Long Shaft vs Short Shaft Outboard Motors [Which?]

The correct shaft length ensures that the outboard motor operates efficiently and effectively. A motor with a shaft that is too short may cavitate, leading to poor performance and potential damage, while one that is too long can cause the boat to drag or handle poorly.

Measuring the right shaft length involves considering the transom height of the boat and the water conditions where the boat will typically be used.

A proper understanding of the differences between long and short-shaft outboard motors and how to measure them is key for any potential motor buyer or boat owner.

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, optimally that distance should be 4 inches (10cm).

Long Shaft Outboard Motors

Long shaft outboard motors are designed for optimal performance in specific boating scenarios, offering benefits and having certain limitations that are crucial to consider.

Increased Efficiency in Rough Waters: They tend to perform better in choppy conditions. The extended length allows the propeller to remain well-submerged, providing consistent propulsion and better handling.

Compatibility with a Range of Boats: These motors are well-suited for sailboats and heavier hull designs like pontoon boats. A longer shaft maintains an optimal position of the propeller relative to the waterline, maximizing the power and efficiency.

Potential Mounting Challenges: The additional length can make the process of attaching and detaching the motor more cumbersome. If the transom bracket is not designed for a long shaft motor, modifications may be needed.

Less Suitable for Shallow Waters: Maneuvering in shallow regions can be difficult since the propeller sits lower in the water, raising the risk of underwater hazards impacting the motor.

Ideal for Taller Transoms: Boats with taller transoms require a long shaft outboard to ensure the motor’s mounting bracket aligns properly and the propeller is sufficiently submerged.

Commonly Used in Larger Recreational Boats and Sailboats: Due to their stability and power generation, long shaft motors are commonly chosen for larger boats where the hull design calls for a motor that can efficiently handle greater water displacement.

Short Shaft Outboard Motors

Short shaft outboards are often suited to smaller boats. They are generally more compact and cost-effective compared to their longer counterparts.

Compact Design: Short shaft outboards are designed for smaller boats with shallower drafts—their more compact size results in an outboard that is easier to handle.

Maneuverability: A notable advantage is the improved control they offer, especially in tight turns or when navigating through congested waters. This makes them ideal for boating in environments where agility is crucial.

Cost-Effectiveness: They tend to be less expensive than long shaft or extra-long shaft outboards, making them a financially accessible option for new boaters or those looking to purchase a new motor on a budget.

Ventilation Issues: These motors might be prone to “ventilation,” which occurs when the propeller is too close to the water surface, causing it to suck in air and lose thrust.

Limited Compatibility: Short shaft outboards are less suitable for high aft transom boats, which may cause an incorrect fit and reduced efficiency in water displacement.

Smaller Boats: Due to their design, short shaft outboard motors offer optimal performance for boats such as dinghies, tenders, or small fishing boats. The motor’s ventilation plate should align with the boat’s bottom for efficient propeller immersion and thrust.

Electric Options: A short shaft variant can be an effective choice, combining the benefits of quiet operation with the inherent advantages of short shaft design.

Importance of Correct Shaft Length

To determine the appropriate shaft length for an outboard motor, you must measure the transom height of the boat.

The transom height is the vertical distance from the top of the boat’s transom to the bottom, where it meets the hull.

Ideally, the anti-ventilation plate of the outboard should be in line with the bottom of the boat when mounted.

  • Short Shaft: Usually for transom heights of approximately 15 inches.
  • Long Shaft: Typically used for transom heights around 20 inches.

Avoid Propellor Cavitation

A shaft that’s too short and operates in turbulent water, will 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.

Steps To Calculate The Required Length

Let’s use an example of existing popular electric outboard motors so this isn’t all just hypothetical. I’ve thrown in a gasoline Mercury 4 stroke for brevity. Can you spot the difference between shaft categories between electric and gasoline?

EPropulsionSpirit One PlusTorqeedoTravel 603ElcoEP-5 Electric OutboardMercury3.5 hp 4/stroke
Extra Short Shaft20.7 in. (52.5 cm)n/an/an/a
Short Shaft24.6 in. (62.5 cm)24.6 in. (62.5 cm)17.15 in. (43.6 cm)17.1 in. (43.5 cm)
Long Shaft29.5 in. (75.0 cm)n/a22.15 in. (56.3 cm)22.1 in.(56.2 cm)

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 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 propeller radius is then measured. This is the distance from the propellor hub’s center to the blade’s edge.

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).

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.

Frequently Asked

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.

  • Driveshaft
  • Gearbox
  • Clutch (clutch dog)
  • Prop shaft
  • Pinion gear
  • Cooling System (including the impeller)
  • Exhaust

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.

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 bottom.

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.

I’m the founder and chief editor here at Kite Ship. The electrification of boating is the most exciting thing to happen to the marine industry in a generation! Welcome, and I hope that we can provide the portal you need to dive into the world of electric propulsion and power.