Sail design , area ratios of mainsail & jib ?

Discussion in 'Boat Design' started by dale.frahm, May 5, 2010.

  1. dale.frahm
    Joined: Feb 2010
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    dale.frahm Junior Member

    I'm designing a 75cm model racing yacht , sail area 5000cm2, ballast ratio 0.5 , designed for an upwind race .

    Regarding the area ratios of mainsail and jib , what performance difference would there be with say a 60:40 ratio, and an 80:20 ratio ?

    Our mainsail is the one we can get the best airfoil shape out of, would it not make sense to have this as the dominant driving sail ? Given that the CE is in line with the CLR without any spacial issues .
  2. gonzo
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    gonzo Senior Member

    If the formula specifies a maximum sail area, a single main will give you maximum performance. A small jib may be used to deflect air flow and allow the main to be trimmed a bit more.
  3. Stumble
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    Stumble Senior Member

    What Gonzo is suggestig is very reminicent of a lot of the high performance racing boats coming out these days. Boats lie the Farr 40, Melgeus 32, ect. Many are going with a 3/4 fractional headsail, normally with the largest jib being a 115% or smaller, and the mast shoved very close to the bow relative to more traditional boats.

    The downside to this approach is that it leaves a very small J diminsion which is the normal length of the spinnaker pole. Now a lot of boats are getting around this with asymetrical spinnakers going to the masthead, which regains a lot of the otherwise lost sail area headed downwind, but I don't know if this is a solution for a RC boat.

    If the race is truely just upwind then it would seem to me that a cat rigged boat with a high aspect main would be the best performer, or a very large main relative to the size of the jib. Something like from the 32nd AC.
  4. idkfa
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    idkfa Senior Member

    A jib stalls first; indicating to you to adjust sheeting and possibly rudder, (hopefully without stalling the main too.) Keeps you sailing faster than if you stalled the only (main) sail you have.

    Aspect ratio, mostly determines efficiency.

    I'm no expert but for a given sail plan, splitting it into two (or more, over lapping) foils, increases lift. And also drag but not as much, so therefore more efficient too.

    My understanding is that those boats are fully powered-up at lower wind speeds, approx 10k. One can depower (flatten: lower drag) the main in a gust, where as the jib requires changing. Hence you can sail fast in lighter winds too, and you average speed improves, which ultimately wins the race.

  5. messabout
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    messabout Senior Member

    Good input by previous posts. Presuming that you model will be radio controlled, there is another advantage to the una rig. Simplicity for starters, but more importantly your ability to respond to and trim to shifts, puffs and lulls. Superior helmsmanship will defeat faster boats almost every time. That implies that you build your boat very soon and practice for hours. When you are doing RC sailing, your thumbs are the difference between success and failure. You need to train your thumbs well.

    The most sage advice is to get your sail or sails from an accomplished sail maker. Do not try to build them yourself unless the rules prohibit professionally made sails. The difference in performance of amateur sails versus very good sails is astounding. With excellent sails you can defeat a competitor with better boat design. Some of the best model sailors and suppliers in the world are right there in the UK. Find them.

    There you have it. Una Rig, Good sails, Cheat by getting plenty of practice.
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