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What is a Propeller Shaft?

By Lori Kilchermann
Updated May 23, 2024
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A propeller shaft is a device on which a propeller is attached to and transfers the power from the engine to the propeller. This terminology is typically used when discussing a ship or boat propeller as an airplane's version is typically mounted to a hub. The propeller shaft runs from the engine through a seal in an inboard engine application. It then runs through the hull and into a bearing just ahead of the propeller. The shaft must run true and straight, free of any bends or it will vibrate the vessel as well as pre-maturely wear out the bearings and seals.

The propeller shaft is typically made of hardened steel and incorporates a spline on the end of the shaft where the propeller mounts. This spline allows the propeller to be mounted onto the shaft without slipping or spinning free. Along with the spline, a threaded section of the shaft incorporates a nut and washer to tighten and secure the propeller to the shaft. This nut is usually a castle nut, which is a nut that resembles the tower of a castle and uses a cotter pin. The pin is placed through a hole drilled through the shaft, and then bent over to prevent the nut from loosening or backing off.

In an outboard engine application, the principle of the propeller shaft remains the same, but on a much shorter scale. The shaft on an outboard engine runs from a beveled gear in the engine's lower unit, through a sealed bearing and out of the casing. The same spline system is used to retain the propeller as with the castle nut.

In many outboard systems, a shear pin is used to prevent an object from coming in contact with the propeller and breaking the propeller shaft. A shear pin is a small metal bolt-like pin which is made of softer metal than the propeller shaft. When the propeller comes in contact with a hard object much like a rock or large underwater stone, the pin will shear off. This allows the shaft to continue rotating while the propeller stays still.

It is an easy task to raise the engine's lower unit out of the water and install a new shear pin with minor hand tools. Once the pin is replaced, the boat can once again be operated. If not for the shear pin, the shaft might be severely damaged or the propeller could be broken or damaged beyond use.

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Discussion Comments
By cardsfan27 — On Jul 08, 2011

@titans62 - I've experienced the same thing before, and it definitely is not fun. While you can repair your own propeller if the damage is minor, I would highly suggest you take it to a professional or buy a new one unless you are certain of what you are doing.

When I bent my propeller, I tried to straighten it out myself. I thought it looked pretty good, but the next time we took it out on the water the slight bend that was left was too much and I ended up damaging the shaft seal, which ended up costing much more than if I had simply broken down for a new propeller.

Also, since it sounds like you were moving pretty quickly, I would also recommend taking the boat to someone and have them look at the propeller shaft and seals just in case.

By titans62 — On Jul 08, 2011

Has anyone here ever tried to fix a damaged propeller? I was boating last week and was going a little too fast near the shore and ran into a log or rock that I couldn't see. It bent the propeller a little, but one of my friends said that he thought I could probably straighten out the propeller myself.

The trip back to the dock was pretty bumpy, but I think the prop shaft is still fine as far as I can tell. Also, the propeller isn't cracked, just bent.

By matthewc23 — On Jul 07, 2011

Wow, I never realized there was so much involved in making a boat move.

Could someone who knows more than I do about the subject please explain what it means when it says the propeller on an airplane is attached to a hub? Does that just mean that it is connected directly to the engine without a shaft?

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