Center of effort of the A class catamaran sail

Discussion in 'Multihulls' started by Paul Scott, Nov 24, 2022.

  1. Paul Scott
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    Paul Scott Senior Member

    (If this has been discussed before, please forgive and link?) Why does it work so far aft? Even on the classic low riders. The helm stands on or near the daggerboard / front beam upwind, and the CE of the sail looks significantly aft of that point.

    Thanks

    Paul
     
  2. fastwave
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    fastwave Senior Member

    Hi Paul,
    You need to think in 3D.
    Picture viewing from above. Draw the force vector on the sail and the leeward keel.
    Upright you will see that the mast needs to be a lot further back due to the lateral distance between the keel and sails
     
  3. Paul Scott
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    Paul Scott Senior Member

    I can see that downwind, but upwind? Wingmasts tend to pull CE a bit forward, but that much? Henry Bossett and I had a conversation about this few years ago, around how much did I know ( not much), that left me mystified, but that started out concerning an una IC sail I was having him build, and drifted into other realms. I was left with the impression that moving CE aft was an ongoing feast, based on trial and error. Contenders have also done essentially the same thing, only with mast rake. I suppose it could be argued that the ‘real’ CLR could be somewhere between the DB & rudder, but the CLR of hulls is a real thing too, at least in my experience. And then there are cats with jibs, and the main is in a similar longitudinal position.
     
  4. fastwave
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    fastwave Senior Member

    Just draw it.
     
  5. upchurchmr
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    upchurchmr Senior Member

    Well that was unhelpful.
    If he could have drawn it he probably wouldn't have to have asked the question.
     
  6. fastwave
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    fastwave Senior Member

    I was the only one to reply. I explained where his thoughts were wrong. If he spends 5min trying to draw the vectors then it will become clear.
    Why don’t you help before criticizing?
    I think I was more helpful than you.
     
  7. Paul Scott
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    Paul Scott Senior Member

    I did.

    Windsurfing gave me some, granted, intuitive understanding of the dynamics, and whether D1, D2, or longboards, CLR of the hull, not the appendages, seemed to dictate sail (CE) position. From your comments, I’m thinking I don’t understand force vectors of various sails, esp sails that relax leach tension under duress?
     
  8. upchurchmr
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    upchurchmr Senior Member

    The reason the CE is pushed aft has nothing to do with the lateral distance between board cl and sail cl.
    The reason is that A class hulls are very narrow. When you are reaching, the vector of the sail force is far forward.
    This depresses the bow. If you don't want to stuff the bow in the water, you need to get more bouyancy farther forward, and crew weight farther aft.
    If you move the mast aft it gives you maximum bow bouyancy, so you can drive the boat harder, something you would want in competition.
    The relative position of the sail CE and the hull/ board/ rudder CR stays the same with 2 exceptions. First is the point of sail (tacking, reaching, running) and the change in immersion of the bow.
    Sticking the bow into the water means the hull has more effect on CR at the bow.

    None of this has anything to do with the question of how does the angle of total force of the sail/ mast change.

    You don't need one drawing of the force vectors, you need at least 3. And as a part of defining the force vectors, you need to define sail trim position for each. Form that you need sail force vectors for each sail angle of attack to the wind.
     
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  9. fastwave
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    fastwave Senior Member

    The wider the hull spacing the further back the mast needs to be when flying a hull or the upwind daggerboard is raised
     

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  10. Paul Scott
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    Paul Scott Senior Member

  11. upchurchmr
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    upchurchmr Senior Member

    Fastwave,

    Nice picture, but under those conditions the boat would be falling off the wind.
    The line of action of the sails and the hull have to be in line or you are going to be turning.
    Angle of attack of the sail or foil has a great deal to do with where the line of the force is.
    With your main along the centerline you are not going to get much progress, anywhere.

    You continue to ignore that the forces changing radically with the direction you are sailing.
     
  12. Erwan
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    Erwan Senior Member

    Paul,
    As above-mentionned, it is very helpful to draw what it known as a "free body diagram" where forces & moments are balanced for a constant speed (gamma=0, no acceleration)
    You can assume that the weight/buoyancy issue is independent of Sail CE/ Centerboard+rudder blade COE
    Crew position on the gunwhale (not sure it is the correct word) is balanced by the hull buoyancy for a A-Classic & the Foils lift for a foiler (centerboard foil+ rudder T-foil of course).
    You have to consider the moment of the rig too.
    Lateral forces from the rig are balanced the same way by the centerboard + rudder or centerboard foil + rudder blade (not T-foil) whether it is a floater or a foiler.
    Just draw it & the light will come
    Cheers
     
  13. Paul Scott
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    Paul Scott Senior Member

    Arrow Catamaran foil centered between hulls 42E8978A-E6A1-45C0-A69D-405687CDD62B.jpeg
     
  14. Paul Scott
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    Paul Scott Senior Member

    Treat a narrow bow conceptually as a type of foil, at least upwind? Globally, is there a rule of thumb about how much aft placement of the centerboard moves total hull CLR?
     

  15. Erwan
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    Erwan Senior Member

    Being a little bit "Old School" I would split the question: Where is the position of the CoE? into 2 questions:
    1-Longitudinal position of the sail's CoE?
    2-Vertical position of the sail's CoE?
    1-The longitunal CoE depends on the CMO of the "wing section"
    for instance a symetric wing section woulsd have a stable CoE at 25% chord from the leading edge
    a low Reynolds /high lift wing section like the Selig 1223 with aft loading would have its longitinal CoE at 42% of the chord from the leading edge
    In conclusion the CoE is likely to vary between 25% and 42% of the chord.

    2-For the vertical CoE we would assume that the helmsman is not a rookie and has the ability to tune the sail close to its optimum.
    Then the Vertical CoE would be at 42% from the trampoline in low wind* when an elliptical lift distribution is the lowest induced drag solution.
    *(which is around 7m/seconds Apparent Wind Speed for an A-Cat).

    And above this AWS, to achieve the lowest induced drag, the lift distribution will be tuned according to a linear downwash, until "Bell-Shaped Distribution" is achieved where the CoE is at 34% above the trampoline.

    In other words if correctly tuned the CoE of an A-Cat sail is likely to be in the:
    25% - 42% bracket longitudinaly
    42% - 34% bracket Verticaly
    According to the AWS

    Tom Speer (RIP) has mentionned T.Jones' workpaper many time. It addresses the minimum drag under moment constraint, and as long as the crew righting moment is limited. (it is the case for a boat).

    Cheers & Fair Winds
    EK
     
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