Outboard shaft length plays a crucial role in the performance of your boat.
With the outboard shaft length too long, meaning the prop is too deep in the water, the engine won’t be as efficient, and the fuel burn is high.
Also, the boat will be more likely to hit bottom or submerge objects such as rocks or logs.
However, with the outboard shaft length too short, meaning the prop is too close to the surface, the engine is likely to ventilate and therefore slip on acceleration or lose power.
So how to measure outboard shaft length? Is my outboard long or short shaft?
Read the following to get the optimal shaft length for outboard motors!
Skip to:
- Outboard Shaft Length: 4 Popular Sizes
- How to Measure Outboard Shaft Length
- Outboard Engine Shaft Length Chart
- FAQs About Outboard Shaft Lengths
Outboard Motor Shaft Length: 4 Popular Sizes
Before we start measuring the outboard shaft length, let’s get it clear about what’s referring to when we talk about the outboard shaft length.
The outboard shaft length is the vertical distance between the top of the mounting clamp bracket and the bottom of the cavitation plate (aka the ventilation plate).
Here is a picture that shows you exactly where the outboard motor shaft length stands:
Usually the shaft length for outboard motors falls into the following 4 sizes:
- Short Shaft: 15’’
- Long Shaft: 20’’
- X-Long Shaft: 25’’
- XX-Long Shaft: 30’’
In practice, you may find the outboard motor shaft length varies among different brands and manufacturers like the Honda,Suzuki, Yamaha, Torqeedo, etc.
Say the 20’’ shaft is a long shaft length for gas engines, but it is a short shaft length for electric outboard motors. And that’s usually the case since the electric outboards are designed with larger and more efficient propeller that needs longer shaft.
So it’s easier and safer to communicate with the dealer with the exact measurement of outboard shaft length in order to get the right model.
How to Measure Outboard Shaft Length
Should I take the raft onto the water and measure the exact distance from the top of the transom to the water line? Or measure from the main engine prop up to the water line?
A big NO. The shaft length for outboard motors has nothing to do with the water line.
So how to determine the outboard shaft length? There are 2 steps to help get a relatively accurate figure.
Step 1: A Rule of Thumb
You may wonder how deep your prop should be in the water.
And the rule of thumb is that you should keep the cavitation plate level with the bottom or keel of the boat.
So you should just measure the height of the boat’s transom from the center down to the lowest point of the hull, and compare transom height with the outboard shaft length numbers.
Attention: Remember to measure the outboard shaft length straight up and down instead of measuring along the transom.
Step 2: Three Variables to Consider
Oftentimes you will want to adjust the outboard shaft length in order to improve the boat performance such as speed.
Here are several situations on how you should adjust the outboard shaft length:
- Powerboats
Mounting the outboard a little bit higher than the standard point could reduce drag, improve RPM at full throttle, and therefore improve speed and make it easier for the boat to get on plane.
So if you are measuring the outboard shaft length for the powerboat, planing boat, or any boat for which the speed is the priority, take the outboard shaft length 1’’ to 1.5’’ (depending on your boat types) shorter than standard would be better.
- Smaller boats
Mounting the outboard a little bit lower than the standard point could avoid the possible ventilation and slip on acceleration. Although the longer outboard shaft length can lead to lower speed and more drag, the boat can push a heavier load.
So if you are measuring the outboard shaft length for smaller boats, inflatables, dinghies, sailboat or other low-speed work boats, the cavitation plate is often about 2″ below the bottom.
- Transom distance
Usually speaking, the farther the outboard is set off the transom, the higher the propeller should be in the water (or the higher the outboard can be mounted).
When mounting the engine, you want the cavitation plate to be 1″ above the boat bottom for every 12″ away from the transom.
To sum up, there is a simple calculator of the shaft length for outboard motors:
Outboard Shaft Length = Transom Height +/- Factors
*The Factors indicates the adjustment affected by on your boat type, the outboard working environment, and the transom distance as discussed above.
Outboard Shaft Length Chart
Some outboards have a serial number that tells you which shaft length to go for.
For example, you can have the Mercury outboard shaft length by model code.
You can find your Mercury outboard model code on an ID plate or sticker.
| Model Code | Meaning |
| E | Electric start |
| L | Outboard long shaft length (20’’) |
| XL | Outboard extra-long shaft (25’’) |
| PT | Power trim |
| M | Manual start |
Actually there is no designated letter for short shaft.
Check out the full chart of the Mercury outboard model code and its features.
Other FAQs About Outboard Shaft Length
Check out some hot FAQs to help you get the proper outboard shaft lengths. Add yours in the comment and I’ll get back to you asap.
Q1. Is my outboard long or short shaft?
A1: Broadly speaking, the outboard motor shaft length for pontoon boat, bass boat, or other larger boats should be long shaft. However, the shaft length for smaller boats could be long or short depending on its style, speed requirement, etc.
Q2: What to do if my outboard shaft too long?
A2: Instead of changing to the short outboard shaft, you can try the Jack Plate first. It moves an outboard up and down to optimize performance and enable a boat to venture into shallower water than it might otherwise.