when designing a boat for plywood sheeting?

Discussion in 'Powerboats' started by bigisland, Mar 28, 2007.

  1. bigisland
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    bigisland Junior Member

    When you are designing a boat to be built from plywoood sheets what kinds of curves are off limits. how much can it bend to shape? thanks for the help.
     
  2. tom28571
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    tom28571 Senior Member

  3. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    You can use other shapes than cones (maybe I misnderstand "conic projection"), but gaussian curvature must be close to zero. Minimum radius like thickness x 50?
     
  4. PAR
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    PAR Yacht Designer/Builder

    I've had a great deal of success beating the predictions for "allowable" curves in plywood. You can apply a fair amount of set to thin panels and of course you can mold or double plank as well.
     
  5. tom28571
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    tom28571 Senior Member

    Yes you can bend thin 3mm plywood into compound shapes which are not predicted by the conic projection. Trying this with thicker 6mm ply will yeild very little compounding. Compounding even thicker ply is not very workable.

    I recall working on 16' trimaran amas that were tortued into a 3D canoe shape from only two pieces of 3mm occoume. They did not like the tortue very much and groaned and creaked as the last work was done toward the sheer. There was one small rupture in mid panel on one ama that was repaired with fiberglass.

    These amas did prove to be very light and also very strong in service.
     
  6. alan white
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    alan white Senior Member

    Good question. I am doing my fordeck soon. I have both a strong sheer and wish to have a crowned deck. In going though all the ways I might accomplish this compound curvature with plywood, I got practical and decided that a small amount of deck crown could be achieved easily because the deck is almost entirely divided by a bowsprit (about 30" of it is mounted on the deck).
    I figured I would make a very shallow inverted Vee (about a 1/4 inch per ft) using 1/2" plywood and then experiment with clamps and shims to see how I could slightly arch the two side sections, if at all. If I couldn't, I doubt anyone's eye will see how the two sides are dead flat athwartship, or not a continuous curve.
    Plywood resists making "bowl-like" shapes, but thinner plywood, say 3mm, can be wrestled into shallow bowl shapes just enough to curve a long curve into a slight gutter shape. It's called torturing and it works because it's done in a controlled fashion, stretching certain parts of the ply and squeezing other parts.
    The limits of such torturing depend on the thickness of the ply, the material it's made from, and the method used. It wants to spring back to its relaxed position just like a metal hand clicker (a tortured steel device) does.
    It would be helpful if there were tables provided that indicated to a builder exactly what compoundness would crush/ tear the unseen inner fibers of plywood. A good way to test your own plywood might be to cut a 2x2ft square, put about an inch thick, inch wide frame under it around the edges, set it down flat against a concrete floor, and put weight on it until it begins to make noises (them's fibers breaking).
    At half the weight it took to do that, how much lower is the middle of the sheet than the sides? That may be the practical safe structural limit to torture that particular plywood. If the plywood safely depressed a quarter of an inch, a geometry rule (which I don't have in front of me) will tell you the radius average you could achieve. I say average, because there are two radii, and adding to one will subtract from the other. A very large radius in one direction will allow a reasonably small radius across it, for example.
    I would assume the two radii added and then divided by two should not exceed the test deflection.
    and now you know what kind of pressure you must exert on the ply to get a certain coumpoundness, and what your boat structure is being asked to hold in place (over time, the pressure will drop as the wood adjusts----- but you don't want to turn you carefully set up and perfectly aligned boat into a pretzel as it tries to accomodate an over-tortured piece of plywood.

    Alan
     
  7. cookiesa
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    cookiesa Senior Member

    I remember seeing someone building a curved flybridge front once, they placed the sheets pf ply in salt water for a while then screwed them to the front for a few weeks to dry out in the correct shape before epoxy sheathing it. (About 9 years ago and the boat has had nothing done to it since so seems to work. Same idea as steaming really
     
  8. PAR
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    PAR Yacht Designer/Builder

    If you have a fair amount of compound to get out in plywood, the easiest ways are to mold diagonal layers or strip double planked layers over the compounded area.

    As Tom points out, you will have a diminishing set of returns if you force plywood into shapes it doesn't like. I've broken my share of panels too, but the experience has given me a pretty good guide as to how much I can torture a panel of given thickness, without straining the veneers into sheer or rupturing a void, etc.

