center of lateral resistance

Discussion in 'Sailboats' started by Guest, Jul 23, 2003.

  1. Skippy
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    Skippy Senior Member

    The reason for the lead is that the bow wave builds up forward when you're underway, so the CLR is forward of where you would expect it to be based on a level waterline. Also, a monohull especially will develop a weather helm underway as it heels.
     
  2. SuperPiper
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    SuperPiper Men With Little Boats . .

    Mainsail Conversion

    I sail a miniature cruiser - Sandpiper 565 - 18.5ft LOA. When it was time to replace the mainsail, I ordered a used Hobie 16 sail from Atlantic Sail Traders. The old sail dimensions were 19.0 luff x 8.0 foot. The new sail was 24.5 luff x 8.5 foot with full battens and a broader roach. To accommodate the taller sail, the mast was extended by 4.0 ft and the gooseneck was lowered to the mast step (Open 60 style).

    I was worried about this new set up being unbalanced and creating severe weather helm. To my surprise, the rig was not unbalanced and there may even have been a little less weather helm!

    How do you explain that?
     
  3. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Steve - nothing is wrong :)
    The CE must be behind the CLR in 3D, but this translates into a situation that makes it AHEAD in 2D, with all sails on CL (not a realistic condition in itself!)
    This appears to be one of those "Trust me..." situations. Between Eric and myself, we have been designing for over 50 years, with well-balanced boats. All you have to do is pick up a book of designs and check. It works.
    Steve
     
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    Steveh,

    Read the books. If you put the aerodynamic center of the sailplan aft of the hydrodynamic center of the hull, you would have wicked bad weather helm. Also, calculating the true aero- and hydrodynamic centers is nearly impossible without the resources of a wind tunnel and a towing tank, which are used only for the richest design projects. Even then, these centers change so momentarily that it would be difficult to determine the best overall placement of the rig in relation to the hull in any given situation based solely on the aero-hydro measurements. That is, the result would be no better than if we merely rely on the much easier method of comparing the geometric centers of area for the sail plan and the underwater hull profile.

    Over time, designers have determined that the CE must always be forward of the CLP. Neutral helm will occur (when the aerodynamic and hydrodynamic forces are in perfect balance as the boat is heeled over) when the CE is a distance of some percentage of the length of the waterline of the CE forward of the CLP. Lee helm will occur if the CE moves farther forward, and weather helm will occur if the CE moves aft from the neutral position. But CE will still be forward of the CLP even with some weather helm. Therefore, if you moved the CE aft of CLP in the profile view, the boat would have way too much weather helm.

    You can test this in the extreme, particularly with a yawl or ketch. Try sailing the boat with just the jib up--you will likely not be able to tack, the bow will want to swing to leeward, and you will be pushing the tiller all the way to leeward to keep her on course. The opposite applies if you sail with only the mizzen up--you will likely keep rounding up into the wind, and will have to haul the tiller all the way to windward to keep her from going into irons, head to wind.

    Eric
     
  5. chrisg
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    chrisg New Member

    Wow, this thread has been a great tutorial!

    Also, from a beginners perspective, it's great to hear that a few
    basic rules of thumb go a long way in producing a design.
     
  6. Skippy
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    Skippy Senior Member

    I don't understand the 3D/2D differentiation. The hull shape and hydrodynamics may place the CE forward of the CLP, but I've never heard of two kinds of CE.
    [Edit: Sorry, I was thinking CLR]

    And I think you can ignore momentary fluctuations in favor of an average value. It seems to me you should be able to come up with some kind of effective waterline as a function of boatspeed that takes the bow wave into account. Obviously the waterline isn't flat when you're underway.
     
  7. Skippy
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    Skippy Senior Member

    SailDesign: The CE must be behind the CLR in 3D, but this translates into a situation that makes it AHEAD in 2D
    Skippy: I don't understand the 3D/2D differentiation.


    Okay, I think I'm starting to comprehend. When you sheet out, the rig's CE moves forward as you look directly athwartship, so it appears to lead the CLP. But the rig's line of action crosses the centerline farther aft, so there's still a weather helm.

