Discussion in 'Boat Design' started by BertKu, May 10, 2016.

  1. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    Hi PAR, Gonzo, rwatson and other experts , A dagger board goes normally deep and insure that a sailboat does not drift when a jib is raised. My question is: Is the total service area what counts, or only the depth the dagger board is under the yacht. i.e Is 2 meters long x 40 cm depth (0.8 m2 ) doing the same job as 60 cm long x 133 cm deep (also 0.8 m2) dagger board doing the same job ? Thanks for some info. Bert
  2. Richard Woods
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    Richard Woods Woods Designs

    Area is the most important factor, but Aspect Ratio also plays a part, the higher AR will be more efficient, but will stall earlier and put greater loads on the exit point. Sailing is a balance, or equilibrium. If you don't have a big enough side force generated by the board the boat will make more leeway until it has increased the force. Then you are back in equilibrium and the boat sails balanced.

    yesterday I was sailing a dipping lug schooner with no keel at all. Leeway was quite horrible (25deg say) when the wind got up but was OK in light winds. Thats because in stronger winds the sails generated too much force

    Sorry a quick post its a complicated subject

    Richard Woods
  3. Skyak
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    Skyak Senior Member

    Higher aspect ratio has lower induced drag. The lift at the free end of a dagger board must be zero because higher pressure on one side can simply flow around the end. So the pressure profile ramps up from the end to some fraction of the lift that an infinite span would produce.

    Another way to look at it is that the force produced by the foil is equal to the net momentum change in the fluid. The higher aspect foil sweeps more water so for a given angle it produces more force.

    In drag the opposite is true. If the boat is stationary and you push it sideways the the daggerboard closer to 1:1 would provide greater resistance.
  4. tom28571
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    tom28571 Senior Member

    As Richard says, it is complicated but perhaps a couple examples will help without going into so much theory.

    I've raced pretty extensively in long full keel boats as well as boats with only daggerboards. Boats with a combination of long stub keel and DB or CB are a different matter but seem to act more like the shallow keel than the full DB. A long shallow keel will develop more and more leeway as the side force increases but will seldom have a full stall when moving fast (never in my experience). A deep DB can have subtle stall that can go unnoticed except when you see other boats creep to windward while both have the identical sail trim. Much of the "my boat won't point" wailing results from this effect.

    The DB can also develop sudden rapid and repeated slides to leeward in wind and waves as the side force oscillates from wind and wave action. In the extreme a DB can totally stall with sometime disastrous results. Case in point is from racing a Grand Slam 7.9 on a beat in fairly extreme conditions from a sudden storm. The DB stalled completely and the boat actually did a 360 out of control. Fortunately no other boats were hit but the adrenaline was such that it happened once more before common sense took over and the genoa was removed. Have also had a J22 fin keel stall completely while maneuvering sharply in very light wind around docks. The boat just suddenly slid sideways and was only stopped by the crew fending off another moored boat.

  5. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    Thank you Richard. It gives me together with Skyak and Tom a good insight.

    Thank you Skyak

    Thank you Tom. Bert
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