Help please boat stability

Discussion in 'Stability' started by Miller90, Sep 24, 2019.

  1. Miller90
    Joined: Sep 2019
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    Location: Australia

    Miller90 Junior Member

    I have a 6.5m aluminium plate hard top boat when I turn to the right it wants to roll out to the left and feel like could capsize this is all at low speed 6kns or less any ideas as to why it would do this would be much appreciated all this is with nothing in boat but me and one other person and as far as I'm aware it is even weight on ether side
     

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  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    This only happens at low speed, do you say ? It isn't unusual in a boat like this to develop an outward lean on a sharpish low speed turn, especially if the centre of gravity is a bit high, and you do appear to have some high structure there, and you might be able to comment of the weight of that. If that is the case (high COG) I would be more concerned about stability overall, than just in a low speed turn. Does the boat have a self bailing cockpit ? I'm assuming so, that would raise the COG too. When you plane the boat, I expect it banks into turns, rather than the outward lean like a naval destroyer. If you are going into rugged offshore use, be wary of too high COG, that cabin structure does have the appearance of a block of flats, and may have been overbuilt, but you'd know more about it than I do.
     
  3. fallguy
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    fallguy Senior Member

    Based on the pictures, which are inadequate for this discussion, the vessel appears to have very sharp angles on her sides. This drives assumptions that the bottom is very narrow and will not support the weight above. A guess is all.

    Also, the fact you have a higher superstructure by nearly double the freeboard bothers me; especially again assuming it is all aluminum.

    The other question that arises quickly is if the hardtop was added after the fact; was any positive buoyancy added? I have some general doubts.

    These assumptions combined make for a very serious list of questions.

    Will she lay over and sink fast...?

    But this is all conjecture based on poor photos and your statements and my guesses.

    Take some pictures of the bottom from the stern and from the bow. That will help.

    Then do a weight calculation of the hardtop. The weight calc is important. You should be able to make good estimates of the plate amounts and windows and other major metal components. Then an average height assumption of the mass of the hardtop.

    The reality is if the hardtop is all welded aluminum; it might be heavy. If it were made of marine foam the right way; it would be both pos buoyant and lighter.

    Another thing you could do in the US is ask the Coast Guard to look at it for you. They would be able to tell you in a hurry. Not sure if the Aussie coasties would help.
     
  4. DCockey
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    DCockey Senior Member

    I was not aware that the USCG gets involved in assessing stability of private recreational boats.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    He lives in Queensland, Australia, judging by the "Q" in the boat registration number, the place is awash with plate alloy boats of the kind, I doubt it lacks beam, it does seem to have a fairly modest deadrise, as do most of them, but if you have high structure, guys seated on high pedestal seats, self-draining floors etc, the COG can get up, and also the windage issues, I'd be interested to know whether that wheelhouse is a one-off, added to an existing boat. But a tendency to lean outwards noticeably when turning sharply at displacement speeds, is a sign the COG is getting up a bit. I don't even like the idea of centre consoles with overhead hard-top rails for big bozos to hang on to, the boat goes into a broach ,and they grip tighter, and provide a wonderful capsizing moment. The wheelhouse appears out of proportion to the boat size here.
     
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  6. Miller90
    Joined: Sep 2019
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    Miller90 Junior Member

    Yeah that's what I was thinking thanks heaps guys I might cut the cab off and start again going a different style just go with small windscreen ect I brought the boat as it was with no motor and i fitted out sounder ect didn't really think about this stuff to now but you live and learn lol. The hard top has no foam in it and is all aluminium
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I wouldn't rush into anything, firstly calculate the weight of that wheelbouse, or at least that part above gunwale height. Is it glass or Perspex windows, e.g., the gauge of the metal, how much framing etc. It is always hard to tell from one still picture, but your boat does appear to be bow heavy, one wonders what that structure does weigh.
     
    Last edited: Sep 24, 2019
  8. fallguy
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    fallguy Senior Member

    mute, but point taken

    but let me ask one question; would the USCG assist a recreational vessel in distress after the fact??? seems a bit off, no?

    We will help if someone dies; otherwise good frickin luck?
     
  9. fallguy
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    fallguy Senior Member

    Do a weight calc before you cut it all up.

    I can give you some ideas about foam weights if you get the aluminum data and the height now and estimate the average.

    But all aluminum hardtop is a mistake if you can't offset the buoyancy. Boat will sink.

    Nice artwork. What fish?
     
  10. DCockey
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    DCockey Senior Member

    A quick check on static stability is to time the roll period with the boat stationary. The common recommendation is the roll period measured in seconds should be around the beam measured in meters. A considerably longer roll period would indicate the height of the center of gravity may be getting too close to the metacenter. To check the roll period have the boat float unrestrained (loose dock lines) in calm water. Push down on one side of the boat and then abruptly take the weight off. Time several oscillations and divide the time by the number of oscillations.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Never heard that rule of thumb, but certainly worth a check, unless the boat was settling after coming off plane in the picture, it does seem quite bow heavy, too, and the height of that hardtop would be around 2.5m from the bottom of the boat, so if any appreciable weight in it, I don't like it.
     
  12. Miller90
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    Miller90 Junior Member

    I would estimate the the cab would weigh around 250kg it's very over built I think here are some pic of front and back it's all made from 3mm aluminium and 3mm think square tube framework also has a bit of 6mm plate in front of Windows as top of cabin (not the hard top roof) the only foam filed bit is all under floor
     

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  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Where are you intending to use the boat ?
     
  14. Miller90
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    Miller90 Junior Member

    Offshore
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, well you don't want to be going offshore with the residual doubts, you have, and may be justified in having. What is the make of the boat ? You can sometimes get away with a bit of a higher COG with boats that are fuller in the bow, but if she is not that broad shouldered, less so. How many people are you anticipating on board ?
     
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