Ideal CE on Different Points of Sail

Discussion in 'Hydrodynamics and Aerodynamics' started by BMP, Aug 4, 2022 at 2:35 PM.

  1. BMP
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    BMP Junior Member

    If you had a rig with an adjustable CE, how would you change the lead on different points of sail?
    i.e. More lead for running less on a beam reach.
    Is there much advantage to being able to adjust the CE?
    Or is lead more of fix it and forget it?
    Thanks
     
  2. jehardiman
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    jehardiman Senior Member

    Change headsails and reef the fore/main/mizzen.
     
  3. BMP
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    BMP Junior Member

    Thanks,

    I was more interested in what the ideal lead would be on different points of sail, like 2% close hauled, 1% close reach, 0 beam reach, 20% running,etc.
    I know how to do it by feel when I'm sailing but was playing with the geometry of a T-mod crab claw rig with a canting mast which would allow you to move your CE anywhere you want it on any point of sail. I was wondering if it is better to have the CE stay at a fixed distance from the CLR or if it is better to have it move as the sails are moved from vertical to horizontal, or if it needs to be independently adjustable.
     
  4. jehardiman
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    jehardiman Senior Member

    Lead is a constructed concept that only matters when comparing like rigs and hulls. The physics of the matter is that it is best if the aerodynamic body vector of the "sail" passes directly over the hydrodynamic body vector of the hull with the rudder centered. But because both of these body vectors change with heel and speed, you are chasing a moving target. It would be best if the T-mast was located directly over the CG, but even this is subject to change as the rotation of the T-mast changes the location of the CG. You could probably get very close with an fully integrated control system for rudder, T-mast, and "sail" using nulling PID widgets for rudder and T-mast position and troque.
     
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  5. BMP
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    BMP Junior Member

  6. gonzo
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    gonzo Senior Member

    Actually, a bit of weather helm is beneficial when beating. The rudder generates more lift when it is slightly to leeward.
     
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  7. jehardiman
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    jehardiman Senior Member

    True, but that is more a function of the moment caused by heel on the hull, some vessels go to weather with the rudder centered like they were on rails. Suffice it to say, that the maximum performance is extracted when sail driving force (i.e. aerodynamic body vector) and the hull drag (i.e. hydrodynamic body vector) combine to produce minimum moment on the center of mass. All the strings in todays modern sailing vessel, each and every one of them has some effect on the location and direction of the driving force.
     
  8. gonzo
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    gonzo Senior Member

    From racing, we found that all winning boats have enough weather helm that let you "feel" it.
     
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  9. jehardiman
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    jehardiman Senior Member

    Again true, but whether that "feel" comes at 1 degree rudder or 5 degrees of rudder is a function of hull form. But it does point out the whole reason and principles behind PID controllers.
    From the Wiki page on PID controllers
    Edit to add: It is interesting to note that modern AC helmsmen are given "quickening" cues for steering while foiling, as the rules prevent direct system control.
     
  10. gonzo
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    gonzo Senior Member

    It is a function of hull form, and heeling which changes the underwater shape of the hull. I remember controls courses and the PID. Dampening also affects steering. However, returning to the OP's question, I think your first answer is correct. Changing sails and reefing is the tried and true way to change the CE. Also, he needs to take into account that as the boat heels the CE moves outboard. Probably the rigs with the easiest and fastest changing CE are windsurfers and kitesurfers.
     
  11. Skyak
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    Skyak Senior Member

    The 3D force diagram of a keel boat can actually be solved and even 'real time' is well within the capabilities of today's computers. The sails swinging off center, the shift in aero moment, and heel are all significant factors, as are waves at the surface and below.

    The most obvious advantage of being able to adjust CE would be to keep the keel producing the optimal share of lift -presuming it is more efficient by having lower induced drag.

    The rudder is always a balance. If there is no weather helm, the rudder is not resisting leeway for it's drag. In lee helm the rudder lift is adding force to lee, contributing to drag. The optimum is the rudder contributing lift but not so much that induced drag is high. As Gonzo notes, that is done by feel.

    Adjusting CE is done all the time -primarily by heeling the boat to it's "groove". Lifting daggerboards, shifting sail trim...it's a constant adjustment.
     
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  12. gonzo
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    gonzo Senior Member

    I agree. If you are racing, there is constant adjustments being made. For cruising or daysailing, it is possible to be more relaxed.
     
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