Centre of Lateral Resistance, Centre of Effort and lead

Discussion in 'Sailboats' started by cookie munster, Jun 26, 2017.

  1. DCockey
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    DCockey Senior Member

    C. A. (Tony) Marchaj did extensive experimental reseach into the aerodynamics and hydrodynamics of sailboats, and wrote several books including Aero-Hydrodynamics of Sailing and Sailing Theory and Practice.

    The CLR-CLE-Lead method of determining balance does not properly represent the physics. It can be a useful empirical method for deciding where to put a sail rig. However it should not be used to try to deduce the physics.
  2. messabout
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    messabout Senior Member

    Not to forget that the usual drawings show a profile of the boat with locations of CLR and CE indicated or at least estimated. The deal is that the sail while underway is never at the fore and aft position that the drawing shows.. If the main sail is eased then the CE will necessarily move forward....simple geometry....If it is eased some more, say on a close reach, the CE will move farther forward and will be to leeward of the boat. That will cause a turning moment because of the "leverage", usually tending to to turn the boat upwind. There must be some point of trim where the forward movement of the CE is overwhelmed by the turning moment of the eased sail.

    To be sure the CLR of the under body may or may not change positions when the boat is heeled. There are also some other treacheries at work. The shape of the waterlines may become non symetrical which could influence the tendency to turn one way or the other. That is not all. The position of the section centroids, if not in a straight line, could and probably would influence the behavior of the boat. The velocity of the b oat plays into the generated turning forces too. All that jazz is what makes it fun to contemplate and probably fool ourselves into believing that we know what we are doing.
    TANSL likes this.
  3. PAR
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    PAR Yacht Designer/Builder

    Marchaj's book is in my library and one of the first I absorbed, years ago. The physics and geometry are known, tests have been performed, but there's still so many variables to account for, that empirical and observational experience often is the true "tell", plus a lot cheaper to perform. Unless working on something a little (or a lot) odd, maybe a cutting edge racer, you'll be fine with the accepted lead ranges, after accounting for most of the usual suspects that can drag this percentage one way or the other.
  4. cookie munster
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    cookie munster Junior Member

    I just found a very interesting section in J.W. Slooff's book The Aero- and Hydromechanics of Keel Yachts.
    Paragraph 6.7.3: Effects of Heel on Hydrodynamic Yawing Moment, Static Directional Stability and Yaw Balance
    It seems to answer some of my questions. I haven't studied it in enough detail yet, probably need to go over it a couple more times and try to follow the (fairly basic) math on a piece of paper :)
    Was thinking about reading Marchaj too, but thought that it might be a bit outdated. Please correct me if I am wrong there.
    Last edited: Jul 3, 2017
  5. sawmaster
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    sawmaster Senior Member

    The explanation that made the most sense me was that once the boat heels, the center of lateral resistance of the hull moves forward due to the newly immersed portion of the hull---If there were no lead, this would result in an undesirable amount of weather helm ( the center of effort of the sail now being further aft of the CLR). If you sail a dinghy with the daggerboard raised just enough to produce a little lee helm you can see evidence of this-- when hit by a gust or if the boat is heeled to leeward intentionally--the lee helm disappears.
  6. sharpii2
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    sharpii2 Senior Member

    I personally favor the bow wave theory.

    As a displacement boat heels, its bow wave becomes asymmetrical, meaning it is higher on the leeward side than on the windward one. It is also pushed further to the side, as the waterlines on the lee side are wider than those on the windward one.

    This applies far less to planing boats because those push the water down rather than aside.

    Boats with pinched bows and wide sterns should exaggerate this effect, because heeling such boasts tends to push the bow deeper, further exaggerating the bow wave difference. But, if this theory is correct, the effect should be negated if the beginning of the maximum Beam is far enough aft the Effective Center of Lateral Resistance (ECLR). In this case, the boat should turn in the opposite direction when heeled.

    (See attachment) This model is laughably extreme, of course.
    The Length and depth of the keel or 'board has a lot to do with this too.

    If the Keel is long and shallow, its ECLR should be about 0.375 its averaged length aft its leading edge.

    If the Keel is short and very deep, another curious thing happens.

    As the boat heels, its ECLR shifts to windward as Center of lift, which is probably very close to its center of drag, is moved to windward. Because it is producing a lot of lift, it is almost certainly producing a proportionate amount of drag, which is likely to be greater than that of the Hull itself.

    This too is going to cause the boat to want to turn to windward.

    A straight sided scow can be very instructive on this matter. Its straight sides produce no lateral bow wave. So, when heeled, it tends to want to turn down wind, just as a planing boat would tend to turn towards its low side.

