S&G bottom material

Discussion in 'Materials' started by bobbrown, Jul 7, 2021.

  1. bobbrown
    Joined: Jul 2021
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    Location: SE Georgia

    bobbrown New Member

    Planning a small skiff to fish shallow water. Lots of oyster bars. Concerned the one coat glass and epoxy won’t hold up to the scrapping. First...would this be a concern and second..how would you address it. Example..multi coats, Kevlar, graphite?
    Thanks to all
     
  2. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Yes. A valid concern.

    Some guys on boat builder central are using graphite.

    I'd want at least 18 oz glass on the bottom. For real major abuse, graphite.
     
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  3. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Two layers of 10 oz woven cloth on the bottom and one on the sides should do just fine. Add a third layer of 6" tape over the chine. This is bulletproof over 3/8 to 1/2 ply S&G construction. Fill the weave completely with more coats of epoxy, then prime and paint. If you are using fir marine plywood, it is rather soft and can still be dinged a bit. Over fir, I would first fill the grain with two coats of epoxy before draping the glass. This basically makes the cost of the panel about the same as if you just went with okume or meranti to begin with. These stiffer and harder sheets have a better surface finish and don't need the undercoats. This should take care of anything up to about 40 hp.
     
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  4. Kayakmarathon
    Joined: Sep 2014
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    Kayakmarathon Junior Member

    Two kevlar layers. The first layer starts 3-4 inches above the waterline on starboard, and ends at the port waterline. The second layer starts 3-4 inches above the port waterline and ends at the starboard waterline. The final layer is 10 oz fiberglass cloth running 6 inches above the port waterline to 6 inches above the starboard waterline. The fiberglass will help seal the kevlar beard.
     
  5. fredrosse
    Joined: Jan 2005
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    Location: Philadelphia PA

    fredrosse USACE Steam

    Graphite for abrasion resistance? In my experience this is not the stuff for abrasion resistance, an alternate filler for epoxy mix is far better for this service.

    The use of epoxy & fiberglass coatings on plywood hulls is extensive, with many fillers available to mix with the epoxy for various purposes. The graphite option is to act as a solid "lubricant", suitable to reduce sliding friction. But that is not what is needed for a beaching condition, what is needed is a tough material that presents abrasion resistance, and for that duty white powder called "cabosil", mixed into an epoxy paste is far better than the graphite mix for enduring abrasion.

    I went through the process of putting on a heavy coating of graphite powder/epoxy on the keel of my steamer, assuming it was best for grounding, and it has been grounded and beached many times, but I replaced the graphite with cabosil/epoxy mix, very hard stuff.
    [​IMG]
    and this holds up much better than graphite.
     
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  6. latestarter
    Joined: Jul 2010
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    Location: N.W. England

    latestarter Senior Member

    When you have decided on a specification here are some tips on applying it.
     
  7. hoytedow
    Joined: Sep 2009
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    hoytedow Carbon Based Life Form

    To that I would add that I built up the leading edge at the stem with additional glass to form a strong point for initial impact with the bottom.
    https://www.boatdesign.net/attachments/wp_003415-jpg.95391/
     
    Last edited: Sep 14, 2021
  8. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I admit I have had some doubt about using the graphite on oyster bed bottoms. But they keep doing it!
     

  9. cracked_ribs
    Joined: Nov 2018
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    Location: Republic of Vancouver Island

    cracked_ribs Senior Member

    On really harsh bottoms the graphite gets cut up; it works better if you're dragging over sand than over barnacles. It's a pretty good wear surface but not great at resisting penetrative forces. I put graphite on the bottom of a dinghy that got beached roughly 500 times a year and it did well but it's not indestructible, that's for sure.

    Dynel, if you really want something to take a beating. But man, does it soak up epoxy.
     
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