Off my rocker?

Discussion in 'Hydrodynamics and Aerodynamics' started by sailorjim, Mar 23, 2013.

  1. JRD
    Joined: May 2010
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    JRD Senior Member

    You will need a high righting moment and light weight to get a 6m sail boat planing, especially if the transom is dragging.
    If you look at keelboats designed to plane like the VOR 70s and the 6-7m sports boats, they all have the transoms clear of the water at a normal sort of trim, but they do have straight lines aft with minimal rocker.

    Higher performance planing racing dinghys are often designed for a small amount of transom immersion (say 20-30mm) when trimmed flat. At at lower speeds where the wake does not separate cleanly, the crew trim the bow down to lift the transom clear of the water.

    There is a chapter on developing compounded ply hulls in the West System book - see chapter 25
    www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook 061205.pdf

    Once you have made cardboard models as PAR has suggested, move on to a scale model from model aircraft ply, 4mm ply would requrie 0.8 to 1mm birch. Its not cheap either but you will get several models out of a sheet and a fair idea when the break will occur.
     
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  2. sailorjim
    Joined: Nov 2010
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    sailorjim reinventing the wheel

    Thanks PAR, I've been playing that way with sheets of various materials (sheet balsa, 3mm ply, heavy card). Heavy card is good. Pushing the top of the stem a few cm horizontally down the midline towards the rear opens up the forward section, making it more buoyant.
     
  3. sailorjim
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    sailorjim reinventing the wheel

    Thanks JRD, this is exactly the information I was looking for :D. Sheet birch is one material I haven't tried.
     
  4. upchurchmr
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    upchurchmr Senior Member

    Sailorjim,

    Sure is difficult to keep track of this conversation when you are on Woodenboat and Boatdesign at the same time.

    Did you look at the Gougeon book last week when I mentioned it on Woodenboat?

    Never mind replying.
     
  5. sailorjim
    Joined: Nov 2010
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    sailorjim reinventing the wheel

    Oh I don't mind replying. I didn't respond to your post because it didn't address my question. I don't want to do stitch and glue, I don't want to torture plywood. I want to make my own mess.
    Thanks for the tip on Gougeon bros, but I have a copy. May I recommend a French book "Tous les bateaux du monde" (ISBN : 978-2-9014-2140-5), which documents traditional working sailboats from around the world over the past 200 years. You'll see there are crazier designs than mine :)
     
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  6. haribo
    Joined: Oct 2012
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    haribo Junior Member

    they do it like you, but upside down for cats

    http://www.mono-cat.de/bau.htm

    with water, vapor and endurance! the (2,5mm) ply is bendingt to a radius of only 30mm near the bow

    for a 5-6 m dingi you must fit together more than one sheet, maybe it is better to glue together 2*2,5 mm for the bow area and use 4 mm for the next section
     
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  7. sailorjim
    Joined: Nov 2010
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    sailorjim reinventing the wheel

    for a 5-6 m dingi you must fit together more than one sheet, maybe it is better to glue together 2*2,5 mm for the bow area and use 4 mm for the next section[/QUOTE]

    I was thinking of 4 mm + 3 mm.
    It would be useful to know the bending characteristics of different thicknesses of ply. A large sheet of 310x153 cm 3mm okoume will bend to a quarter circle of about 35 cm across its short plane.
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Bending characteristics are well established, though as you've suggested, thickness and species do play a considerable role as well. Also the way the sheet is bent is a consideration, with cross grain and with grain orientation making a significant difference in the radius possible.
     
  9. haribo
    Joined: Oct 2012
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    haribo Junior Member

    [​IMG]
    with "3mm flexo-ply" a radius of 25mm is possible, (o you can easy reinvent a ply-weel for a toy)

    in first aproach the smalest radius is linear to d, but it is quite quite different for different ply, and quite different if the outer layer of the ply is alongside or cross the radius

    some stretched tapes cross the radius on the outside helps also

    a cheaper way is to do the test with smaler (8cm) stripes
     
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  10. Manfred.pech
    Joined: Apr 2010
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    Manfred.pech Senior Member

    Origami

    SailorJim, among the various Yahoo Groups you will find an OrigamiPlyBoatDesign group (http://groups.yahoo.com/group/OrigamiPlyBoatDesign/) with an interesting collection of sketches which shows a lot of possibilities to get compound curves with ply. For instance here: http://www.sail-ho.pl/images/artykuly/20040311krzys/ . To get contacts with other builders, designers, more pics and plans you have to sign in - which is no problem (no danger of spam).

    http://www.sail-ho.pl/images/artykuly/20040311krzys/skiff.gif

    http://www.sail-ho.pl/images/artykuly/20040311krzys/zelazkot.gif

    http://www.jachty.p-net.pl/galeria/galeria.html

    http://www.jachty.p-net.pl/konstrukcje/konstrukcje.html

    http://www.jachty.p-net.pl/metoda/metoda.html

    from Krzys

    [​IMG]

    [​IMG]
     

  11. sailorjim
    Joined: Nov 2010
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    sailorjim reinventing the wheel

    Thanks all for your interest and suggestions. I just got my hands on some 1/16th inch basswood from a hobbyist supplier, that just happens to be to the pecise scale of two large sheets of ply placed edge to edge (8x24" = 620x306 cm). I'm getting some interesting results, that I'll post in a couple of days.
    It really looks like the circle can be squared!
     
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