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Author Topic: Slightly Bent Prop Shaft/New Coupler  (Read 355 times)

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GB

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Slightly Bent Prop Shaft/New Coupler
« on: April 09, 2018, 10:55:58 PM »

Last summer the boat had a lot of excessive vibration when underway and we've been working with the boat yard on correcting the issues.

We're in the process of replacing the cutlass bearing and the feathering propeller is being sent off for reconditioning (the yard has indicated that inside the housing is worn and the blades are no longer as secure as they should be).  As part of this process, the yard used a dial indicator on the prop shaft and found a slight bend of .025.  However, they've stated this is within the allowable spec of .030, so replacing it is a toss up.  I am a tad confused as Mainesail's comment in the following thread indicates a much smaller margin if I understand correctly:

http://c34.org/bbs/index.php/topic,2834.msg39289.html#msg39289

Can anyone clarify?  Or have some advice on whether to replace the prop shaft or not at this time?  The current shaft is stainless steel of some sort, so someone replaced the original bronze shaft at some point in time (maybe when the feathering prop was put on?).

My second question concerns the coupling.  I've read threads on this board and others, but it's unclear to me whether it's worth spending more for a flexible coupling at this time or whether the rigid will suffice.  For the rigid coupling, does it matter if the coupling is OEM or will one of the aftermarket couplings work fine (there seems to be a considerable cost difference between the two)?  In terms of the flexible coupling, is the Federal Marine flexible coupling still the recommended route?  For folks that have replaced the coupling, would they make a different choice than the one they made?  Please note that there is a PSS installed. 

Any insight into the above questions is greatly appreciated.
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1988 C34 #525 Tall Rig Fin Keel, M25XP - Chicago, IL

KWKloeber

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Re: Slightly Bent Prop Shaft/New Coupler
« Reply #1 on: April 09, 2018, 11:54:00 PM »

GB

The yard is frankly either:
Inadvertently dropping a decimal point from 0.003 to 0.030, or
Handing you a load of bovine excrement, or
Doesn't have a clue what they're doing. 
Or all three and/or some combination thereof.

k

PS, for that to be acceptable your shaft would have to be on the order of 20' long!!
« Last Edit: April 10, 2018, 12:01:34 AM by KWKloeber »
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Roc

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Re: Slightly Bent Prop Shaft/New Coupler
« Reply #2 on: April 10, 2018, 01:17:05 PM »

About the second year of owning my boat, I had the yard change out the original bronze shaft with a SS shaft and install a flexible coupling.  I remember them saying the only way to see if the shaft is bent is to lay it down on a level surface.  I had the PSS dripless seal installed already and the only flexible coupling that would fit the space was the federal coupling.  I've been happy with it and feel it's a good investment.
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Ron Hill

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Re: Slightly Bent Prop Shaft/New Coupler
« Reply #3 on: April 10, 2018, 01:40:15 PM »

GB : If you still have the original bronze shaft there wouldn't be any doubt in my mind about changing it out to a stainless shaft. I would have changed it just to rid the boat of the vibration!!

As Ken said I believe that the yard may have dropped the decimal point!!

A few thoughts   :cry4`
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Ron, Apache #788

Indian Falls

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Re: Slightly Bent Prop Shaft/New Coupler
« Reply #4 on: April 15, 2018, 03:44:25 PM »

Food for thought:  My bronze shaft which is cast, so you can only indicate on the machined ends was within .005" runout.  All the vibration in my case was from the prop which I had balanced and 1 blade re-pitched back to normal. If there is any vibration from the shaft it's pale in comparison to what the engine is doing.

 .025" is a lot and it also takes a huge amount of force to bend a ss shaft like that so I would be inclined to think it was like that when they put it in your boat or it was hauled with the strap over your prop shaft. 
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GB

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Re: Slightly Bent Prop Shaft/New Coupler
« Reply #5 on: July 08, 2018, 03:25:57 PM »

Thanks to all for the comments.

I just wanted to follow up and let everyone know how this all turned out.

Basically, we ended up replacing the entire drive train -- new Gori 3-blade folding propeller, cutless bearing, SS shaft, coupler, and new motor mounts (K75s all around).

The vibration and sound is significantly better.  We can motor at higher RPMs without a problem and the companion way steps don't rattle and shake your teeth anymore.

As others have mentioned with respect to the K75 motor mounts, there is a lower RPM range that produces vibration, but throttling up quickly through the range solves the issue quickly and it's very smooth from there on.

Overall, we're very happy that we did the work.
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1988 C34 #525 Tall Rig Fin Keel, M25XP - Chicago, IL
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