Strip-Building on a Cylinder Mold

Discussion in 'Boat Design' started by ancient kayaker, May 18, 2011.

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    I was reading about the cylinder mold method of boat construction and it gave me an idea: does anyone know of this concept or have an opinion?

    In the cylinder molding method a straight mold is constructed with a constant section for one side only. The identical station forms are fastened with longitudinal stringers. Two straight, half hulls are taken off the mold, using layered ply with staggered joints or glass etc. The hull halves are trimmed to a profile to form a left-hand, right-hand pair and forced together until they meet along the keel where they are attached. Some additional shaping and construction is required to complete the hull but this is the general procedure. The method is limited to long, narrow hulls like multi-hulls, but it’s reputed to be fast, economical and relatively easy.

    http://www.multihulldesigns.com/pdf/cm/CYLINDER MOLD MULTIHULL CONSTRUCTION.htm

    I think it could be adapted for strip-built construction to bring the advantages of cylinder mold construction to strippers.

    In this adaptation the straight mold is modified so the stringers or strongback can be removed after the strips are applied, and the station forms have a step at the keel. WRC strips are laid together flat and fastened by a few lengths of adhesive tape across the width, forming a flexible sheet. The strip edges may be beveled, or left square. The WRC sheet is laid over the mold, resting on the keel steps, and attached to the end stations with straps or bindings.

    The sheer strip is stapled to the station forms, the other strips are stapled to the center form only (this may not be necessary in practice) and the adhesive tapes removed. At this point the stringers or strongback are removed; see image. The assembly is now flexible.

    The entire assembly is bent to the required sheer plan curve. Note that all the strips except the sheer strip can slide longitudinally over the forms. This is similar to the cylinder mold process except the assembly is more flexible.

    Epoxy is applied to the strips between the straps so it penetrates the joints between the strips. When the epoxy is set the half-hull is rigid enough to be removed.

    The 2 half-hulls are profiled and attached at the keel as for the cylinder molding method, except that no bending force is required. The hull is completed by glassing like any stripper.

    This method has the potential to allow a beamier hull with the use of a football-shaped bottom panel. It still has the other disadvantages of the cylinder mold method, including limitations of hull shape. I can see problems predicting hull shape but I wonder if FreeShip plate developments could be used to predict the profile shape.
     

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  2. marshmat
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    marshmat Senior Member

    It sounds like it could work... certainly worth a try, at least.
    I agree with you that predicting hull shape will be difficult. (FreeShip's plate developments seem good for developable / conic section panels, but this involves torturing a fair bit of compound curvature into the works.) It doesn't sound like an appropriate method for the type of builder who wants everything to be exact. It's probably more suited to the kind of builder who starts from the plans, then works from the shapes that actually get built.
     
  3. Pericles
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    Pericles Senior Member

  4. gonzo
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    gonzo Senior Member

    It is similar to the Constant Camber method.
     
  5. DCockey
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    DCockey Senior Member

    Terry,

    When are the molds removed - after the "assembly" is bent to the sheer curve and epoxy is applied?
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    Yes. The staples in the sheer strip would be removed, freeing the station forms from the current hull half, and the mold re-assembled for the next hull half.
     
  7. ancient kayaker
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    ancient kayaker aka Terry Haines

    True. I was studying the two different ideas to understand the exact differences which is when the idea came to me. The bilge curve can change if significant force is acceptable while mating the hull halves.
     
  8. ancient kayaker
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    ancient kayaker aka Terry Haines

    The material that carries the concrete until it is wetted down would have to be able to stretch over a compound curve - same problem with cylinder molding. Usng the strips gets around that, friction permitting of course.
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    From what I read hulls made using the cylinder molding method tend to vary, and it's reportedly a less precise method than using a conventional mold. The hull-to-hull variation would depend on the degree of force required to mate the hull halves.

    There should be less force with this approach, in fact if the designer used the constant camber design that Gonzo mentioned the hull halves could be cut along the centerline plane and there would be very little force required to mate them. That would make the resultant hull entirely predictable, but adds another constraint to hull design. The http://www.boatdesign.net/forums/wooden-boat-building-restoration/hybrid-construction-36215.html thread shows such a hull design.
     
  10. DCockey
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    DCockey Senior Member

    How are the results from this method different than what would be achieved by placing the molds in their final location and then applying the strips?
     
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    The strips are assembled flat and draped over the mold instead of stapled one at a time, and having all the station forms the same means they can be placed together and sanded ensuring a fair hull. Also only half as much ply is used for the station forms.

    It may be more of an attractive method for production building than a single boat, but even for a one off - if there are no hidden snags - it should be fast.

    At this point the only snag I can foresee is the actual bending of the strips on the mold to get the sheer planshape; it may bot be as easy as it sounds . . .
     
  12. DCockey
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    DCockey Senior Member

    The tape holding together the strips on what will be the outside surface will need to stretch as the sheet strips is wrapped around the molds. The distance around the outside of the strips after wrapping is longer than when the strips are on a flat surface.

    What holds the strips in place after the tape is removed?

    This can be done whether the molds are initially placed in a straight line or lined up with the sheer. The key to sanding the molds to the same shape is that they are the same shape, not where they are located.

    Build one side at at time and you only need half molds. Again doesn't depend on where the molds are located.

    The problems you forsee can be avoided by placing the half-molds in their final position and then bending the strips around them.
     
  13. CatBuilder

    CatBuilder Previous Member

    I've built a set of catamaran hulls using this method.

    I don't see how it would be advantageous to use the strip planking as compared to plywood. Wouldn't it be quite hard to bend the flat strip planked panel over the mold? Also, wouldn't it take a long time to make panels from planking as opposed to using plywood?

    The pieces laid into a cylinder mold have to be quite supple and "floppy" for it to work, or it will rise up in places and not lie true to the mold.

    Also, I'm just curious... I may have missed it... when does the strip planked panel that you drape over the mold get its shape locked in and using what materials?
     
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  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    Most tape is stretchy enough although stretch is not needed if the strips are bevelled; I have built hollow masts this way and it works like a charm. The tape is removed after the strips are held against the end station forms by a strap. Intermediate straps could also be used but they would get in the way of epoxying and I think they are unnecessary. If the strip/tape assembly is bent in 2 directions the tape will likely peel off and the work to make this happen will increase the force required to bend the half-hull to the sheer plan shape.

    I think I have a solution to the bending problem; the station forms at each end of the mold are doubled - about 6" apart - and connected with battens to form a rigid frame, one at each end of the mold, to provide something to attach levers to pull on while bending.

    Ply would be better structurally than WRC strips and would not have to be glassed. Individual strips are very floppy, typically 0.75 x 0.3" WRC and bend with a few ounces of force; provided they don’t stick together the force for bending N strips should only be N times that plus a friction component. Once epoxied the whole half full is rigid enough to hold its shape, same as the ply product from regular cylinder molding.

    Only experience can show how long the process will take: compared with the cylinder mold process which is reported to be one of the fastest build methods, the time to build the mold should be no more, laying out the strips, taping them together and draping them over the mold should be less than applying 2 layers of plywood with scarf joints. Bending the mold and strips to the correct shape is an unknown but seems simple enough and applying the coat of epoxy should take very little time. Joining the two halves should be easier since there is no force required. If the glassing times does not screw everything up I think it has the potential to be even faster than the cylinder mold process = although there will be lessons to be learned the first time.
     

  15. ancient kayaker
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    ancient kayaker aka Terry Haines

    It occurs to me that the station forms would have to be close together to prevent some of the strips forming flats between them. wonder if fewer forms could be used if the strips were bead and cove type, to keep them in alignment during the bending step.
     
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