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Old 09-12-2014, 11:03   #1
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Prop Shaft Failure

Recently were heading 'south' in our 1975 Alberg 37 yawl (which we've owned since 1982) when we encountered a broken prop shaft (broke inside the coupling). See the following article (This is the saga of SHEARWATER’s ‘Short Southbound Cruise’) for details. Ideas welcome on what caused the break.....

Have been researching Aquamet shafting (looking at going with Aquament 19) along with Vetus 'Bullflex' couplings. Seems like the Vetus Bullflex series are quite good with good shock absorbing capabilities.

Also looking at R & D Marine Steel Split Half Couplings with the R & D Marine Gearbox Output Flange Adaptor with the R & D Flexible Shaft Coupling in between.

Anyone out there who has went to a Vetus Bullflex coupling, or the R & D setup mentioned above?? Comments and suggestions welcome.

Also, can anyone recommend a Vetus dealer who is willing to correspond with a potential buyer? Have had no luck so far in corresponding with a Vetus dealer.

Thanks,
Tom in Kinsale, VA
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Old 09-12-2014, 12:06   #2
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Re: Prop Shaft Failure

It looks to be classic high cycle fatigue, but it could be from a stress riser left from machining, an inclusion in the metal, or a few other things, assuming no sudden impact in the past that you know of?
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Old 09-12-2014, 12:07   #3
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Re: Prop Shaft Failure

You put a zinc inside of the boat on the shaft? Do you do that with this sort of failure in mind? Or am I misunderstanding.
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Old 09-12-2014, 12:14   #4
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Re: Prop Shaft Failure

misalignment and slight bend shafts can cause shaft failure at the coupling edge, Aquadrive CV joints are a wonderful stuff if you have enough clearance between the trany and the shaft log....
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Old 09-12-2014, 12:19   #5
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Re: Prop Shaft Failure

The 'zinc' is used as a collar to prevent the shaft from leaving the boat in the event of a shaft failure. We place it about 1/2" in front of the stuffing box. Have done this since we've owned the boat. On the A-37, the prop would most likely jam the rudder should the shaft break at some point inside the boat as happened to us unless the 'collar' was in place, as the A-37 rudder is a full lenght rudder, not a skeg mounted rudder.
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Old 09-12-2014, 12:23   #6
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Re: Prop Shaft Failure

I like that idea, I think I'll steal it
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Old 19-01-2015, 00:11   #7
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Re: Prop Shaft Failure

Shaft alignment, .003 maximum deflection on all faces of the coupler


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Old 19-01-2015, 01:48   #8
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Re: Prop Shaft Failure

Very interesting failure, though I am sorry about your lost cruise. I would have thought it would fail nearer the mouth of the coupling, where the stress should be higher?

Thanks for the link to your blog and excellent photo's. I have only heard of one other shaft failure like this and it happened just inside the stuffing box as far as I am aware.

I would be looking at your engine mounts and bearers, possibly a problem there caused the misalignment issues that possibly lead to the failure? Was there much vibration issues normally or did it run sweet at all revs?
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Old 19-01-2015, 04:03   #9
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Re: Prop Shaft Failure

Drift ... Hey Tom, sorry to hear about your shaft and I have nothing to add without seeing it but ..... hoping to head south again end of summer and will stop by Kinsale for another oyster feast

Wallace
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Old 19-01-2015, 07:23   #10
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Re: Prop Shaft Failure

Well that's a shame but why did you cancel the entire cruise instead of having the boat fixed and continuing on your way?
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Old 19-01-2015, 08:25   #11
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Re: Prop Shaft Failure

if the shaft was actually 40 years old you may already have your answer. it really looks scored and worn from the pictures but i understand if it ain't broken then don't fix it
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Old 19-01-2015, 09:24   #12
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Re: Prop Shaft Failure

definitely looks like fatigue failure due to misalignment. Align the engine with the new shaft and it will last for more than 40 years without any special couplings.

Nice looking boat!

If you need help, you can trust Kenny Bock.
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Old 19-01-2015, 10:33   #13
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Re: Prop Shaft Failure

I have seen an axle on a half ton pickup fail while taking off gently from a stop sign. Inspection of the break showed that a crack formed half way through the axle and then failure occurred with just a little stress. Half of the fractured area was darker. The fracture had a crystalline structure to the steel. What you have, though, is stainless steel. You can Google stress fractures & stainless to see what you get. If the engine were a bit out of alignment, then repetitive stress could occur and in a chloride environment, there might also be formation of fissures to concentrate the stress. A scratch on the propeller shaft when it was manufactured could also concentrate stresses. You could send the propeller shaft to a metals testing lab and they could look at the crystalline structure of the metal near the point of failure and most likely tell you exactly what happened.
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Old 19-01-2015, 14:27   #14
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Re: Prop Shaft Failure

Wallace,

We'll be sure to same some oysters!!!

tj
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Old 20-01-2015, 07:32   #15
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Re: Prop Shaft Failure

I guess I'm not going to get my question answered.
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