propeller mounting question

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berco

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Jul 14, 2021
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Hi, I am a new member and don't even have a boat. I joined to try to get an answer about propeller mounting. I have a machine shop and a customer asked me to bore out the mounting taper in 2 brass 4 blade props which will replace the existing severely worn out ones. The old ones are 21", these are 22", but bores are smaller, so, he wants me to bore them to size to fit the propshaft.
After boring them out, the keyway will have to be cut. The keys that came out with the old props appear to me to need replacing. They are brass.
They have a groove in the middle of each side running the length of the key.
Are they made this way? If so, for what purpose? I am no expert on drive keys on propellers, but, I would think they should be solid square section to seal out water.:confused:
I don't know the availability of the keys, so, I might have to make them, and want to be sure to do it correctly. I think this is a good place to get advice on this subject and I greatly appreciate any help.
Thanks.
 
berco said:
Hi, I am a new member and don't even have a boat. I joined to try to get an answer about propeller mounting. I have a machine shop and a customer asked me to bore out the mounting taper in 2 brass 4 blade props which will replace the existing severely worn out ones. The old ones are 21", these are 22", but bores are smaller, so, he wants me to bore them to size to fit the propshaft.
After boring them out, the keyway will have to be cut. The keys that came out with the old props appear to me to need replacing. They are brass.
They have a groove in the middle of each side running the length of the key.
Are they made this way? If so, for what purpose? I am no expert on drive keys on propellers, but, I would think they should be solid square section to seal out water.:confused:
I don't know the availability of the keys, so, I might have to make them, and want to be sure to do it correctly. I think this is a good place to get advice on this subject and I greatly appreciate any help.
Thanks.
Click to expand...

I am not an expert either, however the prop including the shaft and keyway are all outboard of the boat, so there is no concern at sealing out water.

~A
 
Please take this in a proper way.

He walks in and says, "I want you to....." and not provide enough information and guidance to do the job.

Remember after boring the diameter and cutting the key way and making a new key, he will want to static and dynamically balance the prop.

Do not be afraid to turn down a job and recommend another shop.
 
Last edited:
You might want to turn this one down. There are several good prop shops in the Bay area that do this routinely, they know what is necessary and what it costs to do it.
 
I agree with the suggestion to turn this over to a competent propeller shop where they're set up to perform this work easily and correctly.
 
Both key and taper fit involve many issues that if not done riight can cause the shaft to shear and loose the prop. My dad was a top machinest and I worry this job will just give you needless headaches.
 
Take a pass on it and refer him to a good prop shop.

By the way, a prop generally does not "wear out". They become damaged, out of pitch, sometimes have cavitation damage and a host of other problems but most can be repaired.

pete
 
Pete Meisinger said:
Take a pass on it and refer him to a good prop shop.

By the way, a prop generally does not "wear out". They become damaged, out of pitch, sometimes have cavitation damage and a host of other problems but most can be repaired.

pete
Click to expand...

They can get very thin as they age or are the subject if corrosion, therefore are called worn out. I've heard that term many times.

You can buy key stock via McMaster-Carr or other industrial supply houses. No need to make. I would want standard keys.
You can find all the specs you need for machining online.
But a prop shop would have all that info and the correct fixtures to keep it centered and square and it's not very expensive to have done.
 
Perhaps, just perhaps the prop owner is trying to do it on the "cheap". What he may learn is, it is going to cost him more money than having it done professionally.
 
The keys on all my boats have been stainless, not brass or bronze.

I understand that the taper is an SAE standard, but I also know it’s important to verify the contact area between the shaft and prop hub with bluing. If you don’t have 80% or more contact, the prop may not seat squarely and you will get a vibration. Don’t ask me how I know.

BTW, all the same applies to the coupler end of the shaft, assuming a taper fit.
 
A grooved shaft key sounds as if the prop is not tight on the taper and is rocking back and forth. The keyway on the prop or the keyseat on the shaft may be cutting into the sides of the key.

If the prop and shaft are lapped with good contact the key sees very little torque. The taper should be enough to hold the prop on the shaft.

For this reason, the keys on tapered shafts are often lighter in strength in size and material (shear strength) than keys on straight shafts.

Note. In industry, I have seen more shaft failures than key failures.
 
Prop tapers are sometimes cut on a metric standard, I’ve seen a few.
This does not fit perfectly with an SAE taper.
Grooves on the key way are a red flag, they are too loose.
I took of a pair of props recently that had been installed with some kind of a waterproofing, (maybe RTV), it required some heat,to remove them, but the insides of the fit were in perfect condition, where there’s usually some corrosion.
 
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