Flexible Hull Material?

Discussion in 'Materials' started by monrosm@shrewsb, Nov 27, 2012.

  1. monrosm@shrewsb
    Joined: Apr 2006
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    monrosm@shrewsb Junior Member

    I am working on a concept project, LOA:12m high speed and needs a high degree of comfort for passengers on board. I became interested in the BMW GINA car concept using fabric as a skin for the car and was wondering if there are any such flexible materials that could be used for boat hulls? A flexible material would allow the hull to subtly change shape to suit it's desired function and allow it to alter it's wetted area. The skin from the GINA is not strong enough to withstand the slamming forces produced by high speeds on a boat. Any suggestions?

  2. Petros
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    Petros Senior Member

    This concept was actually borrowed from "tube and rag" aircraft, which borrowed it from kayak construction. Go study skin-on-frame kayaks. Most who build and own them say the performance is better than a hard shell kayak due to the small amount of flex. It acts like a suspension on a car, too soft or too stiff will harm ride and handling, but the right amount of flex/stiffness improves the performance of the car.

    I have built about 11 or 12 skin-on-frame kayaks, and a number of small sailboats using the construction method. I also have an idea for a pocket cruiser with a small cabin that uses skin-on-frame construction just like a traditional kayak (but larger and with a heavier frame). boats up to 36 ft long have used SOF construction.

    The skin used on kayak, and the dingys I have built, is usually heavy (9 or 10 ox/sq yd) nylon, or polyester fabric, with polyurethane sealant. Nylon has more flex but is subject to relaxing when damp, polyester is dimensionally stable but has almost no stretch. I have also used oil based paint and latex paint as sealant. These are built, the fabric stretched over the frame, and than the sealing paint applied. There are also some that have used PVC coated polyester fabric (like used on heavy truck tarps or sun awnings), but it means you have little give in the fabric so an exact fit is critical.

    Describe your project.
  3. monrosm@shrewsb
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    monrosm@shrewsb Junior Member

    Hi Petros, thanks for the information. I did a little research on 'skin-on-frame kayaks which looks promising.

    The project is a concept project for a 12m powerboat providing an advantageous hull design for different functions.

    The vessel would have a top speed of around 50kn and cruising around 30kn, at both these speeds it must provide a secure interior environment and excellent handeling. Similar to the GINA I want to be able to modify the exterior hull with a series of actuators creating a deeper V for high speed planing and be able to reduce the V for a slower cruising speed, perhaps with the capability of altering the hull into a tunnel hull form in rough conditions.

    While the GINA uses neoprene it endures relatively little force from wind while driving, the force produced on a planing boat are considerably greater hence the need for a stronger material. I was thinking about woven kevlar?

    See Sketch attached

    Attached Files:

  4. jehardiman
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    jehardiman Senior Member

    As petros points out, almost any medium strength flexable material (such as walrus skin or intestine) can be used (even in high speed boats) like in kayaks, umiaks and other skin-on-frame construction. The hard part is making the primary and secondary structure to support the tertiary skin loads without point loading the material. Remember, the reason modern shells are the way they are is because of the material and framing system selected, change the material and you have to re-think the way you frame the shell to support.

    FWIW, making the shell material and framing system flexable can drastically lower your slamming loads...but may not change point impact loads. Sprit of Australia was sunk by hitting a soda can that punched right through the thin plywood shell, not the 270 knt slamming load.
  5. tunnels

    tunnels Previous Member

    Just as a thought !!, once made a kevlar skin for a inflatable hull bottom backed with unidirctionall glass !! ,had flex and heaps of twist !!,rode soft and went great but still had the advantages of a semi ridged hull !!.:idea:
  6. jonr
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    jonr Senior Member

    I have a V hulled inflatable catamaran that provides a nice soft ride.
  7. Petros
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    Petros Senior Member


    I like your concept however there are a couple of limiting factors. You would want a fairly heavy fabric, like 20 to 30 oz/yd2 polyester fabric with a PVC coating (they use this for heavy truck tarps), and it would have to be kept fairly tight to prevent it from fluttering at those speeds. If it flutters it will cause a lot of drag and likely wear out the fabric very fast. YOu would not want to use stretch fabric for the same reason. Also with skin-on-frame you can not have any hollows in the hull design unless you have the skin attached with tension bonding (not a good idea), or better with hard panels screwed down holding the skin in place.

    So you would have to have a mechanical arrangement where when the keel went down to increase the V, the chines would go back up the amount to keep the fabric tension the same. This is all doable, but would make a pretty complex moving frame.

    the structural frame could be done as a lightweight truss (space frame type), with the moving elements and the fabric covering, or at least fabric panels. the fabric panels should be replaceable as well of course. the fabric panels could have bonded or sewn-in battens to stiffen the unsupported parts of the skin.

    It is also possible to use inflatable chambers to tension the skin, but to prevent flutter it would have to be fairly high pressures.

    You might also consider just having hinged hard panels on the underside of the hull to changes its effective shape. Consider that early aircraft used "wing warping" for flight controls, the whole wing was twisted (of course it was light wood frame with fabric covering), it was later that movable flaps were developed to get better control of the wing shape, which aircraft still use to this day. The military has experimented with a flexible composite upper wing surface with a very complex internal control mechanism to change the shape of the wing, they made an F-111 fly without any ailerons or wing flaps, control was done by wing warping. So aircraft have come full circle now, but with highly refined flight controls.

    You might be pioneering the same concept for boat hulls. I think it is a great idea. the question is are the advantages worth the extra cost, complexity (maintenance), weight, etc. That has yet to be seen I guess, but it is worth exploring.
  8. gonzo
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    gonzo Senior Member

    It is a pretty common design. There are inflatables, Ribs and Lindsey Lord's system to name a few.

  9. PAR
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    PAR Yacht Designer/Builder

    Spectra . . . maybe over carbon tube longitudinals for real weight advantages.
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