    Now add a Lee Board, and this effect is further enhanced. I know from experience. The ECLR is on the lee side and it stays there. As with the deep Keel case, lift creates drag. And now this drag is on the leeward side.
  7. cookie munster
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    cookie munster Junior Member

    Ah, yes, the bow wave theory. That makes much sense to me. As for the ECLR, I am not so sure about that having much influence on yaw, as the momentum arm is rather small.
    Slooff writes in his book that the main reason for a heeling wider bodied yacht to turn to windward is the fact that the leeway side of the hull digs in deeper. To understand the mechanism behind this I need to do al lot more reading on a topic I am not familiar with. :confused:
  8. PAR
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    PAR Yacht Designer/Builder

    I'm a firm follower of the leeward bow wave effect, mostly from empirical observation, though thery also comes to play. Small boat sailing has taught me it's importance, as you can literally feel the difference from one set of shapes to the next, if you've had the opportunity to jump from one design to the other. The number of items to evaluate and areas to understand, when making these decisions can be daunting to say the least, which is why rules of thumb have been developed. In reality, if standing well out on the design limb in this regard, anticipation of a considerable mast move or rake and/or an easily adjustable step is the usual and logical course.

    This seems to fly in the face of computer generated design, but in fact it's the cheaper way to go. Let's say you want to build a new America's Cup contestant and have done the best you can on everything, shapes, foils, etc. The software for developing this, just doesn't exist. The reason is the huge number of variables that can't be fully accounted for, so you start making some educated guesses. Yeah, that's right it's cheaper to make these guesses on 10's of million dollar yachts, then to develop software that can cope with the vast number of variables that exist in accurate modeling. So, you build the yacht to the computer generated lines and spec's and go sailing. Of course, the variables thing jumps right up and bites you right in the butt, because the software can't account for it all and you start to make adjustments and recalculations, based on testing, equipment, performance and sensor monitoring, etc. Now, you're back at the machine tweaking the shapes, lines, foils, etc. (again) based on the new parameters, and you incorporate these into the next America's Cup boat, possibly the one you actually use, then again, maybe you'll need a third, before you're satisfied with the yacht's potential.

    So yes, in the end, it's actually cheaper to build a few yachts as development evolves, then to develop software or physics understanding that can contend with the huge number of variables, any modeling routine might need to be truly accurate. Ain't reality cool . . .
  9. Principles

    Principles Previous Member

    Lifting Body

    Angle of Attack (AoA) = heeled sailboat Bow, half entry angle

    Lift = CL3d x Dynamic Pressure x Area
    CL3d = Cl2d / (1 + (2 / AR))
    Cl2d = 0.105 x AoA
    AR = Aspect Ratio

    In the declassified NASA's paper the measured Lift was very similar to the calculated with the theory (Ludwig Prandtl, 1931)

    But in a sailboat may be the hull is a slender body

    CL3d slender body = 0.01745 x AoA x pi x AR / 2

    And The Position of This Force @ 20 degrees Heel may be is around 30% LWL from bow

    An the Area may be a rectangle from bow until the max beam x the Draft of the canoe body

    CLR (sailboat D/L > 200) = Bow + Keel + Rudder

    (1) Bow lateral Force or bow yaw moment

    (2) Keel lateral Force and lift carry over = Keel Force x ch
    ch = 1.8 x (Tc / Tk) + 1
    Tc = Draft of the hull canoe body
    Tk= Keel Draft

    [Keuning, JA, Verwerft, B (2009): A new Method for the Prediction of the Side Force on Keel and Rudder of a Sailing Yacht based on the Results of the Delft Systematic Yacht Hull Series, 19th Chesapeake Sailing Yacht Symposium, Annapolis]

    (3) Rudder lateral Force with effective rudder angle

    effective rudder angle = rudder angle + leeway - keel downwash

    downwash = 0.5 x leeway

    CLR (traditional heavy sailboat, D/L > 300) =

    30% LWL @ 20 degrees Heel
    40% LWL @ 0


    Davidson CE (Davidson Laboratory of Stevens Institute of Technology): 1.7 x Foretriangle, 1 x Mainsail

    Tony Marchaj, Sailing Theory and Practice (1964)
    Last edited: Jul 12, 2017
  10. Principles

    Principles Previous Member

    CLR (ultralight sailboat, D/L < 100) =

    Keel lateral Force + Rudder lateral Force
  11. cookie munster
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    cookie munster Junior Member

  12. Principles

    Principles Previous Member


    Sailboat Balance 1860-2020

    1860, Wonder, Southampton fishing cutter, Itchen Ferry

    Aerodynamic CE @ 38% Lwl

    1.7 x Jib (geometric center)
    1.7 x Staysail (geometric center)
    1 x Main (0.4 x gaff Peak - main Boom, and 0.5 x chord)

