Cold molding wood veneers over steel stringers.

Discussion in 'Wooden Boat Building and Restoration' started by mcm, Oct 24, 2010.

  1. mcm
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    mcm Senior Member

    How is it done?

    Is there an increase in hull strength, and is it worth the effort?

    Dave Gerr doesn't specifically mention scantlings for steel frames when laminating a cold molded wood hull skin.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    It is like any other structure that has more than one material, it behaves as a composite.

    Thus any increase or decrease is dictated by where you place the higher strength material and how much of it.
     
  3. apex1

    apex1 Guest

    Before trying to calculate scantlings one should rethink the idea in general.

    Cold moulding over steel frames is not a good method. Though it was done in the past.
    Too different the thermal behaviour of wood and steel.

    Go either for a steel boat, or for a wood epoxy boat.

    Regards
    Richard
     
  4. dskira

    dskira Previous Member

    Due to the nature of the stringer, wood is the most perfect material you can find.
    Once I saw a aluminum boat with wood stringers and wood frames and keel. the alu plating was screwed to the stringers and frames and keel. It was quite interresting.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Bad combination too.

    As Apex alreday noted above with steel, many other factors also need to be considered. In this instance, the wood -acting like a 'sponge'- will cause poultice corrosion of the ally. Not a good combo!:(
     
  6. wardd
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    wardd Senior Member

    how did they do wooden planking over steel frames?
     
  7. mcm
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    mcm Senior Member

    In other words, gluing a wood furring strip to a steel stringer could result in the glue joint cracking due to the different thermal expansion and contraction rates between wood and steel?

    I suppose if i cut a groove along the wood furring strip and allowed it to slide freely along the edge of a steel stringer without gluing them that might eliminate the fore and aft expansion and contraction problem.

    But then there would still be the side to side contraction to counter.

    Moreover, if one also wanted a ballasted steel swing keel swinging up into a steel case, then one would no doubt use steel floor frames to hold the steel swing keel case

    How would that connect to a wood laminated hull skin without causing expansion and contraction cracking?
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    If you do that, there is no shear connection between the two materials, therefore, each material will act as if the other is not there. In otherwords, no benefit, other than adding weight!
     
  9. Corley
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    Corley epoxy coated

    Are you talking about the final structure having a steel frame inside a cold moulded skin or simply using a steel frame as a mould for the original cold moulding process either way it doesnt seem a good idea. I think most builders would recommend making timber laminated frames if you are looking for strength they are beautiful and extremely strong. I think additionally you will find that swing keel cases in most (all?) cold moulded boats are not steel rather timber or plywood with reinforcement where necessary.
     
  10. mcm
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    mcm Senior Member

    Thanks Ad Hoc, you're right, that would defeat the purpose.

    Thanks everyone for your input, i'll forget the steel.
     
  11. PAR
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    PAR Yacht Designer/Builder

    You don't have to forget the steel, but it's a problematic set of issues you'll need to contend with. This was a fairly common building method until truly waterproof adhesives entered the picture in the mid 20th century. Steel (or better yet bronze) floors is an element I often employ if costs permit (bronze floors are generally lighter then wooden, but cost considerably more).

    A metal frame, wooden skinned boat can be quite light weight, much lighter then conventional or traditional wooden methods (carvel), but not lighter then more modern methods of wooden yacht construction.

    So, in the end you have to try to do service to the design brief or SOR and entertain a build method that works within budget and weight restraints. In other words, clever engineering with modern techniques, usually forces your hand in material selections.
     
  12. gonzo
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    gonzo Senior Member

    Metal engine beds over frames or stringers are still common. They can spread the load without the extra height a wooden bed would have. It is possible to use a combination of materials. However, a monocoque coldmolded wooden hull would be lighter than a metal framed one. If you really want to use that technique, an adhesive that is flexible would work well. Sika and Saba sell products for those applications.
     
  13. mcm
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    mcm Senior Member

    Thanks gonzo, i checked the Sika site, and i see what you mean.
    But like ad hoc warned i'm still concerned about whether something that flexible can still create the necessary strong I-beam like connection between the skin and the stringer.

    Still, i remember that Larry Pardee used steel floors for his carvel planked blue water cruiser.

    Hmmm,,,
     
  14. PAR
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    PAR Yacht Designer/Builder

    Larry thinks epoxy is a fad, so take his comments with a healthy dose of your favorite grog. Though in his defense, steel floors are a good weight saver in traditional wooden construction, naturally bronze is much favored.
     

  15. keith66
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    keith66 Senior Member

    A project of mine is a 1905 Saunders launch, though built of Consuta system sewn plywood she was built with steel ring frames & bracing to her full length engine beds.
    Almost certainly done to reinforce her hull for a large engine for racing,the frames did not actually touch the hull only across the keel, stringers & beamshelf. Other boats often fitted with steel frames were scandinavian meter boats, my point being that on all of these rot is a common problem as the steel will eventually start rusting, on my launch she got away with this as the frames were clear of the planking.
    There will be little difference with steel stringers, different rates of expansion, condensation & eventually rust & rot.
     
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