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Old 23-09-2019, 10:07   #1
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Shaft busted - lessons and questions

We left our favorite neighboring port and started motoring the 13 mile journey back to our slip. Winds were forecast to be 11+ on the nose, but it was dead flat.


About 2 miles into the journey I went below and noticed an unusual vibration. Nothing big, but I've had the boat 25 years so the little differences in behavior are easier to pick up. I put the boat in neutral, checked the engine, oil, mounts, etc. Everything looked OK.



Upon shifting to forward, no movement. Put it in reverse. Same. Uh oh. I then checked the coupling area and voila, the shaft was sheared off forward of the keyway; I could see this because the shaft had backed out a couple inches.


I put lockjaws on the shaft to prevent it from backing out further. This happened to friends of mine; the prop hit the rudder so they couldn't steer, and the boat was taking lots of water...so I didn't want this to happen to us. BTW, just in case I carry wood bungs, which is what they had used to stop the flow and save the boat.



No water was coming through the stuffing box, but some leakage coming from the strut, so there must have been some vibration there.


With no wind, we towed using the dingy and could safely make 3.5 knots. After some time I killed the dingy motor and "went" back to our boat; that thing was coming fast! I couldn't get the tow rope in fast enough so it got behind the keel- no big deal, since no prop was spinning. Also I should note another mistake, when I grabbed the big boat toerail the forward momentum spun the dingy bow around, toward the aft of big boat- if the tow line had been tight it could have flipped the dingy.


Then the wind came up, and we beat tacked back to port, hand steering and constantly adjusting sails...too much like racing!


We made it into port, and less than 1000 ft from the marina the wind died totally. Besides which the go-fast boats were making wake all over. I then used the dinghy to push (rather than pull) and that worked great, easy to make 5 knots.


I didn't take photos, we were too tired and hot. It looks like the shaft slipped and the keyway bolt ate up the forward end of the shaft- if you've run a bridgeport you know what happens to the tool when it's run too fast on hard steel...same thing. But odd thing is the keyway bolt is wired, and hasn't moved, and the keyway slot, except for the bitter end, doesn't look banged or damaged or have indication that the bolt was loose.


So the question is how to determine what caused this. Of course, you can't tell particularly w/o photos, but what should I look for?


Presuming I change the shaft, coupler and cutlass bearing, should/ must I also change the stuffing box?


I hope this helps some folks thing about how to handle such a situation, and thanks in advance for the insights.
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Old 23-09-2019, 10:23   #2
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Re: Shaft busted - lessons and questions

Wow, quite a story. In the past I have used the "tie the dingy to the boat" for towing and it works very well, giving you more control as opposed to actually towing. And you are right at the boat.
I would check the shaft when you remove it for corrosion. Look closely at the surfaces of the break. You may have some stray current causing metal corrosion on the boat, especially if using shore power.
I once saw a shaft that was almost completely gone after 4 months in the water. The boat had sailed the south pacific with no problems. Then moored in a Marina with shore power after rebuilding the shaft and etc. Having the bottom cleaned, the diver said to "haul the boat immediately"! The thru hulls were turned to dust. The shaft looked like table cloth lace, barely any metal left.
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Old 23-09-2019, 10:26   #3
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Re: Shaft busted - lessons and questions

My guess is you had an alignment problem. The coupling is probably ok. The Cutless bearing is probably worn and the stuffing box should be repacked. Have a pro realign the engine and shaft.
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Old 23-09-2019, 10:49   #4
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Re: Shaft busted - lessons and questions

If you have space,putting a spare shaft anode on the shaft inside the boat will prevent it heading out backwards if the coupling fails.
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Old 23-09-2019, 11:00   #5
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Re: Shaft busted - lessons and questions

This broken stainless shaft shows corrosion on the inside.
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Old 23-09-2019, 13:01   #6
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Re: Shaft busted - lessons and questions

Quote:
Originally Posted by boatpoker View Post
This broken stainless shaft shows corrosion on the inside.



It looks not dissimilar. Though the boat is and always has been in fresh water and the anode is fine.


The coupler is only $99 so I'll probably just get a new one anyway. I'm going to have to take it off to remove the remnants and clean anyway.
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Old 23-09-2019, 13:01   #7
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Re: Shaft busted - lessons and questions

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If you have space,putting a spare shaft anode on the shaft inside the boat will prevent it heading out backwards if the coupling fails.



Darn, that's a great idea!
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Old 23-09-2019, 14:24   #8
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Re: Shaft busted - lessons and questions

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Originally Posted by boatpoker View Post
This broken stainless shaft shows corrosion on the inside.
Yeah, that's an interesting break. Having had just enough college level metallurgy to make me dangerous (!) I wonder if that is something related to cold rolling or drawing of the bar the shaft is made of when I see something like that. Strange how it looks like a layer peeled off. Something going on with the grain boundary between that outside layer and the core anyway... corrosion I suppose, but how does it get in there and run longitudinally? Or maybe it's just common because the outer part of the bar is work hardened more than inside from cold rolling...
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Old 23-09-2019, 14:32   #9
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Re: Shaft busted - lessons and questions

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Yeah, that's an interesting break. Having had just enough college level metallurgy to make me dangerous (!) I wonder if that is something related to cold rolling or drawing of the bar the shaft is made of when I see something like that. Strange how it looks like a layer peeled off. Something going on with the grain boundary between that outside layer and the core anyway... corrosion I suppose, but how does it get in there and run longitudinally? Or maybe it's just common because the outer part of the bar is work hardened more than inside from cold rolling...
It was crevice corrosion. The break occurred within the cutless bearing of a boat that had not moved in over five years. Externally only pinhead size pits were visible and did not appear overy dramatic. Here is a single pit on X-ray.
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Old 24-09-2019, 10:41   #10
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Re: Shaft busted - lessons and questions

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If you have space,putting a spare shaft anode on the shaft inside the boat will prevent it heading out backwards if the coupling fails.
Great idea!
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Old 24-09-2019, 10:58   #11
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Re: Shaft busted - lessons and questions

If you've truly been boating for 25 years I'm confident you can think of a number of ways that method of towing could go wrong.

Next time try tying up alongside the mothership. You'll find it easier and safer in many respects (safer and easier to control, safer and easier to board, far safer line configuration, etc...)
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Old 24-09-2019, 13:47   #12
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Re: Shaft busted - lessons and questions

Double on that done it a couple of times in a Formosa 35 and a Catalina 42 side tie the dingy and steer from the mother ship it helps if is is flat seas.
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Old 24-09-2019, 14:29   #13
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Re: Shaft busted - lessons and questions

If you don't have room for an anode (like mine) you can cut a very short length of thick plastic tube in two halves and hold them in place around the shaft with a stainless hose clamp. It only takes half an inch of the shaft.
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Old 24-09-2019, 14:49   #14
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Re: Shaft busted - lessons and questions

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Darn, that's a great idea!
Good idea but wrong component. Get a two-part bronze or SS clamp collar. They will handle significant thrust. A zinc will not.
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