Should you choose a short or long shaft boat motor?
Choosing a boat engine is not easy. You must first choose the propulsion mode (electric, gasoline or diesel boat engine), the type of configuration (outboard, sterndrive or inboard), the engine type (boat engine two stroke or four-stroke), the injection mode (direct or electronic injection boat engine), the power (6hp, 25hp, 50hp, 100hp or even 250hp engine) or even the brand (Mercury, Yamaha, etc.). And once you know the answers to all of these questions, you still need to decide what the right shaft length is for your boat. In the rest of this article, we will explain to you point by point which criteria should be considered in order to choose a short or long shaft boat motor. If you want to read our complete guide to choosing the most suitable boat engine for your boat, you can consult our article: “ Which outboard motor to choose for a boat ».
What is the shaft length of an outboard motor?
The shaft length of an outboard motor is the distance between the anti-cavitation plate (the metal plate horizontally above the propeller) and the motor mount (the lowest point of the propeller). mounting bracket). Not all boats are alike so they won't need the same shaft length. It must therefore be chosen according to the distance between the engine mount and the surface of the water (or according to the height of the transom).
Why choosing a short or long shaft boat motor is crucial?
Depending on the architecture of your boat, you will need more or less shaft length so that the propeller is submerged to the right depth. If the propeller is too low, it may bottom out. If the boat's propeller is too high, it will suck in air and lose performance. The choice between a short or long shaft boat motor is therefore essential.
NB: all outboard motors exist with short shafts and long shafts. Thus, they can adapt to all boat designs.
Which shaft length should you choose for your boat's outboard motor?
So how do you know which shaft length to choose for your boat's outboard motor ? Just measure the distance between the motor mount and the water surface. Then refer to this table:
| Motor mount/water surface distance | Recommended shaft length |
| From 0 to 25 cm | 76 cm |
| From 25 to 40 cm | 92 cm |
| From 40 to 55 cm | 107 cm |
| Over 55 cm | Consult a reseller |
If the outboard motor manufacturer does not give the shaft length in centimeters, then simply measure the height of the transom:
- Short shaft boat motor: 38 cm transom height.
See a selection of short shaft outboard motors
- Long shaft boat motor: 51cm transom height.
See a selection of long shaft outboard motors
How to check if the shaft length of your outboard motor is suitable?
After the outboard motor is installed, you can check if its shaft length is suitable. To do this, just check if the anti-cavitation plate is aligned with the lowest point of the transom.
The ideal is to be perfectly aligned but a margin of error of 25mm maximum below is acceptable.
Generally you will not have to encounter any problems because the transoms have standard dimensions. If you are too high there is not much you can do except change the boat motor to a longer shaft length… However if you are more than 25mm below you can adjust the length shaft by installing wooden spacer blocks on the outboard motor mounting point (to raise it slightly).
Conclusion: How to choose between a short or long shaft boat motor?
The answer to the question "how to choose between a short or long shaft boat motor is very simple. Choose a short shaft boat motor if your transom height is 38cm and long shaft if it is 51cm.
Learn more about boat motors:
Which boat engine consumes the least?
What are the best boat motor brands
