Wide Gap Strip Plank - what problems could this building method cause?

Discussion in 'Wooden Boat Building and Restoration' started by rwatson, Sep 9, 2008.

  1. rwatson
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    rwatson Senior Member

    Initially, I got very interested in the way that some hulls are built with "vertical foam" pieces. The supposed benefits are avoiding all the hard work with longititudinal strips.

    vertical_foam_stripping.jpg

    I stared at the picture for a long time - then I saw the "light".

    The whole setup is laid up on ...... longititudinal strips!!!!!

    The question is - if these strips were placed about 1/4" to 1/2" apart, why couldnt you use epoxy mixed with one of the popular filling materials, to fill in the gaps, and fair off the hull later?

    The hardest bit to building in strip, is to get the strips glued tight to each other, no matter how complex the hull shape. If the gap between planks were wider, creating less problems with tight compound bends, and an epoxy filler was used, followed by 'encapsulation' with suitable strength epoxy and cloth - wouldnt that be even better than foam centres?

    heck - for really tight turns, you could use round section planks, and not even have to worry about orienting the plank in the right direction. Each strip would be separated by a waterproof and non compressible barrier of epoxy mix.

    Considering the price of foam cores, this sounds a viable solution.

    There must be a catch!!!
     
  2. alan white
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    alan white Senior Member

    The cost of epoxy?

    Alan
     
  3. TeddyDiver
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    TeddyDiver Gollywobbler

    As far as I know this tecnique is used but I'm not familiar with it. Anyway the cap isn't that wide, maybe around 1/16 to 1/8.
    The hardest part to me is keeping my fingers of the glue:D
     
  4. rwatson
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    rwatson Senior Member

    Things to think about.
    My poor math puts the epoxy use for a 28 foot, 4 metre circumference hull at about .75 litres, or $50. for an average 1 centimetre gap.
    Plus filler ( probably wood flour) so say $100 per hull perhaps?

    I should go and try a small test section. I suppose the big question is whether the time savings would be as great on a whole hull.
     
  5. rwatson
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    rwatson Senior Member

    Improved Idea

    Thinking hard again. Applying the filler between the wider gaps between the planks could be tricky, as it would get pushed out the other side unless some sort of backing was provided.

    I am toying with the idea of CONVEX plank edges, would mean that you could lay the planks as close as convenient without glue, but there wouldnt be a big gap.

    Then, once all the planks are in place, have been checked for fairness, shape etc - you could apply a thickened epoxy to each hull side in turn, in one go. The open gap would ensure a good glue coverage between the wood surfaces.
    Having the whole hull assembled without the glue would allow final adjustment and fairing before the hull was rigid.

    This should result in
    a) Reduced wrestling of individual glue covered planks, and the waiting for the glue to dry before the next planks are in place.
    b) Fewer high spots to sand off where the sharp square edges are proud of the curve.

    For sections of the hull that need to have some temporary planking alignment while the glue dries, you could leave any bracing in place, glue around it, and then fill any gaps later when the main part of the glue is dry. (like when you are doing stitch and glue, gluing around the stitches)

    I know the waiting for a few planks to dry before you can do the others was a major productivity drain. Also, despite your best efforts, there were always gaps in the glue between tightly laid planks that had to be filled later.

    Those crazy joints where the planks had to intersect were a big time waster as well. better to chop the end of the plank short, and fill the awkward bits up with 'epoxy bog'.

    Might be worth a try!
     

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  6. rfnk
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    rfnk Junior Member

    I don't think epoxy is flexible enough. As a hull flexes, glass, timber or foam take the main stresses while the epoxy or polyester is usually quite thin between the planks or in thin layers over and within glass or foam. If the epoxy is in wider or thicker concentrations it would be subject to more stress. Being inflexible, it would tear or crack the adjoining timber strips, I think, rather than playing its part in spreading the stress more uniformly. Wouldn't it?
     
  7. TeddyDiver
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    TeddyDiver Gollywobbler

    The hull would have S-glassed surfaces to take care of the diagonal (and the wooden strips of the longitudinal) stresses. All epoxy used in places where some strength is needed (like here) it's mixed with a filler (like micro fibers) giving it more. The wideness of the cap itself doesn't add stress that much..
     
  8. rfnk
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    rfnk Junior Member

    I don't see strength as the issue. Even without the filler, the epoxy is stronger than cedar or similar strips. The issue is spreading the stress. If you have a membrane that you want to be flexible, the more `hard lumps' you put in it, the more unevenly distributed is the stress. This puts unfair stress on the joins between the less flexible and the more flexible materials. Surely in a hull you're always aiming for the most even distribution of stress? I'm the furthest thing from an engineer but it seems to me if you're going to fill the gaps between timber strips with an inflexible material then there's a limit to the width of that gap before you create an unfair distribution of stress and introduce a weakness (lower tolerance of stress) that would not be present if the material (epoxy) is used, instead, to glue the more flexible material (timber strips) together. If the whole thing is being held together by glass fibre anyway, then why not just fill the gaps with foam or something? It's probably true that, aesthetics aside, timber strips don't always have to be joined precisely and epoxy can be used to fill quite visible gaps between timber strips but my guess is that if these gaps are too wide the stress will not be evenly distributed.
     
