Propeller shaft key material
- Thread starter DArcy
- Start date
We are talking about corrosion induced by dissimilar metals. The answer is it does not matter any effects are insignificant, the contact surface of the key is much smaller than the surface of the shaft and prop together. What is the engine HP? When something wraps around the prop the most often result in small diesels is that it will stop the engine dead in its tracks. On a well set prop, chances are that the prop when stopped abruptly is not going to spin on the shaft. My personal take is that the key is necessary when the prop is first installed and as an added safety measure should the nut and cotter pin fail.
i concur on the corrosion being minimal. Smear some 2-4-c or whatever grease you want on it. The key is a there to give if needed though. It should be softer than both the shaft and prop. They make "key stock" for a reason. Buy a stick, cut what you need and install it.
Wow, That's a new one to me.It certainly can be scary what you find the PO has done to your boat.So the weirdest thing about my situation (but not completely surprising considering some things I have found on this boat) is that the prop that came installed on the boat was for the wrong sized shaft. I didn't measure, it might be a 1-1/4" and I have a 1" shaft. Whoever installed it made a shim out of what appears to be PTFE.I was, to say the least, questioning this setup.
The previous owner said he thought the 3 bladed prop that was on there was a bit too big and gave me the original 2 bladed prop. I did find I could not get max RPM out of the engine and was not getting the same speed under power as other people with the same boat and engine. I am now very glad I decided to switch the prop, I don't know how long that shim would have held.
I'm guessing the PO got the 3 blade for a good deal and decided to try it.
Anyone want a 3 bladed 16x13 prop for a 1-1/4" (ish) shaft? I certainly have no use for it. Free shim included
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Even splined outboard props have a break away point in the prop in case you hit something extreme, so to answer your question, yes. If not, they'd just weld a bar on the shaft as it would be much cheaper than cutting a key way in a taper.
The worlds larges prop shaft manufacturer advises brass or a soft bronze so that it has some "give" and does not cause the shaft to fracture but rather they key will be damaged first. Most key stock you will find for prop shafting will be a brass based alloy.
So, I guess you are claiming that the prop shaft key has two functions; prevent slippage and act as a shear pin. I have seen many shear pin assemblies in machinery. None of them used a shaft key of the type props have as a shear device. If the prop shaft key was designed as a shear device I would expect to see published standards for each size prop and shaft and replacement shear keys to be carried as spares for each application. My snowblower, for example, has a bag of shear pins attached to the handle bars. Instead, what I see are key cross section choices and key material choices but no shear value choices. Length of keys is not even specified by prop manufacturers, as far as I know. Certainly, if shear protection was an expected function, you would expect prop manufacturers to recommend shear values for these keys. I do not see that.
1.- In my 26 years experience as a hull cleaner, I have seen only a few overidden keys and not a single one of them was caused by an impact to the prop. I do not believe that shaft keys are meant to act as a "shear pin."
2.- Do a little research and you'll find that the mated surface between the shaft and prop tapers is what transfers the torque from the shaft to the prop. That is not the key's function.
3.- Regarding key material- I believe it is irrelevant whether it is bronze or some form of steel. Your prop is almost certainly bronze and your shaft is almost certainly some form of steel. Corrosion is not an issue there and therefore is not an issue regardless of which key material you use.
2.- Do a little research and you'll find that the mated surface between the shaft and prop tapers is what transfers the torque from the shaft to the prop. That is not the key's function.
3.- Regarding key material- I believe it is irrelevant whether it is bronze or some form of steel. Your prop is almost certainly bronze and your shaft is almost certainly some form of steel. Corrosion is not an issue there and therefore is not an issue regardless of which key material you use.