Adding Buoyancy Aft

Discussion in 'Powerboats' started by Mat-C, May 8, 2013.

  1. Mat-C
    Joined: May 2007
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    Mat-C Senior Member

    I have a mate who has a sterndrive powered boat that tends to sit stern down when at rest, to such an extent that waves can slosh over the aft platform and into the boat.
    It also tends to raise its bow quite high when getting onto the plane.

    I was wondering about the idea of extending the bottom either side of the sterndrive under the platform to provide additional lift and buoyancy.
    I've seen a number of boats set up like this, but was wondering if there are any pitfalls he should be aware of....like would it effect the handling of the boat in a detrimental way...etc?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, I'd guess there are two issues here, the behaviour at rest, and the other when underway. Assuming there is no water or waterlogged foam creating an imbalance, you have to wonder whether it is a design fault, or an engine fitted that is beyond the weight that can be accomdated comfortably. The nose-in-the-air stuff can be addressed with trim tabs, with a good chance of success. I would not add any pods each side that would constitute a continuation of the bottom aft, as such, as that may well affect the handling, especially running down-sea. I'd be looking at some kind of pod both sides of the drive leg, that is clear of the water at planing speeds, and if you can squeeze trim tabs in there as well, could be the solution. Can you tell us what make/model it is ?
     
  3. daiquiri
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    daiquiri Engineering and Design

    The real question is - can the situation be any worse than what he has now? :p

    Any detrimental effects of buoyancy transom pods on boat handling, if there are any, are imo less important than the significant safety issue the boat is having right now.

    Cheers
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Might just be a boat with insufficient freeboard for the waters it is being used in, he needs to indicate what brand of boat it is I think.
     
  5. Mat-C
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    Mat-C Senior Member

    Sorry...I'm not sure what sort of boat it is. About 22ft long, aluminium.
    It has trim tabs - those QL interceptor things from Volvo. I reckoned that 'normal' trim tabs would work better at the lower speeds, but that's just a guess. The tabs do improve things, but they wouldn't improve anything whilst the boat is at rest.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    I have visions of this being an old De Havilland Trojan with a diesel sterndrive for some reason !
     
  7. tunnels

    tunnels Previous Member

    ok I guess I should first ask about the boat !!
    what size is it ??
    pictures ??
    what sized motors has it got now ??
    has it always had this problem ??
    if not then what's been changed??
    has any one suggested trim tabs for the back so when it starts to move they will lift the back up and push the nose down !
    There are automatic self levelling trim tabs these days , just set and it thinks all by them selves !!
    When the boat isn't moving then its a weight distribution problem that needs to be addressed !!
    :confused:
    Adding buoyancy aft is not addressing the problem !! its shifting weight from one place to another that need to be looked at !!! Oh yes it may look cool to bounce off a wave and have the back go splash first with the nose pointed in the air but that is dangerous !! and one day you will find out just how dangerous !!
     
  8. PAR
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    PAR Yacht Designer/Builder

    Bingo . . .
     
  9. tunnels

    tunnels Previous Member

    Its really funny guys come with a problem and when you ask a bunch of questions and need the information so you can give a decent sensible answer they disappear never to be heard of again !!. :confused:
     
  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Mat-C is a fairly regular contributor, lets give him the benefit of the doubt and hope he's busy, possibly taking some measurements or researching the boat.
     
  11. Easy Rider
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    Easy Rider Senior Member

    Sounds like an extension of the entire stern aft would be a safe alternative.

    But then what if it was warped all the way aft like a Mainship 34?

    As to his departure .. You basically told him his idea wasn't even as good as putting afterburners on a broomstick.

    And perhaps his fuel tanks and battery is all the way aft. I have a Winner 19 that was designed that way but it's an OB.
     

  12. tunnels

    tunnels Previous Member

    forget bouyancy stick with what you got and shift a few things !!

    its all a matter of balance !! why stack everything in the back for heavens sake . fuel can be further forward ,even up front . batteries also can be shifted ,if its got sterndrives the motors can be shifted forward !! theres a lot of options ,non of them will be cheap then what the hell !! boats are just a hole in the water we keep pouring money into so what ,the bigger the boat the bigger the hole !! :p
     
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