dingy sailer's

Discussion in 'Stability' started by benmww, Dec 29, 2006.

  1. benmww
    Joined: Dec 2006
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    Location: crusing yacht - aussie

    benmww Junior Member

    I live on a cruising yacht and I have decided to design and build a second dingy primarily for sailing while at anchor. I have a windsurfer but it is 30 yrs old and doesn't perform at all.
    It would be of overall length of 3.5 m or 11.5 ft
    I'm only thirteen and so wouldn't mind any advice,
    looking towards 120 kg displacement and high PC.
    how important is the displacment curve, and making it comforming to a sine curve and trochoid at medium speeds eg. lwl:V of 1.4
    any help wanted
  2. CT 249
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    CT 249 Senior Member

    I think most designers largely ignore the displacement curve. Some use it, but many of the best dinghy and skiff designers work largely intuitively. The design of dinghies varies so enormously according to their length (which has a massive effect on DLR in such small boats with heavy crews compared to their displacement and length) that some design principles are extremely hard to take from one boat to the next and almost all vary a great deal depending on rig size etc. PC is a hard one for comparison in dinghies as it varies greatly according to crew weight and position. PCs of good boats varies from about .51 to .68. Boats are fast as the 12 Foot Skiff and the Moth are at each end of that scale. Guys like Julian Bethwaite ignore PC totally.

    A good 30 year old windsurfer, well sailed, will have about the same overall speed as a Laser or Laser Radial and be faster than a 125 which may be the sort of boat you're trying to create.
    Last edited: Jan 1, 2007
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    BOATMIK Deeply flawed human being


    Excellent that you are having a go at designing a boat! You should go ahead and do it - it is great fun and a chance to learn heaps.

    But be aware that one of the things that has held back yacht design in particular has been the thinking that some feature of the boat should match some mathematical feature or some feature found in the natural world.

    Like the distribution of hull displacement should be a sine wave.

    Or that a centreboard shape needs to be an ellipse.

    Or that the leading edge of a mast needs can be some sort of "ideal" shape

    In practice we find that those sort of differences are rather tiny.

    In the past there have been times when sine waves and trochoid waves were declared to be "best".

    Or I can remember a local surfboard manufacturer who created boards with bottom rippled like sand in the shallows at the beach or shaped like the bone of a cuttlefish. Beautiful as pieces of art - but very poor performers in the water.

    Another example was the big way that sailboard (windsurfer) manufacturers used to bring out new models all the time with all sorts of concaves on the bottom - each claiming their mix was superior.

    Then later they found out that a basically flat bottom with a little bit of vee was best by far - after a decade of promoting concaves. People who sail sailing boats could have told them that at the start.

    There are some things that are better understood and the basis of scientific research - like the relationship of the PC to boat speed but sine and trochoid displacement distributions were simply voodoo - someone thought that they were a good idea and promoted them strongly at a point in history.

    PC with boats that have a range of speeds (like sailboats) is a bit more of a flexible area and with simple light boats there is often not as much choice as would be nice.

    As CT says above some designers seem to ignore PC - it is a justifiable argument for boats that sail at the same angle of pitch all the time, but a lot of boats vary trim from heavily bow down to stern down naturally as speed increases or crew crams their weight forward in light winds.

    The PC can vary wildly from the theoretical "best" result when the boat is dead level fore and aft.

    Good luck and have fun.

    Michael Storer
  4. Tim B
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    Tim B Senior Member

    Go and have a look at some other similar dinghies. You're probably in the range of 2-man dinghies from 1960 to 1980. Notably, LARK, 420, Albacore, Scorpion etc. Start with these and you won't go far wrong.

    Tim B.
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    BOATMIK Deeply flawed human being

    Or contemporary wooden boats (they are generally designed for more general use than racing boats) designed by

    Iain Oughtred
    Phil Bolger
    Michael Storer
    Selway Fisher
    John Welsford

    Or class boats that retain their home building roots, Optimist, PD Racer and some others - there may be others that can be suggested.
  6. Ike
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    Ike Senior Member

    What you really need to do, in addition to all of the above, is look at a lot of different designs and compare the features of each. Then after that, take those features you think would work best and go for it. High performance dinghies in that category abound; the OK dinghy, Laser, moth, are just some examples.
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    FAST FRED Senior Member

    For a cruising boat the first concern is how big the dink spars will be , and where/how to store them.

  8. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member


    Josua Slocum (who rebuilt as well as practically redesigned an old oystering sloop and sailed it around the world) coined my favorite design phrase: "Blow of the eye."

    That is imagining natural forces interacting with your design idea and comparing these imaginings with with things you've experienced in real life. We've all seen things break. Remembering what we've seen can help us, say, design a better mast step. Remembering how moving water gets around objects from a rock in a stream to a skipping stone can help us design better hulls.

    I do belive formulas have their place, but I belive their place is in general guidence only. And only if you fully understand that formula's history and original intent. Prismatic Coeficient (Pc), for example was originally meant to compare relatively heavy boats that never planed and usually went below 'hull speed' (1.34 * sqrt of WL in ft in knots). Later, this same coefficient was extrapolated to planing boats which first started to appear in the late 19th century with some sometimes very misleading results.

    For instance, take a shallow dinghy and give it a very long, up sloping pointed, bow and a very short but wide and flat stern that also slopes upward and it will have a very low Pc. Anyone reading just that number and not even seeing the actual hull, will predict that it will never plane, when, in fact, it may well beat planing dinghies with much higer Pc's, which are, according to conventional understanding of this coefficient, should perform much better.

    Now, describe this same proposed dinghy to an experienced planing power boat designer and he/she will tell you in a heart beat that it will plane quite well indeed. That is because it is the aft sections of a planing boat that are most important. The rest, when the boat is planing, are often not even in the water.

    A Pc number would never tell you that.

  9. joz
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    joz Senior Member


    There is a boat that might suit your needs its called Lake Burley Griffin designed by Glen Seeley if you can get your hands on Australian Amateur Boat Builder Ed 56 Jan-Mar 07 on page 87-88 it has it and also its a decanting keel dinghy and made from plywood as well.

  10. benmww
    Joined: Dec 2006
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    benmww Junior Member

    thanks guys for all the info
    about that windsurfer,
    I am only a child and it's a large board with a very large sail,
    i cant use the working sail so i have to use the heavy weather sail
    that is why it doesn't perform.
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