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Old 11-10-2010, 10:33   #1
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Bad Vibrations

So I clearly have something amiss with my prop. Let me enumerate the symptoms:

  • Massive vibration in the prop shaft at certain, but variable rpms (i.e. like there's some kind of a resonance). Light vibration at all RPMs and also a light "clicking" sound.
  • Vibration gets worse after frequent shifts in direction and gets better if continuing in one direction
  • Shaft coupling has come loose twice as a result of said vibrations, and they get worse when the coupling is loose
  • When turning slowly by hand, the shaft will move one direction as I turn. I stop turning and then it slides back to the normal position.
Possible causes:

  1. Loose shaft zinc. The zinc is 25mm in diameter and the shaft is 25.4mm. Perhaps it has come loose? The vibrations have gotten worse and this is my best guess of the problem.
  2. Bad cutlass bearing. The engine was run a bit while in pretty terrible alignment. Age of bearing is unknown. But I don't know how to recognize the symptoms.....
  3. Bad alignment. It may still not be in perfect alignment. However, tweaking the alignment further has not been terribly helpful.
  4. I'm missing something?
Advice would be greatly appreciated. The current plan is to call a diver to remove the shaft zinc. I'm planning on replacing the prop in the spring anyway, and I don't think corrosion is going to be a huge problem and this problem seems an awful lot like an imbalanced prop problem that didn't exist before adding the new shaft zinc.
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Old 11-10-2010, 10:41   #2
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Frank and Billy-Bob Barnacle have come to stay?
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Old 11-10-2010, 13:21   #3
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Sounds like you might have a bent shaft the only way to tst it is to draw the shaft and have a look see
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Old 11-10-2010, 16:36   #4
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Yeah, I also wondered about the bent shaft theory. However, I'm more inclined to the loose zinc theory since a bent shaft should not stabilize at higher rpms. The shaft, cutlass bearing and prop are all getting replaced in the spring anyway....
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Old 11-10-2010, 17:25   #5
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Hey Astra, I had very simalar problems this past year. At higher RPMS, or turning sharply I would hear this bap bap, not terribly loud, but enough to know something was wrong. Having just done a complete mech repower I was convinced it was in the drive train. Three weeks ago when I went to the boat I found water pouring in way down in rear of engine compart, I have center cockpit so drive shaft dissapears thru a brass/bronze tube approx 3 ' long. Other end is shaft log threaded on,which holds the cutlass brg. The shaft log was missing the bottom bolt, and the top was loose, as far as I can tell the whole shootin match was rattling at the end. It was actually good I took the water, else I may have not pulled it out. I just put the whole thing back togethet sunday. Great french design, I had to pull the motor to remove the shaft. sorry so wordy Red
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Old 11-10-2010, 18:22   #6
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ehhh? pull the motor to yank the shaft. Can that be?
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Old 11-10-2010, 21:47   #7
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When turning slowly by hand, the shaft will move one direction as I turn. I stop turning and then it slides back to the normal position.

Do you mean that the shaft moves insde the coupling?
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Old 11-10-2010, 21:54   #8
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Quote:
Originally Posted by Chief Engineer View Post
When turning slowly by hand, the shaft will move one direction as I turn. I stop turning and then it slides back to the normal position.

Do you mean that the shaft moves insde the coupling?
Or moving fore and aft? Is the coupler tight on the trany?
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Old 12-10-2010, 05:08   #9
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No. It's strange and hard to describe. The motor is fairly lightly mounted so the whole motor mount twists up and down slightly (rotates perpendicular to the boat's beam). This amount decreases at higher rpm and as I said, goes away if I turn by hand and leave it parked.
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Old 12-10-2010, 05:19   #10
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And you have checked for barnacles growing on the prop, plastic bags, string, fishing line etc?
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Old 12-10-2010, 05:29   #11
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Nope, haven't checked for barnacles yet. I don't suspect them because barnacles don't generally rattle around. Also, this problem existed (in a lesser form) shortly after haulout when the prop and shaft were cleaned.
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Old 12-10-2010, 05:42   #12
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I'd still be splashing in to have a look before you go get delirious about replacing shafts. Plastic bags and fishing lines just jump from one end of the harbour straight onto my prop.
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Old 12-10-2010, 14:19   #13
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It will probably be obvious when you pull her out.I was freaking until I got her out and was able to see what was going on. Sabray, mine has a 65" shaft that must be inplace B-4 engine is set on mounts. Its a center cockpit.
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Old 12-10-2010, 14:31   #14
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vibes

here iis a shot of shaft from engine compartment
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Old 12-10-2010, 14:32   #15
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Start with the simplest explanations. Hop over the side with a mask and check your zincs, inspect the prop, look for barnacle growth, and so forth.

A diver can easily replace a zinc underwater.
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