How to know if a design's righting moment is enough?

Discussion in 'Stability' started by PhilippeCE, Apr 16, 2022.

  1. PhilippeCE
    Joined: Apr 2022
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    Location: Montreal, Quebec, Canada

    PhilippeCE Junior Member

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  2. AlanX
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    AlanX Senior Member

    I did that, It capsized the first outing, had to add ballast:
    76.jpg

    Worth the effort to realise you have too much sail area.

    It righted easily (builtin water tight bulkheads) but very sore after bailing out a tonne or so of water.

    AlanX
     
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  3. PhilippeCE
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    PhilippeCE Junior Member

  4. AlanX
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    AlanX Senior Member

  5. PhilippeCE
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    PhilippeCE Junior Member

    @AlanX maybe it would have been more stable if you would have kept the width all the way aft. but that would have changed the style of it.
     
  6. AlanX
    Joined: Mar 2022
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    AlanX Senior Member

    @PhilippeCE,

    The far aft section adds little to the righting moment as if has (relatively) little displacement.
    Having said that I would not do a double ended boat again (except for a small cat pontoon or canoe).
    But I am still a fan of the flatiron/sharpie hull shape:
    18' sharpie 6-7-2013 2-45-14 PM.jpg

    Basically, you set boat's balance by setting the bow/stem just in the water.
    As the boat is loaded, the aft/stern just sits lower.
    When heeled, the hull presents a "V" to the water flow.

    Here is a pontoon/catamaran version:
    FlatIronpontoon.png

    Regards AlanX
     
  7. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    100% accurate, easy, cheap, instant and reliable results.
    A formula, hah. Hands on engineering is the way to go.
    And what a learning experience!
    Way to go AlanX.
     
  8. zstine
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    zstine Senior Member

    This is covered in the "Principles of Yacht Design" (great book) chapter on sail and rig design. Righting moment is a function of heel angle and yes, you need a righting moment equal to the overturning moment the sails generate. You need to calculate the angle at which the boat has that righting moment and determine for yourself if it is an acceptable angle. In other words, you need to know the boat's righting moment at 'max' heel angle (the angle you want to sail to weather at ~15deg ish(?)). Then you can calculate the wind speed your sail plan would create an equal moment (this would be the wind speed above which you take a reef or otherwise reduce force like flatten the sail/bend the mast, etc). If this is an acceptable wind speed (~14-19knts) then you are good to go. If not, you need to change sail plan or righting moment/ballast. (Sail lateral force) x (distance sailCE to keelCE) = (Righting moment at heel angleX). Calculations of the sail's lateral force can be involved as this is not the overall force, but the vector in the athwartship direction considering mast interference, induced and viscous drag, interaction with a head sail or mizzen sail, etc.. Though estimates can be fairly straight forward as google will tell you. and software such as FreeShip can provide righting moments at different heel angles, so you don't have to do all the hand calcs, which would be very cumbersome to do 100% by hand.

    One comment the PhilippeCE said was that "as the boat heels, the force of the sails decrease". This is not true. Heel impacts the direction/location of the sail's forces, but not the amount of force. It is the change in location of the sail's force (and keel) that creates more weather helm as heel angle increases.
     
  9. gonzo
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    gonzo Senior Member

    It is correct that as the boat heels the force of the sails decrease. The projected vertical surface area of the sail diminishes as the boat heels. The force applied to the sail is a function of P (pressure) X surface area
     
  10. TANSL
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    TANSL Senior Member

    Thinking logically it is difficult to imagine how the surface of the sail can vary when the boat heels. The sail is an object that remains intact unless the helmsman orders to reef the sail (perhaps that is not the correct word but I hope you understand me). The component of the force on the sail that pushes the ship can vary with changing heel, but the surface of the sail does not change, except that at lower altitudes the wind speed can be lower.
     
  11. gonzo
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    gonzo Senior Member

    Please read before commenting. Projected vertical surface area is reduced when a boat heels.
     

  12. TANSL
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    TANSL Senior Member

    In that I totally agree, I was wrong and I apologize

    I have read, following your wise advice, and in this I do not agree. The total force on the sail, if the total area does not change and the direction of the wind or its speed does not change, cannot vary. The components of that force on the sail will change but not the "force of the sail" (if I'm not wrong again).
     
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