Cedar/Composite & moisture in core.....

Discussion in 'Boat Design' started by xpz319, Jan 19, 2008.

  1. xpz319
    Joined: Jan 2008
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    Location: north carolina

    xpz319 New Member

    I know that most woods expand when their humidity goes up to the point that glass over wood below the water line is not advisable. However, I notice a there a fair number of boats (that are NOT drysailed), that are composite over cedar. Is cedar somehow less vulerable to this issue or is this true of all wood / composite construction? Thanks!
     
  2. alan white
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    alan white Senior Member

    Cedar is indeed less vulnerable in two important ways. First, cedar is inherently less prone to rot than practically any other softwood. Second, cedar has a springy nature (dimensional memory). While many other woods will not fare well when expanded within a finite space, cedar is less apt to suffer the crushed fibers that prevent other woods from returning to their original dimension.
    All carvel boats must suffer a bit of expanding and contracting. Cedar is a superb planking wood where the seams are butted, especially if the boat is hauled seasonally.
    In the case of wet core, if cedar has been used as a coring material, the best practice is to use epoxy not only as a surface resin, but also as a means to adhere individual planks. The function of epoxy in the seams is twofold. The epoxy will not fail due to moisture ingress, and each plank will be hermetically seperated from its neighbors.
    Hence, if the job has be properly done, a puncture or gouge may allow water in, but if that plank or planks is drilled and allowed to dry (e.g., by maintaining a fan blowing on or through it), the repair will be as good as new.
    Like any wood, discoloration can sometimes be seen when the area's drilled---- cleaning the stuff off the drill bit and laying it on a piece of paper tells the story. Is it black (rotted) or just wet cedar? In most cases, cedar is in fine shape, it hasn't lost its bonds, and all it needs is to dry out and be resealed.
    It is only cases where improper construction techniques were used (inferior planking glues, poor fits leaving cavities, sloppy fastening or improper fasteners, wood of too large a dimension, etc.) that repairs can become difficult and overly costly.

    Alan
     
  3. TeddyDiver
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    TeddyDiver Gollywobbler

    In sealed composite construction you can use allmost any wood you wan't to. When wooden hull is covered with GRP only from outside it's (my humble opinion :) ) just not a composite...
     
  4. alan white
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    alan white Senior Member

    True---- I'd say the hull is only sheathed. I do like cedar personally for it's low cost, light weight, stiffness, resiliance, and of course it's resistance to rot.
    I would also consider spruce for planking core for even more strength for its weight, though have yet to see it favored over cedar. Here, spruce is plentiful and it is perhaps the least expensive of all common woods, being even cheaper than pine.

    A.
     
  5. TeddyDiver
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    TeddyDiver Gollywobbler

    I was a bit uncertain what phrase to use:) You know, I'm one of the foreigners using funny english...
    Anyways here the cheapest and easiest are pine and fir. And they got nothing to do with Douglas fir or Oregon pine;)
     

  6. alan white
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    alan white Senior Member

    "Sheathed" means covered. Also, "skinned".
    I think the fir is very similar. The pine may be similar to some of out American pines---- many small round knots? There are so many varieties of each species of pine or fir even within a given geographical area.

    A.
     
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