    I prefer double planking plywood where light weight and strength aren't prime considerations and a molded approach if light weight and stiffness are more important. I find it easier to double plank, because both layers can go down at the same time, where as with a molded assembly you have to put down each layer one at a time.

    Dudley Dix's rolled chine methods may be of some use here as well.
     
  9. sigurd
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    sigurd Pompuous Pangolin

    I heard you can put wood below 60m or something and it will come up soft like spaghetti. ?
     
  10. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I don't think that will work with plywood :)
     
  11. sigurd
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    sigurd Pompuous Pangolin

    why not? will try it one time if I remember.
     
  12. Pericles
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    Pericles Senior Member

    How about kerfing or slitting the marine ply to form tight curves or cross hatching to get compound curves. It's not difficult with a circular saw set to the correct depth. then after the panels have been dry fitted, spread thickened epoxy into the kerfs and secure the ply panels in place. continue with cold moulding strips of marine ply to obtain hull dimensions.

    As you will deduce, marine ply is the only way to go as far as I am concerned.:D I certainly would not entertain soaking it with water. at any stage of boat construction. Building an epoxy encapsulated craft to Lloyds' qualification, will certainly demand BS 1088 quality, with moisture content towards the lower end of the manufacturers' specification.

    Technical Specification of BS1088
    British Standard BS1088:1966 is currently under review and may ultimately be replaced by a "guarantee" policy. However as a manufacturing standard it will continue to be used for the foreseeable future.
    The standard applies to plywood made from untreated tropical hardwood veneers having a suitable level of resistance to fungal attack, with a bond of WBP glue quality between the plies.
    Bonding
    Phenolic formaldehyde WBP to BS EN 314-2 class 3.Stag marine plywood can be obtained Lloyds Type Approved to British Standard BS6566 part 8 (now withdrawn and superceded by BS EN 315)
    Species
    Faces and cores are produced from Okoume/Gaboon (Aucoume klaineana), which is classed as non-durable.
    Veneers may be rotary or sliced cut. The method of cutting is at the option of the manufacturer unless otherwise specified.
    Face veneers shall present a solid surface, free from open defects. They shall be free from knots, other than pin knots, of which there shall be no more than 6 in any area 30 cm square, and not more than an average of 2 per 30 cm square.
    Veneers showing compression failure shall be excluded. Occasional discoloration is permissible.
    Tolerances
    Thickness tolerance - 4mm +.02/-0.6, 6mm +.04/-0.65, 9mm +.06/-0.75, 12mm +.09/-0.82
    15mm +.1/-0.9, 18mm +.12/-0.98, 25mm +1.8/-1.16.
    Multiply Construction applies to boards thicker than 4.8mm - each face veneer shall be a minimum of 1.3mm and not thicker than 3.8mm with a core not exceeding 4.8mm.

    Quality
    Boards will be sanded on both sides evenly, face veneer thickness shall not be less than 1/8" of the total thickness of veneers. Moisture content at the time of leaving the factory shall be between 6 and 14%.


    Pericles
     
  13. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    That's a recommended method.
    You can even buy "bending plywood" that has lots av small kerfes on one (or both?) sides.

    I think the glue in plywood will make it less suited for softening by sinking in deep water. It will probably work for 3-ply but not so good for 5 or 7.
     
  14. Pericles
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    Pericles Senior Member

    Raggi,

    It's the moisture content issue that concerns me. Deep sixing sheets of ply (they'd rather float than sink) seems pointless. I'd take them to a sheet steel rolling press and bung them through a few times. That'd give 'em a nice curve and a smooth surface as well.:) :)

    I can get my hands on flexible ply, which I shall use to create the stylish curves for a stitch and glue vessel, on which I'll cold mould with marine ply strips and perforated ply panels to create appropriate hull scantlings.

    http://www.marineply.com/prices.htm

    I just cannot see any reason to continue to include frames, ribs, stringers etc. in a epoxy ply composite boat. They do not contribute strength, only unnecessary weight. Laminated ply sheer clamps, however, do have some structural purpose in retaining initial hull shape and a strong base to which the decks are secured. Longitudinal bulkheads (berth flats and lockers) supply both stiffness and some of the interior furniture fittings.

    Quite soon I'm also going to put my money where my mouth is, so anyone will be able to watch and comment on progress.

    Pericles.
     

  15. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

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