Shaft vibration

Discussion in 'Inboards' started by Deering, Jun 2, 2019.

  1. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    Hi all. I’m performing sea trials on my catamaran conversion project and have noticed some shaft vibration on one shaft. I’m not sure how much of a problem I have.

    These are pretty long shafts, with about 11 feet between the thrust and cutless bearings. The stuffing box is about midway, but is only connected to the stern tube with heavy rubber hose, no hard connection. At the engine end the shaft is mounted to an Aquadrive, a fixed thrust bearing mounted to the hull that connects to the transmission through a pair of CV joints - engine alignment is decoupled from shaft alignment.

    It’s a 1.5” Aquamet 22 shaft. New. The prop is new as well. The other shaft exhibits no visible vibration.

    I haven’t measured the amplitude at the stuffing box (mid-length) but a rough eyeball estimate might be 1/8 of an inch at most. I don’t hear/feel significant vibration. When I rotate the shaft by hand (no problem with that) I’m measuring about .006 inches of runout, though I can physically flex the shaft a lot more than that.

    I have a number of trips ahead of me soon, and pulling the boat would be problematic. I can dive on it to inspect the prop but I suspect that if it has any damage it’ll be difficult to detect it underwater. It’s Alaska so diving consists of a bit more than just a mask and snorkel.

    So is this a crisis I must deal with immediately? Is there anything else I can do to further evaluate the problem? Any advice welcomed.
     
  2. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    If that long thin shaft of 11 feet has just 1 minor support mid way - that is way way too long for a shaft to be unsupported.
     
  3. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The shaft is pretty long for the diameter. Also, an Aquadrive is not a magic solution to gross misalignment. Make sure the engine/shaft alignment is within the limits of the system.
     
  4. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    Confirmed. Alignment deviation is about 2%, well within limits. But even if it weren’t, I’d expect to see vibration presented in the short shaft between the thrust bearing, which is a fixed monument, and the engine.
     
  5. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    It’s at the outside bounds of the recommendations given in Dave Gerr’s book on boat mechanical systems. Shaft material plays a role - this one is AQ-22. The other identical shaft is performing flawlessly (so far). I can add a fixed bearing into the system, if warranted, but not until the end of the season.
     
  6. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    Can never go wrong with more support, especially on such a long thin shaft.
    Maybe worth checking the balance of the prop too?
     
  7. Deering
    Joined: Feb 2005
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    Location: Juneau, Alaska

    Deering Senior Member

    Thank you. I’ll explore that some more. I’m hoping to put it on a grid in a couple days so I can inspect the prop out of water. I have a spare prop so I might just switch it out to see what happens. Appreciate the advice.

    I could also reinforce the center bearing to make it stiffer. I’ll explore that further too.
     

  8. Sam Smullin
    Joined: Jun 2019
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    Location: Salem, MA

    Sam Smullin New Member

    The max bearing spacing for a 1.5" aquamet 22 shaft at 900rpm is 7.27'. I would add a support bearing sooner rather than later, running with that much unsupported shaft could result in breaking a shaft.

    If your shaft RPM is above 900 the minimum bearing spacing will be reduced.

    sam
     
    Barry likes this.
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