
🔑 Key Takeaways:
1️⃣ The optimal shaft length of the outboard motor is determined by measuring the required height of the transom and then matching this with the most suitable model.
2️⃣ Fitting the most appropriate outboard motor to your boat is essential to ensure that it delivers the best performance possible.
3️⃣ If the shaft is too short, the hull will mask the water flow and prevent the motors from producing the maximum power.
4️⃣ If the shaft is too long, it will be exposed to grounding in shallow water, which may affect the performance.
This article discusses the difference between long and short-shaft outboard motors and the effects of the wrong shaft being installed. We also demonstrate how to measure the correct size shaft and match some brands based on their shaft length.
⚖️ Does The Boat Need A Long Shaft Or Short Shaft Outboard
To work optimally, the boat’s propellor must be in the best position. The propellor should operate in clear water underneath the boat’s hull.
This is measured by ensuring that the propellors radius (the distance from the propeller hub to the outer blade edge) falls below the boat’s hull.
In simple terms, this means that when one of the blades is at its highest point, it should be below the keel line of the boat’s hull.
The optimal distance between the blade and keel is 4 inches (10cm). While also ensuring the following:
- The hull is not blocking the water from reaching the propellor, which means it operates at maximum effectiveness.
- It sits below the turbulent water produced by the hull.
A Shaft Which Is Too Short Will Cause Propellor Cavitation
If the propellor operates in turbulent water, it may result in cavitation.
A propellor generates a positive (high pressure) water pressure in the front of the blade and a negative (low pressure) on the back of the blade. The pressure differential is what drives the boat forward.
The negative pressure on the back of the blade causes the oxygen at the back to evolve into bubbles.
The bubbles collapse, which causes a “hammering” effect on the back of the blades. The forces produced by the hammering may be more than 7 kg/cm2.
Under normal circumstances, there is a complex balance between the following variables.
- The shape of the propellor blades.
- The strength of the water flow.
- The mean depth below the water is relative to the size of the propellor.
If the balance is changed, the flow pattern over the propellor blades is disturbed (called cavitation), which causes an immediate loss of thrust, and over time this can cause pitting damage to the back of the blade.
Ensuring that the propellor’s shaft length is sufficiently long is the most effective way to minimize cavitation.
A Shaft Which Is Too Short May Contact The Boat
If the propellor is not positioned with sufficient spare space below the hull, it may strike to boat when the motor steering is turned.
✅ What Shaft Length Is Right For Your Boat?
To decide whether a short or long shaft engine outboard is needed, you need three measurements.
The Distance Between The Transom And The Keel
The transom is the reinforced position at the stern of the boat onto which the outboard motor is installed.
The transom is designed to carry the outboard motor and resist the forces it transfers to the boat.
The height of the transom is defined as the distance between the topmost part of the transom and the bottom of the boat keel.
The Shape Of The Boats Hull
The hull’s shape must be considered when measuring the transom height because it is one of the primary influencers of the boat’s draught.
The draught is the measurement between the waterline on the boat’s hull and the bottom of the hull.
A boat with a deep-V hull has a much bigger draught than a flat-hulled boat.
The Outboard Motor’s Mounting Bracket
The type of mounting bracket used will affect how low the outboard motor sits in the water.
🔢 Steps To Follow To Calculate The Required Length Shaft
The following are the steps needed to ascertain the right shaft length of the outboard.
1. Measure The Transom Length
This is the measurement between the transom and the lowest point of the boat’s keel.
2. Measure The Propellor
The propellors radius is then measured. This is the distance from the propellor hub’s center to the blade’s edge.
3. Add A Further Length
To ensure that the propellor is in free water and not shielded by the boat’s hull, and add an additional 4 inches (10cm) to the shaft length. This will help ensure it is outside the hull’s water turbulence and prevent the propellor from cavitating.
The following example assumes the following dimensions.
- Transom Height 19 inches (53 cm).
- Propellor Radius 5.5 inches (24 cm).
- Margin 4 inches (10 cm).
The shaft length should be 28.5 inches (72.4 cm). The following lists the shaft length of some popular electric outboard motor manufacturers
|EPropulsionSpirit One Plus||TorqeedoTravel 603||ElcoEP-5 Electric Outboard||Mercury3.5 hp 4/stroke|
|Extra Short Shaft||20.7 in. (52.5 cm)||n/a||n/a||n/a|
|Short Shaft||24.6 in. (62.5 cm)||24.6 in. (62.5 cm)||17.15 in. (43.6 cm)||17.1 in. (43.5 cm)|
|Long Shaft||29.5 in. (75.0 cm)||n/a||22.15 in. (56.3 cm)||22.1 in.(56.2 cm)|
Based on the example’s 28.5 inches (72.4 cm) shaft length, the only compatible long-shaft electric outboard motor would be the EPropulsion Spirit One Plus.
📏 Do I Need A Short Shaft Or Long Shaft Outboard?
You need the most appropriate size shaft outboard to conform to your boat’s dimensions.
The following transom measurements with matching shaft lengths from several manufacturers are listed below.
|Transom Height||EPropulsionSpirit One Plus||TorqeedoTravel 603||ElcoEP-5 Electric Outboard||Mercury3.5 hp 4/stroke|
|17 inches (43.2 cm)||n/a||n/a||Short Shaft||Short Shaft|
|20.0 inches (50.8 cm)||Extra Short Shaft||n/a||n/a||n/a|
|22.0 inches (55.8 cm)||Short Shaft||Short Shaft||Long Shaft||Long Shaft|
|25.0 inches (63.5 cm)||Short Shaft||Long Shaft||n/a||n/a|
|28.5 inches (72.4 cm)||Long Shaft||n/a||n/a||n/a|
The following comparisons of electric outboard motors may help.
🤷♂️ What Happens If You Put A Long-Shaft Motor On A Short-Shaft Boat?
If long shaft outboards are installed on a boat with a “short shaft” transom height, and dependent on the boat’s draft, there will be a danger of the prop striking the lake or river floor.
The shaft length protruding below the boat’s keel may also cause additional drag. This can affect top-end speed and fuel economy (or battery endurance.)
The longer shaft may also impact the boat’s steering.
✂️ Can You Change An Outboard From Long Shaft To Short Shaft?
It is easier to reduce the shaft length on an electric outboard motor than on a gas-powered model.
Gas-powered motors have the following components installed in the shaft and lower unit.
- Clutch (clutch dog)
- Prop shaft
- Pinion gear
- Cooling System (including the impeller)
These are precisely sized, and if the shaft were to be reduced, it would involve a completely new shaft assembly and replacement of the relevant components.
Electric outboard motors generally have the motor mounted on the lower unit; The only connections are the electric power cables. Depending on the motor model, the new shaft may be installed and the wiring shortened to match.
🔑 Key Takeaways
The shaft length of an outboard motor needs to be carefully calculated. Once the boat’s dimensions have been measured and the optimal shaft length is known, the model motor can be selected.
1️⃣ A shaft that’s too short can cause the hull to obstruct water flow, reducing the engine’s power output.
2️⃣ On the other hand, a long shaft can be at risk of grounding in shallow water, potentially impacting performance.
Once you have measured the transom and calculated the required shaft length, we recommend you investigate which manufacturers have the optimal electric propulsion systems for your needs.