  9. Petros
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    Petros Senior Member

    I can see several problems with it: already noted is that hard brittle filler between planks would eventually, from flexing, cause damage to both the softer wood strips and the covering fiberglass; also I would think it would be difficult to sand smooth since you would be taking off the wood faster than the hard filler between planks, and eventual the more brittle filler would crack and crumble away, leaving sharp edged chucks under the fiberglass skin that will likely eventually chew their way though it.

    I have actually thought of a similar idea but with different materials. Skin-on-frame kayaks, and some canvas canoes use the fabric to hold out the water. The fabric itself is flexible to allow the frame members to flex, and all of the structural loads are taken by the frame (usually wood, but it does not have to be wood). The only function of the fabric is to "seal" the hull.

    What if you built a full sized cruising boat (25 to 40 ft loa) with a structural frame and planks applied with a 1 inch to 1/4 inch gap (less fitting and faster hull covering). The strips must be sanded and fared, and then apply a very heavy (like 28 oz) polyester or nylon fabric skin, with polyurethane or even hypalon paint sealant applied after the fabric is secured. It would have noticeable "lines" on the hull, but not too much since the planks are rounded to match the curve of the adjoining planks. The skin fastening could be held on and or covered with metal or wood (or even fiberglass) trim strips or rub rails.

    To service the hull the skin could be partially removed, repairs done, and then replaced. If the skin wears out or gets badly damaged it can be replaced. This construction method would be much faster and less expensive than conventional plank hulls.

    The only disadvantage is you can not have hollows in the hull design (you can actually but you would have hand stitch the fabric into the hollows between the planks, but it is not pretty). Also the inside of the hull will have a lot of little gapes to to catch debris and garbage, so it would be hard to keep clean (fill these gaps from the inside after the skin is on with silicone or latex caulk perhaps?)

    This is already done on deck topsides, and it is done on canoes and kayaks, why not much larger boats? Some day I was thinking of trying this out. I have built 7 sea kayaks this way, and one 14 foot sloop.
     
  10. rfnk
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    rfnk Junior Member

    I think there'd be three problems with a `skin on frame' approach to building a cruising yacht as opposed to a skiff or kayak. Firstly, the design provides little resistance to impact, especially from anything sharp, unless some sort of bulletproof membrane could be made. Secondly, assuming there's a skin on the inside as well, you'd have a whole lot of voids. The expansion and contraction of the air in these voids from temperature changes and the everyday pressures of sailing would introduce huge amounts of water into the structure, causing all kinds of problems. Thirdly, wouldn't the hull be too flexible if constructed in this way (affecting performance)?

    If the skin is so thick that it's impact resistant, so strong that it can't allow water ingress from `pumping' and so stiff that it can't flex too much, isn't it then just a GRP boat? In other words, a neat way to create a mold. You could peel the strip planks out altogether afterwards.

    People have sailed skin on frame boats from Ireland to Greenland etc. but as far as I know they didn't have much success selling tickets to anyone wanting to go along for the ride!
     
  11. rwatson
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    rwatson Senior Member

    Thanks for all the input guys. The comments have given me food for thought.

    The points about dissimilar sanding rates are very valid. Though, epoxy, mixed with Microballons and such is pretty close to light timber strips, so that doesnt worry me unduly.

    The comment about putting foam between the planks is valid - this is essentially what filling with an epoxy mix would do - but without all the cutting out and fitting - and probably expense.

    The advantage a light cored hull has over just a pure fibreglass one is that the thickness bewteen two outside f'glass skins makes it more rigid for less weight.

    I would never consider just a flexible skin, as the puncture potential is not worth it.

    The biggest point I need to test is if the two dissimilar materials do create hard spots that would crack the outside layers.

    I guess I would have to set up two test panels on some kind of mechanical torture machine, and see if I get early failure using the method.

    I know foam cores put a lot of emphasis on mechanical properties to support the two outer layers, so perhaps using dissimilar materials could cause problems in the long run.

    I am going to do some panel testing!
     
  12. TeddyDiver
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    TeddyDiver Gollywobbler

    Thats exactly what causes cracking...
     
  13. Butch .H
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    Butch .H Senior Member

    Ok you could have started this thread sooner you know then I would not have stressed out so much:mad: . Answerd most of my questions:D
     
  14. srimes
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    srimes Senior Member

    What if you think about it from the other direction. Here you're talking about taking a core and starting to space it out. What if you take conventional construction solid fiberglass hull with wood stringer and start moving the stringers closer together? From this viewpoint the space between the stringers isn't filled with thickened expoxy or foam, it's just fiberglass that alternates between solid and "cored."

    If done right I think this could result in a pretty boat that isn't too "plastic" looking.
     

  15. rfnk
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    rfnk Junior Member

    Except that the stringers can't really be integral - there has to be an edge where the glass stops and the timber begins, and this is where the cracking is likely to occur. You can reinforce a GRP hull with timber stringers but they don't interrupt the GRP; they're fastened onto it rather than through it. If the `stringers' run through the structure, you're creating edges all over the place.
     
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