Is my hull shape decent?

Discussion in 'Boat Design' started by stonedpirate, Feb 7, 2012.

  1. ancient kayaker
    Joined: Aug 2006
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    ancient kayaker aka Terry Haines

    - those observations both seem counter-intuitive to me. Given that righting factor increases (roughly) as beam^3 and a deeper keel has a greater moment arm, I would expect the stubby boat with a shallow keel to have more righting moment and less damping resulting in more rapid and sustained rolling.
     
  2. Stumble
    Joined: Oct 2008
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    Stumble Senior Member

    Let's for a moment talk about some quantifiable issues for this design...

    By necessity this boat is going to have a terrible 'comfort factor'. Since due to its imposed design leingth limit it can't have appreciable over hangs, and must be relatively wide for its leingth. At 10' long and 4' wide this boat will have dimensions closer to a multihull than a typical mono. This is going to make the highly uncomfortable, reaching levels proven to cause crew fatigue over moderate periods of time.

    'Capsize Risk' is also heavily dependent on a narrow boat. Again this boat will be very wide for its displacement, so it will be highly prone to capsizing. So because this boat will capsize easily additional stress wil be placed on the crew.

    In even a moderate 2-3 day storm it is completely possible that the crew would find it close to impossible to sleep, increasing the possibility of poor decision making, and poor reactions to emergencies. This to some extent can be mitigated by high levels of experience on boats, and in small boats in particular. This is why in the mini class sailors have to qualify by resume, not for racing experience or results, but for miles sailed, and sailed alone.

    Compare this to the OP, who as I remember has no off shore experience, no heavy weather experience, and no real sailing experience, who is planning on embarking on a voyage that would make expereinced sailors hesitant. If he said he was planning on buying a Mini class and going off shore to learn what he was infore, I would have a whole host of recommendations, like learning to go days without sleeping more than 15 minutes at a time, learning how to rebuild sails, ect... But he doesn't even have the qualifications to learn what he needs to know.

    Compound this with the idea that those of us who have been off shore a great deal are just acting like old women, unwilling to take the risk... Well guess what, I have seen people die off shore, i have seen people loose limbs, and have permanent injury, and loose limbs, and I still go off shore sailing for fun. But I never loose sight of the realities of how dangerous this sport can be. Because it is only with a reasonable assesment of the risks that good decisions can be made.



    The best off shore sailor I know, who before he passed away had wracked up 5 circumnavigations, and was brought on as a heavy weather specalist for Whitbread cape roundings, is also the only person I know who ALWAYS, ALWAYS wore a harness when off shore. He was more scared of heavy weather than anyone I have ever known, but loved being there. The reason he and his crew lived through those situations was two fold, his incredible skill both preparing a boat and crew for the conditions, and his incredible fear of the sea.
     
  3. TrustedShips
    Joined: Feb 2012
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    TrustedShips Mr.

    That whole comment is written so nicely. Hope the person who started this thread learns something from it.

    :cool:
     
  4. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    I know it's counter intuitive.

    A short, deep keel acts like an airplane wing. Up to two thirds its lift is coming from the windward side, as the water passing over it is pulled to leeward., just as the wind passing over the top of an airplane wing is pulled downward.

    In the condition know as a 'stall', the wind going over the top of the wing breaks away from the top of the wing and tries to go directly aft. It fails, of course, breaking up into greater and lessor whorls, which only add drag.

    When this happens, the wing loses up to two thirds its lift, causing the plane to drop.

    The same thing can happen with a short, deep keel. The boat can abruptly slide sideways, as if half the keel had suddenly broken off.

    When it comes to rolling, if the short, deep keel stalls, it loses up to one half its dampening ability as its weight and long lever arm pull it straight down, causing the boat to roll sharply upright.

    Add to this a hull that is light and shallow for its Beam and, therefore, has a greater buoyancy shift when it heels, you get a boat that rolls upright abruptly after a knock down in smooth water. Not bad.

    Now, take this same boat into rougher water where it is trying to conform to an uneven surface all the while the deep keel is trying to keep it dead upright, with its weight and lever arm, and trying to dampen the very rolls that the shallow hull creates (failing sometimes due to stalls), and you end up with sharp, unpredictable rolls that the human body has difficulty adjusting to.

