Transverse stability newbie

Discussion in 'Hydrodynamics and Aerodynamics' started by alesserfate, Feb 27, 2017.

  1. alesserfate
    Joined: Feb 2017
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    alesserfate Junior Member

    Hey folks,

    New here, from the PNW and somewhat veteran boater.

    In center of buoyancy and center of gravity considerations, as long as G (CoG) is below the M (rotation metacenter) and above the B (center of buoyancy), will the vessel absolutely always have positive equilibrium no matter the heel angle ?

    As in, even if it has tender roll (sluggish, slow return to upright position, close G to M and short upright lever (G to Z), will it always return to upright no matter how much you try to roll it, even if its past 90deg heel ?

    I know its rare, but also what if the G is below the M, but both G and M are above the W-L (waterline), how would this affect the stability ?

    [​IMG]

    [​IMG]

    I used this article for reference to understand what I'm talking about here: http://www.splashmaritime.com.au/Marops/data/less/Shipk/Stab/Longitudinal.htm#_1.__Transverse

    Thanks in advance! :confused:

    EDIT: Sorry I posted in wrong part of the forum, should have posted in the 'stability' section. If someone could move it there, that would be great thanks!
     
  2. alesserfate
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    alesserfate Junior Member

    Thanks, sorry that question didn't make sense if its the center of the submerged part. I edited it out.
     
  3. Joakim
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    Joakim Senior Member

    Only the position of CoG is constant, when heel angle is varied. M and CoB will change with the heeling angle.

    For most hull forms M can be assumed to be constant at small heel angles, but not at higher heel angles. So purely from M, B, G nothing can be said about the stability of the vessel at higher heel angles.

    CoB can't be above waterline, since CoB is the center of the under WL hull volume. M is typically clearly above WL and CoG can be "anywhere" from well above WL to well below CoB, but it must be well below M for the vessel to have initial stability.
     
  4. alesserfate
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    alesserfate Junior Member

    Thanks Joakim, I appreciate it.

    I built an 150lb aluminum roof approximately 9ft above the waterline on my 26ft powerboat, although it felt slightly more 'tender' and less stable, it has been 2 years and many rough seas later it never capsized. I even caught it heeling 45 deg on camera when I was slammed by a wave from the side while on the plane at 20kts and it managed to right itself.

    That said, I have recently added a 300lb battery and a water tank which when filled also weighs about 300lbs in the lower part of my boat approximately 1 foot below the water line, in an effort to increase transverse stability by moving the CofG lower (and presumably away from M). My question is, being 1ft below the waterline vs 9ft above, are my efforts to recover stability in vain ?

    The second reason for adding the heavy batteries in the forward part of my boat (about 4ft from the bow), my boat is stern heavy and I have trouble cruising on the plane without having the trim tabs on the transom full down, so I am trying to add useful weight on the bow to counter the need for the trim tabs.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    How much beam has your 26 foot planing boat got ?
     
  6. alesserfate
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    alesserfate Junior Member

    Beam 8ft. Draft about 2ft. Modified V hull.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, 150 lbs that high certainly does not help stability, but I have seen a few boats that size and beam with flybridges over the years, and they seemed to have stayed upright for the most part. That has to be worse than your situation. Would you describe your boat as full and broad in the bow area ? That helps a lot. Slender bows with a fine entry would be a concern, imo.
     
  8. Joakim
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    Joakim Senior Member

    Having not yet capsized is a poor proof of safety. Stability at planing speeds is a totally different thing and the physics discussed earlier apply only to stationary (or slow displacement speed) vessels.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I have to say I get more concerned by big bozos hanging on to overhead pipe structure on boats of this kind, running barred inlets and the like, if things get a bit hairy, their hanging on to it like grim death will go a long way toward flipping the boat. Of course what Joakim says is correct, a typical monohull planing hull has different, and sometimes better, stability characteristics on plane, than off plane.
     
  10. alesserfate
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    alesserfate Junior Member

    Alright so lets say im not planing and just using it as a displacement hull or stationary, is the weight I'm adding sufficient or do I have to add more?
     
  11. Heimfried
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    Heimfried Senior Member

    May be this could give you an idea of mass changes on a different, but may be not too different boat:

    http://www.bootsphysik.de/rechner/bootx.php

    Click button top right: "English". Buttons with "i" show information.

    The locus of CoB, CoG, M are shown. You can put in new load and set the location of the new load by typing distances to CoG along the axes of the ship fixed coordinates.
     
  12. alesserfate
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    alesserfate Junior Member

    Thanks I appreciate that.
     

  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Unless your boat is inherently dodgy, there is nothing to panic about, I'd say, there were a good number of trailable 23' Bertram flybridges sold in Australia (2.5m beam), I don't know of any horror stories and they'd certainly have been used in some demanding conditions. I don't like the idea, very different to having a 25 footer with 10 foot beam and the flybridge, but any flybridge is going to weigh a bit even with no-one up there.
     
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