Mast & Keel Positioning / Brasil student

Discussion in 'Sailboats' started by Jeandupont, Apr 9, 2006.

  1. Jeandupont
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    Jeandupont Designer/Boatbuilder

    Hi,,,
    which is the ideal position of the mast and keel in a sailing ship?
    I make ability of industrial design,
    I am developing a sailboat of high performance for the end of the course.
    He is 5 meters in length and 2.15 Beam...
    Any help will be thankful.
    Thanks,
    Jean Dupont - São Paulo - Brasil
     
  2. frosh
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    frosh Senior Member

    What type of sailboat?

    Hi, Is it a ballasted keel boat or lightweight centreboarder capable of planing?
    It is best to give more information than less. You should design the rig first and calculate the Centre of Effort when sailing upwind. Then you design keel/centreboard and rudder and calculate the Centre of Lateral Resistance. As a general rule if you position the CLR just in front of the CE the boat will probably sail upwind with slight weather helm which is the ideal. This is over-simplified but the best I can do with the information you have given.
     
  3. Jeandupont
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    Jeandupont Designer/Boatbuilder

    Is it a lightweight centreboarder capable of planing..
     
  4. sorenfdk
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    sorenfdk Yacht Designer

    No - it's the other way round. The CLR should be aft of the CE.
     
  5. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Soren,
    Thanks for that - you beat me to the punch. It is the most misunderstood concept in yacht design, and yet the MOST important in terms of handling.
    Steve
     
  6. frosh
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    frosh Senior Member

    Soren, my experience with lightweight centreboard yachts tells me that an ideal balance occurs with the CE just behind the CLR. Therefore I have to respectfully disagree with you. The one boat that I built that had the CE in front of the CLR which unusually had no movable rudder, refused to go upwind and with stronger wind pressure went more and more off the wind. This was rectified by reducing the rudder blade area, therefore moving the CLR forward.
     
  7. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Yabbut, frosh - you're in the Southern Hemisphere, mate. We all know things are the wrong way round down there.... :)
    Seriously, though, how are you calculating your CE and your CLR? Do you use the full rudder, or only a part? Are you including roach with the CE? Such things make big differences....
    Steve
     
  8. frosh
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    frosh Senior Member

    :confused: Steve, we do have a plague of kangaroos in the main street and koalas in the ceiling, but a boat is a boat in both hemispheres. I am including the roach and about 50% of rudder area. The boat I built that was extremely sensitive to balance was an "ocean going" 6.2 meter sailboard for two sailors, but in the shape of a long narrow sailing dinghy. As both sailors stand and hold the booms with both hands steering is done with foot pressure and subtle differentials in sheeting of fore and aft sails. This boat if not extremely well balanced would not sail in straight line at all, as it has a deep centreboard as well as a fixed rear mounted fin. I have built and sailed about a dozen more conventional high performance dinghys each with the CE aft of the CLR and all worked fine at the first attempt. A few of these were my own design. I thought that a slight weather helm was the ideal situation, as letting the tiller go results in the boat gradually turning into the wind. Maybe the current thinking is that lee helm is better!
     
  9. RHough
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    RHough Retro Dude

    It kind of depends on how you come up with CLR doesn't it? :)

    Centre Lateral Area?

    Centre Lateral Pressure?

    For lifting foils of A:R < 3:1 the CE of the keel/fin is about 1/6 chord. For A:R >3 it is about 1/4 Chord. The centre of area is 1/2 chord.

    Weather helm is created when the total yaw moment of the sail plan acts to turn the boat into the wind. That means at zero heel the lateral force of the sails must be behind the lateral force of the hull and keel/foil.

    A design may well have "Lead" where the geometric centre of the sail plan is ahead of the geometric centre of the underbody, but for the boat to have weather helm at zero heel the sail effort must act behind the underbody effort that opposes it.

    Heel adds to the weather going yaw moment, a design that will sail heeled at 15-20 degrees may well require enough "Lead" to have lee helm at small heel angles in order reduce the heeled weather helm.
     
  10. frosh
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    frosh Senior Member

    Hi Retro Dude, It sounds that you are giving my viewpoint qualified support.
    In my field of planing boats I always try to sail them flat when powered up. Therefore the greater yaw affect that occurs when heeled to leeward is not something I take into consideration as a heeled planing dinghy is not being sailed properly. Sounds a lot like keel boat theory which is not what the thread originator is interested in.
     
  11. RHough
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    RHough Retro Dude

    Unqualified support. :) If CLR is defined as the point around which the hull rotates to weather, the CE of the sails MUST be aft of CLR for the system to develop weather helm. Just like a wind vane.

    Locating the CLR is the trick, simple geometry does not do a very good job. Weighting the rudder area at some percentage of it's true lateral area has the effect of moving the Centre of Lateral Area forward, closer to where aerodynamic theory says it should be.

    Of course the same holds true for the sails. The Aerodynamic Centre of Effort is not the geometric centre of area. The Sail CE is forward of the Centre of Area just as the Hull CLR is forward of its Centre of Area.

    From "The Rules"
    "In flat-bottomed boats, putting the CE directly over the CLR seems to achieve good balance just about every time; however it is different in boats with a deep vee-section near the bows." Jim Michalak
     
  12. gypsiemarine
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    gypsiemarine Gypsie Marine

    Gypsie Marine

    Mate, just go back to basics. Make an accurate flat cardboard cutout of the hull and balance it on a pencil untill you have the centre. The rest is easy, centreboard/keel position,rudder profile, sail power requirements and rig her to suit.
     
  13. frosh
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    frosh Senior Member

    Late April Fools day

    Hi Gypsiemarine, you are kidding!! I would think?;)
     
  14. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Frosh - I think we may have a case of "the exception proves the rule" here. "May"
    If you are including the roach, then your "CE" is going to be further aft than that calculated by the part of the world that doesn't use roach. That, and the fact that your boats are primarily upright-sailing planing dinghies, whose skippers can adjust CLR easily by moving weight around and CE by tweaking sail set.
    Whichever works for your boat, then that is obviously right for your boat, but I'll stand by my rule-of-thumb for my keelboats.
    Steve
     

  15. Cliff Pope
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    Cliff Pope Junior Member

    My understanding from extensive reading is that CE should be slightly (6" in 20 '?) AHEAD of CLR, when measuered statically. Then when the wind starts to push, the boat moves ahead but begins to pay off the wind. However at the same time it will heel slightly, and this immediately puts a counteracting turning effect, back INTO the wind. The result, if set up correctly, will be a nice self-correcting balance, making the boat light on the helm but with an overall slight weather tendency for safety, which will normally be countered with slight rudder correction.
    Any calculations or measurements done in 2-D will immediately change as soon as the boat heels and it becomes a 3-D situation, and I think that is what confuses this kind of discussion.
     
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