Keel and mast position

Discussion in 'Sailboats' started by Panos_na, Feb 14, 2010.

  1. Panos_na
    Joined: Sep 2004
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    Panos_na Junior Member

    Hi there!

    I am dealing with a sailboat design and I have designed the hull.

    Now I want to move to the next step. According to some books now I have to deal with the keel and rudder design.

    As known, the horizontal distance between the Centre of Lateral Resistance of the underwater body (CLR) and the Centre of Effort of the sails (CE) is very important for the balance of the boat.

    I have found some methods to calculate those two, but I dont know how to start.

    Should I put the keel at a longitudinal position, having in mind similar boats? Or is there a thumb of rule that I can follow?
  2. Perm Stress
    Joined: Sep 2009
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    Perm Stress Senior Member

    For initial design you could use the following:

    *Keel center of effort (not center of area!) is ~22% from leading edge, ~45% down from the hull bottom
    *This COE has to be at 45%, +/- 2% aft from fore end of LWL.

    Then appropriate method for placing the rig relative to keel COE, or center of underwater area, or whatever, could be used.

    By "appropriate" I mean method, developed for (or derived from) boats of similar general architecture: type keel and rudder, hull form and proportions, type of rig.
    For example, several methods I know have to be used completely differently for masthead rig and 3/4 rig.

    As you do not mention weight balance and trim problem I assume that weight calculation is done, and you know how much ballast and what longitudinal position is required.

  3. capt vimes
    Joined: Apr 2009
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    capt vimes Senior Member

    seems to me he hasn't done it yet... otherwise he would know the CG of his design and therfore the position of keel and mast...

    if you have the hull you know the CB at 0° heel... CG has to be inline (longitudinal) with your CB... take your layout and calculate your CG... that will give you an approximate keel (knowing your displ./ballast ratio) and mast position... compute your max RM which will give you your max sail area the vessel can carry... calculate CE for lateral plan and sails... it will give you the lead and position for the mast in relation to your keel and therefore the lead... redo your CG calculation and start again from there...

    that's at least the way i understood the design process of a sailing yacht... ;)

  4. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    This can be a complicated question depending on the design of the hull and keel, and the sails.

    If you are designing a conventional hull with a rudder on the trailing edge of the keel, with conventional marconi type sail rig, most designers simply balance the centroid of the projected areas as an approximation. And than they plan on doing fine tuning once the boat is in the water with mast sweep and sail adjustments. this is the most common method used by far.

    Modern high performance sails with deep bulb keels, spade rudder, and with a constant foil section keel will be something different again. There is no easy method for exact results with modern hull and sail designs.

    Perhaps if you give more details of your design, size, hull type, etc. or even drawings someone can suggest a method.

    What part of Greece are you from?
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