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Old 19-04-2019, 13:57   #1
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stripped thread on prop shaft

When the marina removed the prop to change the cutlass bearing, they found the threads ruined on both the shaft and the hub. They say that the prop is not the original prop for the boat, that it was not the correct size for the shaft, and that by forcing the brass hub onto the stainless steel shaft, someone ruined both. Photos attached.

I have had the boat since 2005 and have never removed the prop, so this was installed by a previous owner. The marina says I may need to change the shaft (I fear this may be expensive) but they are also looking for other options for reattaching the propeller, which would entail me signing a liability waiver in case the prop falls off in route one day.

Any advice from the community on all of this?

Thanks,
Joe
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Old 19-04-2019, 14:05   #2
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Re: stripped thread on prop shaft

You could get a machine shop to cut a new taper and thread if you have the room for the prop to be 1 1/2 further forward.

Thats what I would likely consider.
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Old 19-04-2019, 14:54   #3
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Re: stripped thread on prop shaft

The shaft is chewed up some, but a die might chase most of the threads back to shape. The hub looks bad, so it needs to be replaced. Sounds like you might need a new prop too.
The shaft thread doesn't appear to be continuously damaged, only from about 1/4 (of the entire thread length) to about the middle. Kind of odd pattern, one might expect the entire thread to be damaged if the hub was the wrong thread, or the free end of the thread to be damaged if somebody started the hub crossthreaded. Btw, the hub is probably bronze, not brass.
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Old 19-04-2019, 14:55   #4
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stripped thread on prop shaft

I think your buying a new shaft.
Is that a bronze shaft?
That is an unusual looking shaft nut, what do you mean by the hub threads were ruined?
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Old 19-04-2019, 21:19   #5
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Re: stripped thread on prop shaft

Am I missing something? What prop threads onto the shaft?
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Old 19-04-2019, 23:02   #6
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Re: stripped thread on prop shaft

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Originally Posted by fstbttms View Post
Am I missing something? What prop threads onto the shaft?
Good question... I don't understand either!

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Old 19-04-2019, 23:24   #7
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Re: stripped thread on prop shaft

No use speculating how it happened, my guess is galling but not really sure.

Repair is possible by an experienced welder and someone that can turn a new thread once material has been returned to the shaft. Depending on what sort of stainless it is, it may need annealing to weld and machine ... or not.

Experienced welder is the key word. Cutting the thread is easy. The nut needs to be made new with special attention to the type of bronze used to avoid galling. Silicon bronze nut most likely but further enquiries required.

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I like the marina asking for a disclaimer in case they stuff up. Suggest for them to install a chain and a ring in one of the blades to stop the propeller from falling off?
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Old 19-04-2019, 23:53   #8
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Re: stripped thread on prop shaft

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Originally Posted by Joe500 View Post
When the marina removed the prop to change the cutlass bearing, they found the threads ruined on both the shaft and the hub. They say that the prop is not the original prop for the boat, that it was not the correct size for the shaft, and that by forcing the brass hub onto the stainless steel shaft, someone ruined both. Photos attached.

I have had the boat since 2005 and have never removed the prop, so this was installed by a previous owner. The marina says I may need to change the shaft (I fear this may be expensive) but they are also looking for other options for reattaching the propeller, which would entail me signing a liability waiver in case the prop falls off in route one day.

Any advice from the community on all of this?

Thanks,
Joe
Joel, I think I would find the correct fitting prop, or if the performance of the prop that you have is OK then have it machined to fit the shaft correctly so that the end of the boss is at the beginning of the thread, then run a dienut down the shaft thread to clean it up as best as possible, fit the prop and secure with a new castellated nut, This will require the drilling of a hole through the shaft to accomodate a split pin. There appears to be sufficient thread to allow this. Install everything, including new shaft zinc, launch and go sailing.
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Old 19-04-2019, 23:58   #9
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Re: stripped thread on prop shaft

The shaft is not that expensive. Add a new coupling also and it will be about $300 to $400 depending on length and diameter. Of course that doesn't include labor.
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Old 20-04-2019, 00:19   #10
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Re: stripped thread on prop shaft

Don't worry , if you zoom in it still looks acceptable , run over it with the same thread cutting die to fresh it up , .... job done . Most of the time you're in forward gear , there's no strain on the thread .
But do get a new prop. nut in place .
Also some thread lock will reinforce the lot .

New shaft is 300 - 400$ , money down the drain .
But then it's your money .

Good luck
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Old 20-04-2019, 02:22   #11
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Re: stripped thread on prop shaft

Quote:
Originally Posted by bobs View Post
Don't worry , if you zoom in it still looks acceptable , run over it with the same thread cutting die to fresh it up , .... job done . Most of the time you're in forward gear , there's no strain on the thread .
But do get a new prop. nut in place .
Also some thread lock will reinforce the lot .

New shaft is 300 - 400$ , money down the drain .
But then it's your money .

Good luck
As kmacdonald said a new shaft is not that expensive, so I would go for this solution. It is also true that the thread could be refreshed using a thread cutting tool (like in the picture here below) but I see that parts of the shaft and nut threads are gone, so I would not take this risk, if you loose your prop while maneuvering in a port it's a damned bad situation !
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Old 20-04-2019, 02:58   #12
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Re: stripped thread on prop shaft

Chase the threads then have another look at it. I think when you clean it up it will look much better. The scale on the parts where the nut was not riding makes the wastage look worse than it is.

If you decide to reuse the shaft use bronze nuts to hold the prop on. Stainless on stainless can gall and seize. Because of the thread damage your shaft might be more susceptible to that.
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Old 20-04-2019, 05:12   #13
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Re: stripped thread on prop shaft


Does this mean Joe500's boat has a faring piece that was also the nut but without a cotter pin?


Seems like the important bits (prop, nut) were not ever right for the shaft. If it's a good prop the could the shaft could get remade to suit a decent nut, pin, and faring piece?
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Old 20-04-2019, 05:30   #14
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Re: stripped thread on prop shaft

You may be able to repair the threads, then get a new nut.

First you would have to accuratly measure the threads.

If you have to buy a thread die ask for a thread repair die, much less expensive than a thread cutting die.
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Old 20-04-2019, 09:38   #15
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Re: stripped thread on prop shaft

Quote:
Originally Posted by Joe500 View Post
When the marina removed the prop to change the cutlass bearing, they found the threads ruined on both the shaft and the hub. They say that the prop is not the original prop for the boat, that it was not the correct size for the shaft, and that by forcing the brass hub onto the stainless steel shaft, someone ruined both. Photos attached.

I have had the boat since 2005 and have never removed the prop, so this was installed by a previous owner. The marina says I may need to change the shaft (I fear this may be expensive) but they are also looking for other options for reattaching the propeller, which would entail me signing a liability waiver in case the prop falls off in route one day.

Any advice from the community on all of this.
Thanks,
Joe
The shaft and nut are silicon bronze not stainless. Run a die on the shaft and buy a new castle nut drill a hole in the end of the shaft for a cotter pin. If the yard told you the shaft was stainless I would take their advice with a large grain of salt.
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