Input desired on 20ft row/kite boat design

Discussion in 'Boat Design' started by W9GFO, Oct 15, 2015.

  1. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    If a little sailboat tips over, it stops sailing, this one will keep going ! Be careful.
     
  2. CT249
    Joined: May 2003
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    Location: Sydney Australia

    CT249 Senior Member

    Nice post.
     

  3. CT249
    Joined: May 2003
    Posts: 1,228
    Likes: 66, Points: 48, Legacy Rep: 215
    Location: Sydney Australia

    CT249 Senior Member

    I'm not a designer, but I study the history of sailing designs and because of my job I've been able to talk to many of the best sailboat designers. To distil what I think they would say;

    If you don't add volume forward, you don't actually add anything of value. Imagine taking a typical 20' boat and adding a 30' long extension 1mm wide and 1mm high - is it now going to perform like a 50' boat? Nope.

    The length forward has to have some significant volume if it is to add extra buoyancy or to increase hull speed by increasing the distance between the bow and stern waves. If the hull is too fine to create a significant bow wave then it's largely adding wetted surface and weight without doing anything else. Yes, rowing sculls and racing kayaks seem to have very fine ends, but they are not actually very fine IN RELATION TO THEIR VERY NARROW MIDSHIP SECTIONS - a racing kayak has a CP of around .66 which means it actually has finer ends than a cruising sea kayak (around .45). Sure, a very fine bow may slice through chop, but normally the extra wetted surface and weight doesn't make that worthwhile. (EDIT - modern racing cats are also around .7 PC, I think - ie much fatter in the ends than a conventional mono.

    The amount of dynamic (ie planing) lift a hull develops is related to how flat it is, because the planing lift is created by the re-direction of water particles at right angles to the hull. A very Veed bow like yours does not direct the water particles down, but mainly out to the side, therefore it does not develop much planing lift.

    How flat the stern is doesn't really matter in terms of developing lift, because by the time the water particles reach the stern, they are running along the hull skin - therefore even a flat stern will not re-direct them down. And even if somehow your stern did develop all the planing lift then you'll just nosedive.

    Check out the stuff about Dan Savitsky's work on the mechanism of planing - there should be some stuff in the net from this pioneer.

    Yes, there are some wedge-shaped sailboats on the net with deep Vee bows - but which ones? There are those like the 49er, but they have very large rigs and therefore are inherently fast, so they can sacrifice planing lift and suffer increased wetted surface in exchange for a better ride in chop - a rowing boat can't.

    Look at the development classes that have smaller rigs and you'll see U-shaped sections and finer sterns, because those boats cannot afford to suffer the extra wetted surface drag and reduced planing lift from a Vee/wedge shape.

    You mentioned cats - look at the modern cat designs. The forward sections are very U shaped (to develop lift, among other things) and the sterns are getting narrower because they've realised that having extra volume aft causes nosediving.
     
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