    But I still don't think that's quite it. The lead is measured with the sails flat along the centerline; it's not determined by the sheeted-out position of the CE. The real culprit is much sneakier. It lies in the fact that almost all rigs rotate about their leading edge, or at least some point forward of the CE. That makes the weather helm increase as you sheet out. So even if there's a lead, i.e. lee helm, with the sails along the centerline, there can still be a weather helm once you're underway.
     
  8. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Skippy, my boy - you've got it in one!
    Congratulations - you may now proceed to the next level (if you can gind one...)
    These things are difficult to explain in words, sometimes, but can eventually be hashed out.
    Steve
     
  9. terabika
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    terabika Junior Member

    It was nice to see an NA admit that they guess at the relationship between Center of effort and center of lateral resistance! Kudos!! I have known that the terms were missused and that the numbers they used were just close guesses as to what was really happening in the physics!! Some NA's talk as though all this stuff is culculable to the tenth decimal point and that this precision is needed or you shall surely sink and die of the clap....
     
  10. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Did you mention that the driving force from the sails is not in the longitudinal center of the boat? Look at the boat from behind, the sails are pushing forward far to the lee side while the keel is on the other side. That makes the boat turn to windward. The more you heel, the more you have to compensate with the rudder.
     
  11. Cliff Pope
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    Cliff Pope Junior Member

    My understanding of this topic has always been that there must be a certain amount of "lead" of the CE forward of the CLR. That gives a very small degree of LEE helm. But as soon as the boat begins to move, and heel slightly, the CE immediately shifts slightly in another plane, and moves to leeward. This has a pivotting effect which tends to turn the boat back towards the wind. As it does so, it will heal slightly less, so the effect will be self-correcting.

    That I think is what makes a boat sail in a stable way, and not either turn up or pay away uncontrollably. In the best balanced boats, it means they will self-steer with a lashed tiller.
    Usually the whole process is adjusted to give, overall, a slight weather helm for the reasons someone outlined.
     
  12. chandler
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    chandler Senior Member

    Has everyone in this forum just graduated with a m. e. and a 6 gig laptop?
     
  13. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    You can guess, and if you know what angle of heel you will sail with, then you can estimated the lead quite accurately :)
     
  14. Cliff Pope
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    Cliff Pope Junior Member

    You can actually watch the shifting effect by playing with a model sailing yacht on a pond. Balance the hull so that you have the CE an inch or so ahead of the CLR. Then when you press the sail at that point very gently, the bow pays off from the "wind". But as you press harder, and the boat heels, the leverage effect of the now-outboard CE comes into play, and the boat starts to come up more towards the wind again. You can feel it turning through your hand pressing the sail.
    Then try with a real wind. Most well-balanced models will sail a straight course, but because they are so light they heel, and turn, very quickly with every puff of wind, so they usually weave a somewhat erratic course, but broadly in the right direction.
    I am remembering this from years ago as a boy, but I have ever since been very ready to accept that, as others have asserted, a CE leading the CLR will actually give weather helm because of the shifting 3-D effect.
    Playing around with sails and ballast in my real boat has confirmed this.
     

  15. percyff
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    percyff percyff

    It is really only the wind from the beam up to close-hauled that balance really matters.

    If you take a full bodied hull - large merchant or warship sail from the last few centuries for example - the hull is a vertical stubby foil, with a trim-tab (rudder).
    When a foil has an angle of attack, the point at which it generates torqueless lift (the point at which the forces are considered to act) is about 25% of the length back from the bow. If you did the calculation of the CLR by area you would get a figure more like 50%. The fact that the hull is short and stubby means that the best you could get is about 35-40%, but that is still a movement. It explains why the sail plan of a multimasted and multisailed tall ship is so offset forward. There is nothing aft of the stern, but several large headsails on a bowsprit which may be half the length of the hull again. Balance is all important.

    You may also realise that if you get the balance so that the hull is 'flying' upwind, you can cancel out leeway. Our ancestors knew what they were doing!
     
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