    Not to say this type of boat is un seaworthy. It can be quite seaworthy, if it holds together. If the keel and hull, often working at cross purposes, don't break apart from one another.

    Now take the boat that is heavier for its Beam. The buoyancy shift, as it heels, is no where near as great. And the heavier hull has more momentum and inertia, so accelerates slower given the same impute (which, due to it's lesser buoyancy shift, it doesn't get), so has a much slower, more predictable roll, when it recovers from a knockdown, even if it has the same short, deep keel the shallower, lighter hull section has.

    Now add to this, the long shallow keel. The long shallow keel gets a far smaller amount of its lift on the windward side (which, if it were an airplane wing, would be its top surface) This is because the water on the lee surface sneaks under the bottom edge of the keel, like a kid jumping a fence to take a short cut. For this reason. a long, shallow keel needs considerably more area to work adequately. It works more like an angled snow plow than a wing.

    For this reason, it never stalls. It never stalls because, by definition, it is ALWAYS in a stall. For this reason, its dampening effect is far more predictable than that of the short deep keel. Thus, when this hull keel combination meets an uneven water surface, or gets a sudden knock down, it reacts much slower and much more predictably, allowing the human body to adjust.

    As for Beam itself, you seem to have a major misconception. It appears that you believe that Meta Centric Height is directly proportionate to Beam.

    This is simply not the case.

    Meta Centric Height is proportionate to one half Beam cubed times two times Length divided by volume. For this reason, it is possible for a boat with a four foot Beam to have a higher Meta Centric height than a boat with an eight foot beam. If the wider boat has very deep sections and the narrower boat has very shallow sections the four foot wide boat could well end up with a higher Meta Center.

    I hope I haven't confused you too much.
     

  5. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    Hi, Stumble.

    Trying very hard not to impugn your long experience at sea, I will try to point out a few things.

    When it comes to initial sail carrying capability, initial stability is king. A boat with a low Heft Factor has a great deal of that per given Beam. A boat can be very long in proportion to its Beam and have a low HF, giving it great initial stability. It can also be very short in proportion to its Beam and have a very high HF, giving it very low initial stability. Therefore, Beam to Length ratio, though often a good indicator of sea kindliness, can also be very misleading. When you compare two boats with similar hull sections (which is usually the case), the Beam to Length ratio works as a predictor. But when you have vastly differently proportioned hull sections, this can be terribly misleading.

    I think if you gave this fellow's proposed design the same Length and Beam as, say, a San Juan 28, keeping its hull section the same proportions as the original boat, you will probably find it would displace more than three times as much.

    Add to that the fact (I hope you agree), that in rough seas, the buoyancy of the hull, from keelson to shear clamp counts, not just the volume below the waterline. For this reason, overhangs, from side to side, as long as they aren't extreme, don't count nearly as much as one might think.

    As far as your offshore advice goes, I find it spot on, if not a bit reproachful.

    I don't get the impression that this fellow is planning on going of on his round the world voyage as soon as his boat's keel kisses the water.

    Instead, I get the impression that his is something he MIGHT do, after he gets himself better acquainted with it.

    You had to start from some place, just as he will have to. I'm sure you weren't born with off shore experience.

    I wasn't born with sailing experience, but I taught myself how to sail. And on a far from perfect boat I built myself, with no help from anyone or anything else, other than some shitty sailing books I had, which didn't give me a clue of what I was doing.

    I don't think it's a good idea to trash this fellow's dream. Because, I get the impression that that's what it is mostly about.

    Maybe he just wants to own a boat that COULD cross an ocean, even though it probably never will.

    For what its worth, my number one rule of seamanship is: KNOW YOUR BOAT.

    Know what it is going to do in a given situation.

    If I were to build my FOOTBALL design, I would first take it day sailing in good weather. Then I would try sleeping aboard, tied to a dock. Then I would take it sailing in rougher weather, further testing its limits. Then i would take it on an overnight trip. And so on.

    At one point, I would take it on a short off shore passage.

    After owning it and testing it for at least three seasons, I might consider taking it on a longer off shore passage.
     
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