Spotted gum and fg hull extension.

Discussion in 'Materials' started by d. right, Jan 6, 2018.

  1. d. right
    Joined: Jan 2018
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    Location: kuranda,australia

    d. right Junior Member

    Hello all. New member here. I am doing a hull extension on a 5.3 meter Swiftcraft Dominater.I was going to use marine ply for stringers and transom rebuild but have noticed I can get spotted gum kd decking timber from bunnings. Proposed plan was to extend stringers through slots cut in transom and build hull extension on this. Thoughts please.?
    Cheers, d.r
     
  2. rwatson
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    Location: Tasmania,Australia

    rwatson Senior Member

    EDIT : oops, got the context wrong in the original post, it IS durable, as you would expect for decking. but
    http://www.parksidetimber.com.au/pdf/spotted-gum.pdf has it at about 1 tonne per cubic metre, whereas plywood is less than half that.

    Check out Kiri ( Paulonia) in your area, Port Phillip Plantation Shutters in melbourne are a good source. Its nearly as light as Balsa, has similar strength and bend as Western Red Cedar, rotproof as Huon Pine, and is plantation grown. It's a joy to plane and work, with non carcinogenic dust.

    EDIT PS "The wood (spotted gum) is slightly greasy and gum veins are common" so Epoxy adherence might be iffy. Kiri has no such problem.
     
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  3. d. right
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    d. right Junior Member

    Tar mate, the s/g I was referring to is sold as pencil rounded 135x19 decking. I have worked on old trawlers built out of this timber and thought if it was sold as spotted gum in oz then thats what it was. Thanks for the options.
     
  4. d. right
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    d. right Junior Member

    Bunnings, hmmmm.
     
  5. rwatson
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    rwatson Senior Member

    Check out my edited response, I read the wrong info about its longevity.

    The spotted gum weight is a problem, as its greasiness, but timber is a lot stronger than ply.
     
  6. d. right
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    d. right Junior Member

    Understand the weight issue but i dont want to worry about rot, anyway I ll only be using about 20 l/metres of this product. Adhesion od poly resin is my only issue.
     
  7. d. right
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    d. right Junior Member

    When I was 12 my dad and I built an Arafura Cadet cat. I seem to remember a red powder mixed with water to make the glue, was this an early form of resorcinol. ?
     
  8. PAR
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    I work with a number of Australian builders and the solid lumber you want to use is a pretty common choice, though can be troublesome to epoxy encapsulate, which you'll want to do to insure longevity and water tightness. You'll want to strip out the oils, likely with a mix of xylene/toluene and acetone and a soft scrub brush. Lather it on, a light scrub, then let it sit to evaporate, taking the oils with it. Once this is done, which also kills spores and other "beasties" that might be present, you can use epoxy normally as a coating and/or an adhesive. This stuff (spotted gum) is pretty tough stuff and I wish I had the hardwood choices you folks do down there.

    Yep, the glue you remember was likely resorcinol and this can be temperamental to work with, requiring very good joint fits, lots of clamping pressure and temperature considerations, plus product shelf life concerns. Epoxy and to a lesser degree, polyurethanes have all but replaced this stuff, but it is still available if you want to play with it. The nice thing about epoxy is you don't need tight joints and the only clamping pressure you need, is just enough to make the epoxy contact both surfaces.

    As to you idea, it should work, though you don't have to notch the transom for the stringers. In a perfict world, these should be continuous, but often they can't be, so knees, metal brackets and other additional reinforcements are used, mostly to save time and effort, particularly on out drive setups. Knees are commonly used on outboard arrangements.
     
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  9. jorgepease
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    jorgepease Senior Member

    Why wood, you don't want rot, use foam light density for stringers and Coosa high density for transom, no rot.
     
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  10. PAR
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    PAR Yacht Designer/Builder

    I hear this all the time and the answer is always the same, price up a 1"x4"x8' length of solid timber, then do the same for Coosa, foam and the fabric you'll need to mimic what the timber does, all my itself and the logic becomes clear.
     
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  11. rwatson
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    rwatson Senior Member

    I wouldn't even THINK about Poly with timber. Epoxy is the ONLY solution with timber. It isn't an issue price wise, really, but IF you end up using a potential rot core, you should NEVER use non-waterproof Poly resin.

    I also think jorge has a great answer, use high density foam with Poly. Par reckons that the price compared to timber is a factor, but for such a relatively small use of core material, I don't think it will work out much of an issue. Dont forget, you waste a lot more timber with odd shapes, and cutting is more problematic. Think about being able to say to the next buyer of the boat "I used un-rottable heavy foam" instead of "there is timber in there, the surveyor will need to do some test drills"
     
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  12. d. right
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    d. right Junior Member

    Thanks Par, always respected and enjoyed your erudite comments. The stringers will be carried through the transom and be incorporated into the hull extension.
     
  13. d. right
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    d. right Junior Member

    Thanks rw,
    Im hearing you rw, when I demolished the stringers they appeared to be meranti glassed over from an earlier rebuild. As a carpenter I have learned that timber like meranti , spruce oregon etc have no resistance to freshwater moisture at all, in fact oregon is banned from exterior use in Queensland. Having said that I did build a hollow mast for my cat out of aircraft grade spruce (i believe) which my dad bought.One thing my dad told me, saltwater wont rot a boat. Pars suggestion of stripping oils and tannins from the spotted gum makes sense to me, also a bit of weathering in our fierce NQ sun might help. Im on a budget and did look at epoxy, might still go down that road. As far as foam for this project goes, doesnt seem to me a good idea to introduce a lot of exotic materials into this small project. Additional information, my other 18 footer was still in regular use. Found a soft spot in the deck which when investigated extended to rotten stringers and framing from bow to stern.Also rw I think isopthalic resins( according to the interwebby ) are an improvement over the earlier polys. Still seeking knowledge. Thanks to all.
    d.right

     
  14. d. right
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    d. right Junior Member

    Thanks jorge, aussie spotted gum is a premium hardwood, used for wharves, pylons and bridgedecking before concrete became commonplace, this stuff will not rot.
    Cheers, d. right.
     

  15. PAR
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    PAR Yacht Designer/Builder

    Consider epoxy. The less than desirable, rot prone species, suddenly become very rot resistant, once buttered in goo, particularly with 'glass fabrics too. It's also the best adhesive for bonding wooden elements too. Since these stringers will be buried in the bilge, you'll need to tab them onto the hull shell, so epoxy is the reasonable choice.
     
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