Boat resonanting speed

Discussion in 'Boat Design' started by mbous, Aug 1, 2007.

  1. mbous
    Joined: Aug 2007
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    Location: Louisiana

    mbous New Member

    I recently had a aluminum bass boat built. At 3500 rpm, the boat slowly begin to porpoise. It starts with a slight tap then progresses to a very distinct bounce. I have tried to adjust the trim of the outboard motor but can't stop the bounce. I have adjusted the height of the motor on the jack plate but no success. Any speed before or after the boat runs smooth.

    1. Is there a placement of weight in the boat to stop the resonanting- bounce of the boat?

    2. Is there any design equation to help figure this out?
     
  2. stonebreaker
    Joined: May 2006
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    stonebreaker Senior Member

    If it's truly a resonance, changing the length of the boat will help. Trim tabs, perhaps?
     
  3. Gilbert
    Joined: Aug 2004
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    Gilbert Senior Member

    Very puzzling.
    I am going to guess that it is only happening in smooth water. ?? I don't think it would happen in choppy water. I am sure you will correct me if that is not right.
    I am further going to guess that there is something about the shape of the boat and/or the weight distribution combined with the amount of power or thrust at 3500 rpm that makes the boat want to break free of the 'sticky' smooth water, but it just can't quite do it at that rpm and so it settles back down and then the oscillations begin. And amplify. And my theory just might be all wet.
     
  4. mbous
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    mbous New Member

    Reply to Gilbert

    You are correct on your observations. So, what is the solution?
     
  5. timshwak
    Joined: Apr 2007
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    timshwak Junior Member

    What happens if you exceed that rpm/speed combination?
     
  6. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    Go to the top bar, click on search and enter "porpoising". There are a bunch of threads, there must be an answer.
     
  7. mbous
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    mbous New Member

    Reply to Timshwak

    The boat performs great at speed/rpm before and after 3500rpm. The 3500rpm is the problem.
     
  8. timshwak
    Joined: Apr 2007
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    timshwak Junior Member

    I worked on some larger cat's that were jet drives and they had some bad RPM zones that you just had to stay away from. It was kind of a pain at first, but after a while you just got used to it. It was a cavitation problem in the jets that made a lot of racket and could have caused problems if you operated in that zone.

    Trim tabs could be the answer.
     

  9. Gilbert
    Joined: Aug 2004
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    Gilbert Senior Member

    For most planing boats the boat should balance at a point about 1.25 times the waterline beam ahead of the transom. If it is much different than that weight distribution might be the problem. The only other thing I can suggest is to not run at 3500. But I suppose that is the speed you would usually like to go. It can be a perverse world, can't it?
     
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