    1888, Alexander Lawrence, New York pilot schooner No. 4

    Aerodynamic CE @ 38% Lwl

    1.7 x Jib (geometric center)
    1.7 x Staysail (geometric center)
    1 x foresail (0.4 x gaff Peak - Boom, and 0.5 x chord)
    0.5 x Main (0.4 x gaff Peak - main Boom, and 0.5 x chord)

    1994, YD-40, Son of IOR

    Aerodynamic CE @ 38% Lwl

    1.7 x Big Genoa (geometric center)
    1 x main (0.4 x P, 0.5 x chord)

    2020, W-20, Son of the Wind

    Aerodynamic CE @ 25% Daggerboard chord or may be 0% chord
    (Daggerboard Hydrodynamic Center Of Pressure = 0.25 x chord)

    1.7 x Jib (geometric center)
    1 x Main (0.4 P, 0.5 chord)

    1860 - 2020 Aerodynamic CE - Hydrodynamic CLR without Heel and without Rudder =

    +/- 0%

    If you like you can say +2%
  13. gonzo
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    gonzo Senior Member

    On a sailboat, it is possible to change the balance by trimming the sails. This is a dynamic adjustment that needs to be accounted for. Seems like most computer programs assume the crew is not handling the boat. For example, if the wind increases with the corresponding increase in lee helm, the CE can be move forward in the mainsail by tightening the Cunningham. Moving the jib cars aft will have a similar result.
  14. Principles

    Principles Previous Member

    Ignorantia Iuris Non Excusat

    Ignorantia juris non excusat - Wikipedia https://en.wikipedia.org/wiki/Ignorantia_juris_non_excusat

    The so called "lead" (= geometric CE - CLP [Center of Lateral Plane]) is The measured of Ignorance

    CLP = Center of Lateral Plane, geometric Center of Lateral Plane
    CLR = Center of Lateral Resistance, Hydrodynamic Center of Lateral Resistance

    If you will design a good balance sailboat you can follow two ways

    (Way 1): calculate the aerodynamic Drag angle, and calculate the hydrodynamic Drag angle, and ... and ... and ... and ... and ... and ...

    (Way 2): you can follow a Rule with paper and pencil: lead (= Aerodynamic CE - Hydrodynamic CLR without Heel and Rudder) = 0

    The OP question was the so called "lead" and the CLR

    And the short Answer is Aero-Hydrodynamic "lead" = 0

    Traditional Long Keel: Davidson CE @ 38% Lwl
    Modern light sailboat: Davidson CE @ 0% Daggerboard chord

  15. Principles

    Principles Previous Member

    Max Michael Munk

    Sailboat Balance 1860-2020, Downwind in a Gale

    Naval architects tend to draw bows to the right
    Aeronautical engineers tend to draw noses to the left

    The aircrafts are right
    The boats are wrong


    The boats are wrong, fundamentally wrong, because the boats have a original sin: the Hydrodynamic Center is ahead of the Center of Gravity

    The traditional boat is a good boat, but be careful, you can not surf a big wave with security and comfort, because the Hydrodynamic Center is ahead of the Center of Gravity, the boat wants to turn around, protets and complains (yaw, broach): she wants to be an airplane, she wants to sail like an arrow

    The tradicional boat in bad wether must stop and hove-to, and the traditional saliboat hove-to very very very well

    There is a classic solution for the traditional boat:
    if yaw ... the Hydrodynamic Center move backward, and there is a reconciliation between CLR and GC:


    The video title said "sinks", No, the boat crossed the Atlantic back alone, without the crew

    Sons of IOR, heavy, D/L > 200

    be careful with the heavy Sons of IOR downwind in very bad weather,
    be careful with the position of the keel,
    because you must fight a formidable army:

    + the Max Michael Munk Moment (MMMM)
    + the lateral force of the heeled Bow (Lifting Body)
    + the hydrodinamic center of the keel ahead of the gravity center (GC)

    Light, D/L 150 and D/L < 150

    From around D/L 150 the boat glides with pleasure

    Ultralight, D/L 100

    A very good surf board

    but, be carfeul, you can not stop, you can not hove-to with security and comfort

    The IMOCA Hugo Boss, a monster, stopped, hove-to, and overturned, capsize in F8

    "Such a vessel will be able to carry plenty of sail, will be very stiff under canvas
    The great danger of a ship which is all shoulder and has no underwater body
    is that across a sharp and breaking sea she is liable to capsize"

    (Scott Russel, On the Rolling of Ships as influenced by their Forms, 1863)
    (CA Marchaj, Seaworthiness, The Forgotten Factor, 1986-1996)

    How can we stop and hove-to with a light surfboard ?
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