Reinforcing a sail boat for power

Discussion in 'Materials' started by Carlfly, Mar 10, 2019.

?

Epoxy fiberglass over marine paint

  1. Yes it is doable

    0 vote(s)
    0.0%
  2. No the paint must be stripped back

    4 vote(s)
    100.0%
Multiple votes are allowed.
  1. Carlfly
    Joined: Mar 2019
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    Carlfly Junior Member

    I am converting a FRP sailing dingy to power and want to reinforce the hull to cope with 45 knots for my WIG project. Will be adding extra bulkheads.
    Hoping to lay extra layers of epoxy fiberglass over the existing internal paint, will strip it all back if required reluctantly.
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Do you know what that paint is ?
     
  3. Carlfly
    Joined: Mar 2019
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    Carlfly Junior Member

    Not sure of the paint, it is grey non slip, gritty, the undercoat is white the sanding dust
    has a very chemical smell. could make some inquires have the factory name if still in there
    after 20 years.
     
  4. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    I’d say that it’s a bad idea to glass over paint, even if the paint is chemically compatible with your resin.
    You want to join the new laminate to the old, and that cannot occur with a membrane between them.
    I understand your desire to strengthen the hull, a sailing dinghy was never designed for your target speed.
    Do you know for a fact that the hull will do that without dangerous consequenses?
     
    Mr Efficiency likes this.
  5. Carlfly
    Joined: Mar 2019
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    Carlfly Junior Member

    Yes your reasoning is sound, looks like I will be making some noise and dust in the backyard soon. Not sure what the maximum speed is for the hull, the world record is 16 knots for the type. Am taking hints from speed boat design to strengthen it. Chose the dingy for its more aerodynamic shape and lightness.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    You;d expect it to be flow-coat (polyester), but may have been painted over later, too hard to say.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Sure needs a thorough audit before attempting breakneck speeds, and unless it is a bit of a secret, might be best to illustrate what he is attempting, before anything else.
     
  8. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Strip it back for the bulkhead tabbing. You probably need about a foot with no paint. If you are adding frames every foot, that means grind everything. You probably want to build the skin up on the outside. So sand that down to glass as well. Yes, this is the easy way, of the ways that actually work. You can do a little fairing work under the exterior skin to make the laminating easier, but keep it minimal as possible. This takes only a little longer than building a boat from scratch, BTW. But the lofting is done for you, and the material budget is bit less. And try to find a hardware store that is going out of business and buy all their sandpaper. Your going to want about 1000 sheets.
     
  9. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member


    Use the coarsest grit you can find. 36 works well.
     
  10. philSweet
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    philSweet Senior Member

    Yup, a 36 grit Norton 10" PSA disc, 140 PSI of shop air, and a deft touch gets the demo done in a hurry. Then the fun begins.
     
  11. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    When I need to do aggressive removal, I go to 16 grit on a 7” angle grinder, it makes short work of it and leaves a distinctly “scratched” texture that improves bond.
    It also throws a good bit of shrapnel, so be careful to use appropriate ppe and keep it away from your body parts!
    The discs will gum up, so I keep the used ones in a jar of resin to soak so they can be reused several times.
     
  12. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I have found that a sandblaster is the best tool. It will remove paint and gelcoat without cutting the fibers.
     
    rwatson likes this.
  13. kapnD
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    kapnD Senior Member

    Oops, in post#11 I meant to say I keep the used discs in a jar of acetone, not resin!
     
  14. Carlfly
    Joined: Mar 2019
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    Carlfly Junior Member

    Thank you all for your replies, I will keep you posted of the results especially
    like sand blasting, will experiment with that. This is going to be a WIG craft so
    no hull in the water at 45 knots or less with a larger wing.

    Carlfly
     

  15. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    So are you saying that any hull for your intended application is just a structure to carry the foils?
    This only reinforces my opinion that a sailboat hull is a poor candidate for this.
    If an engine is to be used, the craft must accommodate a power plant, running gear, fuel, and more, again, not a sailboats’ forte.
    It will struggle through transitional stages, and if something breaks at high speed your problems will be compounded by a hull that will not even glide gracefully at great speed.

    Hint hint
     
    Wayne Grabow likes this.
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