Many new boat owners forget to choose the right shaft length for their Outboard Motors. It's important to know that the length of the shaft has a direct effect on how well the outboard motor works, how safe it is to sail, and how long it will last. If you choose the improper shaft length, your engine may not run as fast or last as long. In the worst circumstances, it can even cause sailboats to crash. This tutorial will give boat owners all the information they need regarding outboard motor shaft length, including how to choose the right length.
What Is The Length Of An Outboard Motor Shaft?
The length of the outboard motor shaft affects how deep the propeller goes into the water. It means the space between the mounting bracket and the flat plate above the propeller.
Classification Of Outboard Motors By Shaft Length
| Type | Shaft Length |
| Short Shaft Outboard Motor | 38 CM |
| Long Shaft Outboard Motor | 51 CM |
| Extra Long Shaft Outboard Motor | 63 CM |
Why Is It Important To Have The Right Shaft Length?
To put it simply, the boat can go quicker with the correct shaft length. The right shaft length lets the propeller grab the water at the right depth and lets the engine's cooling system work effectively. If the shaft is too short, the propeller won't be able to fully immerse in the water, which will produce cavitation and power loss. If the shaft is too long, the propeller will be submerged too deeply, which will make the water resistance and fuel consumption go up.
How To Measure The Height Of The Sternplate?
The sternplate is the baffle that is at the back of a ship. The height of the sternplate is the vertical distance from the top of the sternplate to the bottom of the hull. Measuring the sternplate height vertically is very important. To get an accurate measurement, the ship should be parked on flat ground. Find the centre of the sternplate and measure straight down from the top of the sternplate to the keel line at the bottom of the hull. The resulting measurement is the sternplate height.
If the height of the sternplate is about 38 cm, consider an outboard engine with a short shaft.
Choose a long-shaft boat motor if the sternplate height is about 51 cm.
Choose an extra-long-shaft boat engine if the sternplate height is about 63 cm.
| Type | Shaft Length | Sternplate Height |
| Short Shaft Outboard Motor | 38 CM | 38 CM |
| Long Shaft Outboard Motor | 51 CM | 51 CM |
| Extra Long Shaft Outboard Motor | 63 CM | 63 CM |
To check if the shaft length choice is right, look at where the Anti-Ventilation Plate is. Above the propeller, there is a big horizontal metal plate called the Anti-Ventilation Plate. It is okay if the Anti-Ventilation Plate is level with the hull's lowest point or 1–2 cm higher or lower than that.
During the choosing process, stay away from these three mistakes:
I think a longer shaft is safer:
This is the most common thing that new people do wrong. They consider that a longer shaft equals more water getting into the propeller, which is not a bad thing. Not right! A shaft that is too long might provide a lot of leverage, which puts more stress on the stern plate. This can cause cracks in the stern plate over time.
Not paying attention to how water flows:
If the hull has a lot of bumps (like sensor brackets or patches), the water flow becomes choppy and has bubbles in it. In this scenario, the shaft length might need to be a little longer to get past the bubble layer and get deeper, harder water.
If you want to install two engines, the hull is normally V-shaped and the installation point is off-center. In this instance, you will probably require a longer shaft than you would for a single-engine installation.
Conclusion
Always measure the boat first and then think about the engines when you choose a shaft length. Don't take any chances when you choose a model. Keep in mind that the right shaft length can save you up to 10% on gas. The above makes it easy for the boat owner to handle. It's not hard to pick the appropriate length for an outboard motor shaft, but it's quite important. Don't forget that the shaft length should match the height of the sternplate and that the propeller should always be fully submerged in water. If you use the boat correctly, it will run more smoothly, faster, and more efficiently, and it will be less likely to have difficulties in the future.
Still not sure which outboard motor fits your boat? Contact KLS Supplier today for expert advice and factory-direct pricing:
• +8615094051894
• Room 1705, Yihe Blue Diamond, No. 154, West Section of Second Ring South Road, Yanta District, Xi'an City
FAQ
Q1: Is it okay to use a long shaft on a short sternplate?
A1: You shouldn't do it. This will make the car slower, more difficult to handle, and cause more drag.
Q2: Can the height of the installation make up for the shaft not being long enough?
A2: No, the range of adjustment for installation height is quite tiny. You can't fully fix a shaft length that is wrong.
Q3: How can I tell if my shaft is too short?
A3: The shaft is probably too short if the propeller is above water or the engine rpm is too high.
Q4: What is the best length for the shaft on a 15 hp outboard motor?
A4: Not horsepower, but the height of the sternplate. Always measure first.
Q5: Should I pick a short or long shaft if my sternplate height is 17 inches?
A5: This height is strange. A 15-inch short shaft might not be deep enough and let air in, while a 20-inch shaft... A shaft that is 10 inches long is too deep. A 20-inch-long shaft is best, but you can raise the engine 2–3 inches by putting a shim on the stern plate to get it just right.
References
Wikipedia Outboard Motor