Large Depressions in Hull

Discussion in 'Fiberglass and Composite Boat Building' started by SailingWithFriends, Jan 16, 2021.

  1. wet feet
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    wet feet Senior Member

    That style of trailer support is notorious for causing damage.It gets worse if tight straps are holding the boat down while weight accumulates.Before attempting anything the boat needs to be well supported along the keel line and as the OP has learned some lateral support should be provided where bulkheads or framing is located.Before getting too far down the road of applying heat I would be inclined to get inside the boat with a few plywood pads a post and a small bottle jack.It may well be that there is enough flexibility in the laminate to push out at least some of the distortion.If some can be recovered,I'd leave the force applied for a few days and then try to get the rest about right.Once all the available gains have been made a longitudinal foam stiffener with some drain tubes through it and glass over it should retain the shape.It would be easier to nudge the laminate into place when warm,but I'd shy away from the kind of temperature that would turn the gelcoat brown.

    I do know of an IOR era hull that was fitted out while having inadequate keel support and the plywood fronts of the quarter berths were bonded in over this kind of depression-locking the hollows in.
     
  2. gonzo
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    gonzo Senior Member

    Go sailing. The fish won't mind what the bottom looks like. All polymers will creep (deform over time when a constant force is applied). It is possible to heat up the areas and push them back, but you need to be careful not to set the boat on fire. Polyester resin is a thermoset. That means that heat can get it to "glass transition temperature" at which is softens. Beyond that, the cured resin will burn but never liquify.
     
  3. SailingWithFriends
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    SailingWithFriends Junior Member

    The boat is going to be for yacht racing against the local J24 fleet.
     
  4. SailingWithFriends
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    SailingWithFriends Junior Member

    Does that means I can use my heat gun to soften the FRP shape it and then it will reharden afterwards?
     
  5. bajansailor
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    bajansailor Marine Surveyor

    Do you know yet what sort of rating you will have, as to if you will be competitive against them, or are you not too worried about that aspect?

    And have you acquired the boat already, or are you still considering it?

    If the latter, then it might be easier to look for a J 24 instead, and then you can enjoy true one design racing?

    I found a couple in Oz for sale (although the one in Sydney is 'under contract') -
    J Boats For Sale in Australia | Boats Online https://www.boatsonline.com.au/boats-for-sale/j-boats/
     
  6. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    No. It does not soften like a thermoplastic, which can be heated and shaped. Then when cooled retains that shape.

    Heating it will allow you to move it a bit easier, it needs to be permanently secured in place or it will return to the warped shape.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Just seems too hard, trying to use heat on large areas, if you don't burn the boat, you'll probably burn yourself, I wouldn't even try, I would be more inclined to try and install some extra internal framing, and force it into place where the depressions are. AT least you could try in one area, and see what happens, and it not successful it isn't the end of the world.
     
  8. SailingWithFriends
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    SailingWithFriends Junior Member

    I have the boat already. 1500 lbs against 3000 lbs and same sail area, same length. I'm thinking I might be in with a chance. The older j24 here in Australia is built by a different builder and about 200 to 300 lb overweight. The correct weight boats are exy compared to my RL and the RL is a development class.

    The RL is a great boat with 500 in the USA. Maybe we can chat about this aspect in a different thread?
     
  9. rxcomposite
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    rxcomposite Senior Member

    Agree. Thermoset plastics has a "memory" and returns to its original shape after a time. If you want to hasten the process, heat it locally.
     
  10. gonzo
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    gonzo Senior Member

    Yes, it will take the new shape.
     
  11. SailingWithFriends
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    SailingWithFriends Junior Member

    Sooo... plan A then is to heat the area with a heat gun... install some thing that holds the shape from the inside like a big flywheel puller in reverse, leave for a few hours, then remove the shaping device and hey presto we have a convex hull shape there... does this hold water? Will I be needing the shaping device or will once heated it will go to its design shape?
     
  12. gonzo
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    gonzo Senior Member

    All you need is a few sticks or boards and maybe wedges to hold the shape until it cools. Install the sticks to push the hollows out and then heat the area.
     
  13. SailingWithFriends
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    SailingWithFriends Junior Member

    The big question is ... thanks for your answers by the way... which of the answerers have actually done this before?
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    I have never heard of anyone doing it, but if Ondarvr speaks of it, I'm saying it isn't an urban myth. It puts me in mind of that technique of using suction to get hail dents out of car bodies, I wonder whether a similar vacuum effect might work.
     

  15. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    I would only use heat with great caution.I have had depressions caused by over tight trailer straps return to normal after a few days of warm weather and without any force being applied.The process was aided by having no load at those points.If I had to change the shape of this particular hull the process has been described in my earlier post.Mechanical force,spread over a reasonable area and then bond some reinforcement in place.It could even be that simply leaving the boat afloat for a few days would be sufficient.Those trailer supports caused the problem and will do so again unless they bear on a surface with less chance of deflection or unless you replace them with something that spreads the load.It is fundamental to supporting any hull that you support the keel to bear most of the weight and then get lateral stability by chocking at points where there may be bulkheads or substantial support framing.The summary is that somebody bought a trailer and pushed the rollers in until they touched the hull-without checking what structure was in place.Maybe the rollers could have been moved to a point where there was some internal structure.
     
    bajansailor likes this.
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