Cedar strip / epoxy

Discussion in 'Materials' started by sighmoon, Oct 17, 2007.

  1. sighmoon
    Joined: Oct 2007
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    sighmoon Junior Member

    Hi all,

    I'm looking at building a light displacement sailing yacht, 6m long, 2.4 m wide, from my own (developing) design. It's a bit like a scaled down Mini650.

    This is my first project, and I know very little. The plan is to build in epoxy saturated cedar speed strip, as (I'm told), it's relativey easy to achieve compound curvature with this technique.

    My first drawings give it a displacement of 750Kg(ish). Which, subtracting 250kg for ballast, and 150kg for crew, leaves 350Kg for the tub. Is this realistic?

    Can anybody give me a figure for the density of Epoxy saturated Cedar? And point me in the right direction to work out how thick it ought to be?

    I've read on the net that Cedar is soft, so it needs to be glassed in, but is this really the case if it's epoxy saturated? Having tried sanding Epoxy in the past, I suspect not.

    Can anybody reccomed any books on building with this technique?

    Are there other techniques / materials I ought to look at? I fancy some sort of wood, as the inside of a wooden boat is a beautiful place.

  2. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    Epoxy saturated cedar----- meaning glassed inside and out...
    The cedar is about 26 lbs per cubic foot. The glass and epoxy schedule should be done to a small sample piece of cedar and then weighed and calculated. Different weight glasses will absorb differing amounts of resin.
    Quick test: A (e.g.) 20" stick of any uniform cross sectional dimension that's either epoxied or sealed with a wax coating is up-ended in a bucket using a loose pipe to hold it vertical. It comes to rest with a certain amount below the water and a certain amount above the water.
    The ratio above to below tells you the weight of the stick per cubic foot. If, for example, the stick is exactly half submerged. that means the hull material will weigh exactly 31 lbs per cu. ft., half that of water. A typical cedar strip and glass hull test piece will weigh about that.

    Any material weighing less than water can be weighed this way. It is a specific gravity test.
  3. nero
    Joined: Aug 2003
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    nero Senior Member

    For $15 you can get MacNaughten's Strip planking scantling booklet. It will help you to calculate the strip thickness and glass schedule. Well worth the money.
  4. Gilbert
    Joined: Aug 2004
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    Location: Cathlamet, WA

    Gilbert Senior Member

    I don't think 350kg for the hull weight will be far off. I believe 1/2" thick strips will be about right. If you like stout go 5/8". You will definitely need to glass inside and out unless you use frames. If you lay 90 degree biaxial cloth, lay it at about 45 degrees to the centerline so both the warp and weft are working together to keep the strips aligned. The strips provide all the longitudinal strength you need. Of course you need to engineer your keel attachment structure and your mast step and rigging attachments.

  5. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    This 'saturated cedar' technique isnt a recognised boat building methodology. I am just finishing off a cedar boat, and my experience is that cedar that doesnt have fibre in it would be very prone to fractures.
    The trick would be to make a couple of test panels, one with fibre, one without. My experience is that impacts on non fibre samples will fracture like glass (and the impact zone will get pushed into the timber). With fibre, the load is spread out and the damage is much less intense.
    There are a lot of comments on this site about how unfibred epoxy develops these stress fractures if it is unreinforced, thereby letting in water.
    If weight is a consideration in your design, thin cedar planks epoxy/fibred on both sides is the way to go.
    Dont forget, that even using epoxy as a glue, the strength of the plank joins can be a much smaller percentage than the strength of the timber, and it will vary on how lucky you are with the accuracy of the plank joins. Without a webbing of fibre, the impact strength is very small on thin soft cedar planks.
    I dont think leaving fibre out of a design will save significant amounts of weight or money, and if you have .25 tonne of ballast, then the weight of fibre is a minor concern.
    The previous suggestion of using frames inside instead of a layer of fibre/epoxy has to be weighed up against certainty of the cedar getting wet in the inevitable breaks in varnish/paint/glue in the interior, that let moisture in, especially in a small craft that is bound to get wet inside.
    I found the Gougen Bros book on the prevention of rot/mold etc emphasised the desirability of complete encapsulation for both strength and rot proofing.
    The thicker planks (5/8") would also have the advantage of allowing more 'intense' fairing, both inside and out. Its amazing how much timber gets sanded back off.
    Slightly off the subject, I am wondering if it is almost worth putting a layer of packing tape ( that epoxy doesnt stick to) on the inner side of each plank before installing it, would save a hell of a lot of cleanup work on hull turnover. Fairing and cleaning the inside of the hull is reeeeealllly tiresome!
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