Keel abrasive protection, HDPE

Discussion in 'Boatbuilding' started by nimblemotors, Mar 6, 2013.

  1. nimblemotors
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    nimblemotors Senior Member

    I am resurfacing the bottom of my boat. It is a shallow draft catamaran which may often sit on the ground at low tide. I'm looking for suggestions on what I might add to the keel to prevent damage.

    Was thinking hdpe, but how to attach it, and what about anti-fouling?

    [​IMG]
     
  2. Stumble
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    Stumble Senior Member

    Metal strike plates also come to mind. And they could be epoxied into place.
     
  3. michael pierzga
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    michael pierzga Senior Member

    Metal deforms, dents and is hard to paint.

    Why not chafe plates made of Eglass ? Its tough stuff. Lay up several layers of 12oz biax on the bottom over mylar tape, remove, trim to size, then Adhere them..no fasteners. Perhaps 5200 to the bottom.

    use copper powder in the layup and you need not antifoul.

    when they are old and damaged remove and replace.
     
  4. nimblemotors
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    nimblemotors Senior Member

    Something like this:

    [​IMG]

    Being able to replace it once it becomes worn is a useful feature.
    One thought is to epoxy in some threads in a subsurface and
    then attach strips of HDPE via screws, so no through-hull.

    Otherwise, what about "truck liner" coatings?

    [​IMG]
     
  5. pauloman
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    pauloman Epoxy Vendor

    cheap fix is epoxy mixed with sand. (rock against rock). when it wears down just mix up some more
     
  6. PAR
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    PAR Yacht Designer/Builder

    Decomposed granite is the stuff I've used for this sort of thing. It's good on appendage leading edges too.

    Paul I tried to call today - will try again Monday.
     
  7. MikeJohns
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    MikeJohns Senior Member

    Paul(s)

    Do you mix some other filler along with the hard material to fair the surface ? I'd hate to drag the dinghy up some of the lovely lauching ramps if it were effectively abrasive grit 40.
     
  8. michael pierzga
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    michael pierzga Senior Member

    If you have to drag it over concrete use sacrifical wooded skids on the bottom. Avoid fasteners, use an adhesive.
    The pictured hull with sheets of HDP mechanically fastened to the bottom will suffer when the fasteners begin to move and leak. It will have a short life.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Coating a bottom with a stone filled epoxy is a bit over the top and a real ***** to repair or shape. Under the cured goo, it's as rough as you want it. It can tend to be a bit like wood dust from a sander and clumpy or nugget filled, so smoothing it out with a tool is difficult, but I tend to cast these types of things, which permits other options, like a few base coats in the mold, with straight epoxy, presenting a smooth, clean surface on the exterior of the piece when cast.

    Wood is the traditional material and works okay, though metals and plastics can be very effective. In all cases, you need to do several things to insure no new issues afterward. The first is lightly fastening the sacrificial pieces, regardless of material they might be. I most often use a #6 flat head for these, knowing in a really hard strike they'll break off. Well bedding the piece should go without saying, but epoxy bonded fastener holes is another concern, that can prevent leaks and other problems from occurring. With plastics, you need elongated slots for the fasteners, so material expansion which is high in the plastics can occur without ripping out the fasteners.
     
  10. pauloman
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    pauloman Epoxy Vendor

    the sand in epoxy approach is fairly common in places like the pacific NW where river fisherman drag their boats up on gravel sandbars etc. It eventually wears off and more is slapped on.... You can get some pretty fine grained sand to mix with the epoxy, so it can be pretty smooth. Also, a quick topcoat of straight epoxy would remove any texture.

    again, this is a cheap, on the fly type of fix that will not rust, easily come off etc. If necessary you would probably remove with a heat gun which would soften the epoxy and let you scrap it off.
     
  11. nimblemotors
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    nimblemotors Senior Member

    The hulls currently epoxy over ply, to add another layer of epoxy seems a problem of just making it thicker, so it can't be removed without taking off all the existing epoxy.
    The bottom is basically flat now, so was thinking of add a protruding sacrificial strip.

    Another thought was a strip of copper sheet?
     
  12. michael pierzga
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    michael pierzga Senior Member

    How will you hold the metal chafe strips to the bottom ?
     
  13. nimblemotors
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    nimblemotors Senior Member

    PAR suggest small screws earlier, bronze screws I gather for Cu sheet.
    But if the sheet is very thin the screw heads would protrude slowing the boat. So the copper would need some thickness, starting to sound very expensive now. Epoxy won't stick to copper?
     
  14. jonr
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    jonr Senior Member

    Use aluminum oxide if you are going to add a powder.
     

  15. PAR
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    PAR Yacht Designer/Builder

    I wouldn't screw sheet metal to the bottom of a boat. By metal, I usually mean sacrificial strips, rather then sheet metal. Half ovals of bronze or stainless are the usual choices, though I've used 1/8" aluminum flat bar stock too. Epoxy sticks to copper very well.

    The logic behind light fasteners is so they break off, rather then tear up the bottom of the boat as they resist being beat up.

    Aluminum oxide isn't the best "hardening" filler material you can employ. It's typical use is for as a thermal break and temperature control, not as abrasion resistant filler. Harder materials (aluminum is very soft) are the usual choices. Iron fillings make an excellent choice, though they can rust if exposed. Copper is relatively soft, but much better than aluminum. Bronze and monel dust is good, but costly, as are Kevlar fibers. Sand and stone are cost effective alternatives. Sand is typically sandstone and quartz, both relatively soft compared to decomposed granite and other, more noble stone types.
     
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