Off my rocker?

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

  1. sailorjim
    Joined: Nov 2010
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    sailorjim reinventing the wheel

    Hello to all. I’m obsessed by building a sailing boat of 5 or 6 meters by simply bending whole plywood panels around each side of a laminated keel (simple to build, hydrodynamic cone section). The photo shows the principle for a 3-meter version. Ze problem is that plywood can basically only be developed in one direction, so I can’t get any after rocker and the base of the transom will be a few centimeters below the water line. That means I’ll be trailing several buckets of water, welling up around the transom. My question to my superiors on this site (i.e. all other members...) is does this really matter that much for a planing hull?

    https://plus.google.com/photos/1114...81873015282?banner=pwa&authkey=CIuMxdnjhcvbUg
     
  2. gonzo
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    gonzo Senior Member

    Firstly, what do you mean by a planing hull? No-one in this forum has been able to define it, unfortunately. What speed are you trying to achieve? Are the design parameters that all plywood sheets must be whole?
     
  3. PAR
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    PAR Yacht Designer/Builder

    Welcome to the forum Jim.

    Actually, you can force some compound into the panels and with a carefully arranged seam, down the centerline, you can incorporate some rocker into the profile of the keel. The amount of compound you can get, is dependent on several factors. You'd be best advised to develop these shapes in software, that will display the "developability" of the panels and the associated stresses, that come with this technique. Lastly, this sort of "free form" building and shape development is literally a crap shoot, in regard to prismatic coefficient, preferred full plane mode shapes, volume distribution, etc. At best this is a very difficult way to design a performance dinghy that actually has the level of performance you desire.
     
  4. sailorjim
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    sailorjim reinventing the wheel

    thanks for your interest Gonzo. I think Uffa Fox knew a bit about planing hulls, but I'm no Uffa Fox... Yes, for simplicity and hydrodynamicity (...), the panels must remain whole.
     
  5. gonzo
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    gonzo Senior Member

    If you are looking for simplicity and hydrodynamic efficiency, full panels are the wrong way to go. As a challenge on how to build a hull without cutting curves on panels, it is interesting though.
     
  6. sailorjim
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    sailorjim reinventing the wheel

    True, you can get some compound from large panels, but if I can't get enough to lift the transom out of the water, how much will it matter?
    I don't know what you mean by crap shoot, but I suspect it's "hit and miss". If so, I can only agree. The main problem is the curvature of the stem, which is inevitably spooned. In fact, building boats this way takes a dose of sculpture! .
     
  7. PAR
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    PAR Yacht Designer/Builder

    A simple slit about a 1/3 to a 1/2 a meter long on the bow's centerline and you could force that panel to close up. Playing with these sort of "more than developed" surfaces is really all about experience with the material. No software in the world will predict how much you can get. Only the guy bending the piece can tell how far is too far, usually an observation, just preceded by a loud bang. Pre-loading panels like this decreases their ability to carry the strains of a boat, particularly rigging loads.
     
  8. sailorjim
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    sailorjim reinventing the wheel

    In fact the slit has to be a meter long, or one-third of the panel: any less and the (4mm) ply would split (ok, it did split on my first attempt...). From what you say, you obviously have a lot of experience with bending ply; yes, it's pretty unnerving waiting for the "bang" - 150 euros down the drain. As for strain-bearing capacity, I dont know about rigging (the mast is free-standing and fixed to the bulkhead), but the convex surfaces are very resistant to external pressure. The roll benches are 2x3mm ply and have no internal structure, but no way could you cave them in.
     
  9. gonzo
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    gonzo Senior Member

    What PAR is saying, is that you need to add the forces or stress of the rigging to the ones caused by the bending. They need to be less than what makes the plywood break. Also, you need a percentage for safety.
     
  10. El_Guero

    El_Guero Previous Member

    "SAFETY? We don't need no stinking safety!"

    You gotta love it when you need to remind people about safety (fudge) factors.

    I need to go visit the water soooooon! I am becoming land locked.
     
  11. sailorjim
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    sailorjim reinventing the wheel

    Thanks for the safety reminder, El_Guero.
    Is anyone able to answer my initial question?
     
  12. gonzo
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    gonzo Senior Member

    The problem is that the question is not properly formulated. You need to define what you mean. For example, give the dimensions of the hull, sail area, target speed, total weight, etc. Basically, you are trying to use a method that will produce a bad hull. However, you state that is is for "for simplicity and hydrodynamicity ", which doesn't make any sense.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Simply put, the cone section you've stumbled across, isn't especially efficient, though to some extent fairly simple. What you're doing can be done, though to get a reasonable shape, you'll need to cut the panel for bending, rather then just trying to make a cone. You can slit the after portions of the centerline and get some rocker, as well as the forward portions. You'll need a pie wedge or two taken out and likely some curves cut forward to offer some sort of bow.

    The best thing you can do, is play with some stiff cardboard (not corrugated). This will quickly show you what's possible and where you need to make cuts.
     
  14. haribo
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    haribo Junior Member

    [​IMG]
    there is no planning without a transom under the waterline
    your problem is not the transom, the problem is to bring the bow out of the water

    so for your round bilg hull you need a bigger engine than for the table

    the fastest stressed plywood sailboat is a tornado catamaran
     

  15. PAR
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    PAR Yacht Designer/Builder

    If you play with the cardboard as I suggested, you'll find this is a good starting point. A 4'x8' sheet is shown.
     

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