Finer details of C.L.P. ??

Discussion in 'Sailboats' started by Garry Hartshorn, Oct 12, 2009.

  1. Garry Hartshorn
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    Garry Hartshorn Junior Member

    In my investigation of placement of CLP I have come across a couple of theories regarding whether or not to use the rudder or portion of in the cutout.

    1. do not used rudder at all
    2. use rudder
    3. use 50% rudder
    4. if rudder is balanced use equal amount of area aft of rudder stock as in front.

    Any ideas ???
  2. MikeJohns
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    MikeJohns Senior Member

    As a rough rule of thumb (and others will have a slightly different approach)

    If the rudder is separate to the keel balanced or not then ignore it for CLP. If its hung on a skeg take the skeg area and half the rudder area. Similalry if it's hung on the keel take half the rudder area.

    It's a good idea to look at similar succesfull vessels since there is far more to dynamic centre of latteral resistance than a static centre of areas.

    Hope this helps
  3. gonzo
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    gonzo Senior Member

    The rudder provides lift at an angle which makes it disproportional to its size in comparison to the keel. That is, unless you have canards on the keel
  4. Brent Swain
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    Brent Swain Member

    Do not use any part of the hull which doesn't have any real leeway resistance , like well rounded forefoot, rounded stern sections etc. Use only keels, rudder and skegs as lateral resistance, as they are the only parts of the hull which do.
    As any ten year old who has played with model air planes will tell you, the centre of area of a wing is definitely not the centre of lift. That is a point about 20% back from the leading edge.
    Best throw away the numbers game and base a design on that which as balanced well in the past. Mathematical calculations on balance have an abysmal track record of predicting how well a boat will balance. Past experience is a far better predictor.
  5. Brent Swain
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    Brent Swain Member

    Best rule of thumb for moderate aspect ratio fin keels, with moderate sized fortriangle - mainsail ratios, sloop or cutter rig, is to step the mast where the leading edge of the keel meets the hull.

  6. Perm Stress
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    Perm Stress Senior Member

    "Finer details of C.L.P. ??"

    It depend on the purpose you will use it for.

    Let's assume, you need to design a spade rudder and stock.

    Than, for normal sailing, C.L.P., or rather, Center of Effort, CoE, will be at about 20-22% from leading edge. It has to be kept in mind, however, that this will be so only up to stall angle (~10 degrees for thin (~6-10%) high aspect rudders, 20, may be 30 for thick (~14-18%) )ones, but not after. after stall, it would safe to assume, that CoE is closer to center of area.

    While, if you want to check the strength of rudder stock against impact breaking wave crest impact, it sure good assumption to place CoE in center of area.

    C.L.P., Center of Effort for sails, etc., is only meaningful when used in connection with quite large database, collected for similar (at least broadly similar) boats in EXACTLY THE SAME MANNER.
    The reason is all simplified calculations are very far away from actual physics at work in real world.
    Let's assume, you try to design a sailboat of certain type, eg. fin keel, separate rudder with skeg (or whatewer), and want to balance a rig against underwater body.
    Let's assume, you have got some recommendation from book, or manual, or suggestion of someone, whose opinion you thrust: CoE of sails X% forward of CoE of underwater body (keel only, keel and rudder, full underwater profile.....).
    Deciding points here are:
    1. Is your design really similar the designs this recommendation is derived from? If no, recommendation is at best not suitable for direct application.
    2. If yes, than you calculate all the areas and CoE exactly the same way for your design and prototype, and compare numbers.
    Than you could guess, how different your design is , and make allowance for this.

    Hope this will help your question.
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