hairline cracks

Discussion in 'Sailboats' started by Guest, Oct 3, 2003.

  1. Guest

    Guest Guest

    hi. i was wondering if any1 has any surgestion of how to deal with them in a fiberglass hull.
    thnx
    el
     
  2. gonzo
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    gonzo Senior Member

    It depends on the cracks. Cosmetic gelcoat cracks can be opened and filled with gelcoat and then sanded and polished. If the cracks are structural and reach the laminate, it will take grinding and relaminating.
     
  3. Tim B
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    Tim B Senior Member

    Guest,

    Gonzo's quite right. Cut back the gelcoat with an old chisel, then clean it with some acetone (wash it off quickly) and then inpsect the area.

    Press either side of the origenal crack. If there is a difference in movement, then there is a structural crack. If so, enlarge the area, only enough to work effectively, then lay in glass/carbon/kevlar as appropriate (I use a layer of kevlar and three glass). Then finish with gelcoat.

    If it is just a superficial crack, patch it with gelcoat and finish it nicely.

    If the laminate is too thin to cut back to a working depth of about 2.5 to 3mm (1/8 inch max) then two options are possible...

    1)enlarge the crack and place a backplate behind, or

    2)if it is not easy to get to the back of the repair, finish with glass and spray paint.

    I used method two on my LARK, owing to the presence of a bouyancy tank and thin laminate, It has been perfectly alright, but needs checking after it's second season for a touch-up spray.

    Incidentally, if the hull is a double-skinned cored layup, the two skins will be much thinner (1mm, not 3mm), a small crack in the skin can be repaired by clearing the core and injecting resin into the void, then finishing with cloth outside. If you have a large crack in this kind of layup, take it to a boat-builder.

    Good Luck,

    Tim B.
     
  4. Tohbi
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    Tohbi Senior Member

    if it is structural, laying up a big ole patch on the inside of the hull gives strength without uglifying the outside.
     
  5. gonzo
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    gonzo Senior Member

    Just adding more fiberglass to the inside will make a hard spot. Ultimately it will cause a structural failure. More can be weaker. A well made patch will not show.
     
  6. kayaxi
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    kayaxi Junior Member

    Mushy spot

    I posted this quetion in another thread but this seems more appropriate.....

    Mushy spot next to a mast step. It's a small area and presses down about 1/4 inch or so when I put my full weight on it. I think it's coming from a crack where the mast step meets the deck....... only on one side of the step though. The rest of the deck in that area seems perfectly firm.
    I just wonder if this is something that can be done myself if I don't mind getting a little itchy. Someone told me that I could inject something into it but the more I read the more I find that this won't do since the soggy balsa (if it is in fact soggy) will never dry out if I don't cut away the bad spots.
    Also, did Chrysler use balsa core in the '76 or '77 models?
     
  7. Tohbi
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    Tohbi Senior Member

    gonzo, i have to wonder about a "hard spot" causing a structural failure. seems the hull would have to be awfully flexible to begin with to have any affect at all. my 21' glass cutter has hard spots from bulkheads with no structural failures [built '67] but some crazing at those points. the patch should taper from thin on the ends to thicker in the middle but, even if it didn't, i would have to see such a structural failure to believe it. we're just talking about reinforcing the area behind a crack anyway. but, then, your experience is probably greater than mine.
     
  8. gonzo
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    gonzo Senior Member

    Tohbi:
    The crack nedding repair is a structural failure. The important thing is to understand that adding more is not always the solution. A hard spot localizes the stress and may cause a problem. Also, a boat built in '67 is probably solid fiberglass and heavily built. With cores, hard spots can cause delamination because they bend at a smaller area. I think it depends on the design and construction. Your boat can probably take an inside patch and do fine.
     
  9. safewalrus
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    safewalrus Ancient Marriner

    Paint and sell?
     
  10. Tim B
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    Tim B Senior Member

    you'd get through boats at a fair rate if you painted 'em up and sold them each time a crack developed.

    There is no problem with putting a patch behind a laminate to effect a good repair. I did it on the Lark not so long ago... glass fibre mat laid up on cardboard, then pulled against the failed laminate (this was for a stiffner) then just layed up woven and mat until I'd matched the thickness, then a generous size patch over the top. and white spraypaint to matchthe white interior. No problems at all, it is a perfectly sound method.

    For a smaller crack or hole, you're really just trying to put in something to spread the load and let you work on it.

    Tim B.
     

  11. safewalrus
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    safewalrus Ancient Marriner

    Tim
    it's nice to see you have a sense of humour! Now please arrange the following into a well known phase or saying - "life get a"

    It was a joke - as you state you would get through a lot of boats by this method! and of course would anybody who surveyed the boat allow it to happen (looks like they might if you were doing the job!)
     
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