Side Slipping & Multihull Capsize Force

Discussion in 'Stability' started by Iridian, Oct 25, 2021.

  1. Iridian
    Joined: Jan 2020
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    Iridian Junior Member

    In reading John Shuttleworth's piece on Multihull considerations for Seaworthiness:
    Considerations for Seaworthiness https://shuttleworthdesign.com/NESTalk.html

    And Chris White's page on the Capsize of Catamaran Anna:
    How much wind was needed to capsize Anna? https://www.chriswhitedesigns.com/25-news/114-how-much-wind-was-needed-to-capsize-anna

    Several forum posts were discussing how having only the windward board down in heavy winds could potentially save a boat from flipping, as the boat would slip/surf sideways, alleviating the force on the rig.

    Considering there would still be significant resistance provided by the hulls alone, how much of a difference would this make?

    Is it as simple as the reduction in wind speed (apparent wind) by surfing? That'd likely be in the 1-2 kts range, and probably not that significant, or is there something else I am missing?
     
    Last edited: Oct 27, 2021
  2. fallguy
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    fallguy Senior Member

    First, I am not a sailor. Second, I can see how some physics could play here.

    If you consider the boat like a seesaw and the wind the applied force, then keeping the board away from the wind down does seem to have the potential for the board becoming a lever point. The wind gets under the lee hull and the lever point is far enough away that less force is required to pick up the lee side of the seesaw than the force that would be required for tipping the boat over from the lee board.

    So, from my perspective and simple physics, it is less about anything magical from the lee board and more about the dangers of the board away which would be down in the case of both boards.

    And it is probably something one can calculate and get into far more serious conversations with righting moments. But in the simplest case of the seesaw. Consider the boat to be a seesaw and the chosen dagger to be the fulcrum. Let's say the boat is 5 meters wide and each hull is one meter wide. The fulcrum for the away board to the other side of the hull is 4.5M and the fulcrum for the lee board is 0.5M. Applying the same force to each side would require 9 times the force to lift the boat against the lee fulcrum point.

    I apologize as I realize my answer is amateurish, but this response should kick off some better.
     
  3. Robert Biegler
    Joined: Jun 2017
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    Robert Biegler Junior Member

    Perhaps it has less to do with force on the rig, but with wave-induced rotation:
    1) Breaking crest hits windward hull. If any board is down, the boat can't go sideways much, so the crest can only push the windward hull up. That both increases heel angle and gives the boat rotational momentum.
    2) Crest hits leeward hull. If the leeward board is down, the boat still can't go sideways, and the lever arm between the hull that is now being pushed and the board further down adds more rotational momentum, perhaps just enough to tip the boat over. If instead no board is down, or if the windward board is ineffective, either because it is out of the water or because it is in bubbly water, then the crest can smack the leeward hull further to lee, counteracting the rotational momentum the boat was given a moment earlier.
    This is speculation only, not backed up by anything.
    If I correctly remember how torque works, this argument doesn't work. What matters are where the force vectors go, not their origins. As long as the two boards of a catamaran are vertical to the waterline, each board's force vector passes through the centre of effort of the other (when viewed along the longitudinal axis). That gives them identical effects on heeling moment, for as long as both are in the water and experience the same flow conditions. In the scenario I outlined, that is no longer true, which is why lateral placement can have an effect. But it has nothing to do with lever arm.
     
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  4. fallguy
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    fallguy Senior Member

    I figured we'd get better answers!
     
  5. Iridian
    Joined: Jan 2020
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    Iridian Junior Member

    Thanks for the comments guys. I can definitely see that wave rotation induced capsize and lee vs windward board making a difference..

    With @fallguy 's lever example though, that might still have something to it but maybe slightly adjusted..

    If the fulcrum(daggerboard) was set on the inside of the hull and took a significant amount of the heeling force vs on the outside of the hull, it could actually make a bit of a difference... Probably not huge but still significant.

    If the daggerboard was angled diagonally toward the center of the boat, that'd move the fulcrum closer to the center as well. Whether or not the reduction in resistance by having that angle would make up for the movement in the fulcrum position, I am not sure.

    This makes me thinks curved lifting daggerboards might also move the fulcrum inward and reduce stability, by moving the lift more toward the center.
     
  6. tlouth7
    Joined: Jun 2013
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    tlouth7 Senior Member

    An angled (inward) or curved board has a line of force that points upwards. This reduces the overturning moment generated. At the extreme this line of force points directly at the centre of effort of the rig and there is no propensity to capsize due to wind loads (such boards have a specific name which escapes me at the moment).

    If the boards are vertical and well submerged then it makes no difference where they are positioned in the transverse direction; the resulting overturning moment is the same. To be honest the seesaw analogy is not great for visualising this.

    In terms of the idea of only having the windward board down I do see the appeal. As the boat heels the lateral resistance will decrease, so leeway must go up to reach a new equilibrium. This increased leeway will reduce the angle of attack on the sails, so the force on the rig ought to be reduced, thus reducing the overturning moment. Whether this effect is significant is hard to say.
     
  7. Iridian
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    Iridian Junior Member

    If you could happen to remember or have an article discussing this I'd love to see it. Been looking around for a while but I haven't found anything on foils cancelling the overturning moment.
     

  8. tlouth7
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    tlouth7 Senior Member

    Aha! The concept is called a Bruce foil.

    https://en.wikipedia.org/wiki/Bruce_foil

    To quote from the linked article "a foil on the windward side presents an unstable situation, since any lifting of the foil out of the water, by mechanisms such as heeling or wave action, will result in reduced downwards force, potentially leading to capsize. A lee foil provides more stability, as loss of lift will force the foil back into the water."
     
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