Leeward Ama Generates Leehelm ?

Discussion in 'Multihulls' started by mcm, Jul 26, 2011.

  1. Gary Baigent
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    Gary Baigent Senior Member

    It is true, you can get away with murder on board/foil positions in relation to sail CoE. To bookish bible advocates (the Marchaj quoters), this is anathema, goes against the rules, but many multihulls have their main hull board position forward of the mast, forward of CoE, admittedly angled aft but still with the CLR ahead of sail balance point (Peyron's Fujicolor, the earlier one, was an extreme case )... and then there are others with the board angled acutely forward from a position aft of the mast (Steinlager 1)... so lifting the board in both cases makes quite dramatic balance shifts ... yet, with differing headsail setups, crews keep their boats in balance. With board forward, angled aft, even in heavy weather, a headsail has to be carried for balance, whereas positioned aft, angled forward, just the main can be used in similar conditions. On original form Flash Harry, with angled foils well forward of the wing mast/soft sail una rig, (to windward sailing) the supposedly imbalance should have produced intolerable weather helm, but such was definitely not the case. An interesting modern development is the curved foil position (with no conventional daggerboards) on the new Nacra 20 foot cats, foils set well forward of sail balance point, (because they don't want to lift sterns and nosedive) so much so that the platforms rear up when lift becomes too high. So as Cav says, there is art involved as well as science, very confusing for some. On a spreadout multihull platform, single dimension profile drawings do not add up and you have to take into account the wide platform and the pivoting around the board/foil position way out to leeward in conjunction with the heeling and sail CoE position. Also if you carry a wing mast, drive appears to move the complete sail CoE even further forward (because of better efficiency in wing mast design). Yes, it is confusing. Unfortunately, there is some suck and see involved. Just IMO of course.
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    It's actually just according the rules of physics as also you would know if you accidently had read your "Marchaj" :D
    What it's against are the rules of thumb..
    Some good points thou :)
    BR Teddy
  3. Gary Baigent
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    Gary Baigent Senior Member

    Quite true, Teddy (I've got a couple of M's works) it was more of a swipe at certain steel boat believers who, when they find something unusual, not in "the book", then they righteously quote. Not Machaj's fault. I shouldn't have used his name. Apologies.

  4. cavalier mk2
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    cavalier mk2 Senior Member

    Another way of thinking of the CLR , CE, buoyancy centers, lever arms etc.....as balance spheres. We know the only time those zones are static is when looking at the plans. Add ripples and waves and varying wind, crew movement, sail shape trim and course and all the centers are moving around in a spherical zone as the boat sails off into the sunset.. Like juggling, when the spheres get too big or in the wrong zone loss of speed, control or capsizes can happen. A good example is of a monohull doing death rolls under a oscillating chute. It is physics but the art is in balancing the zones in the design and making them easy to manage when you have things in conflict like wanting too much sail up to blast past the competition. As sailing becomes more techie in the racing maybe we will see boat crews with goggles tied in with load sensors to view the virtual projected zones as they trim the boat. Tis why ephemeral arties do well going by the seat of their pants.
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