HELP! Hull is Sagging! I don't know what to do!

Discussion in 'Boatbuilding' started by CatBuilder, Mar 21, 2011.

  1. JRD
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    JRD Senior Member

    Cat, Instead of wrapping slings around the hull, can you just tie to your bulkheads? Surely these are attached solidly to the inside of the hull. You could still have a couple of slings wrapped around in case a bulk head was to shift during the lift.

    For your next lamination is it possible to laminate a complete half shell and cut the non-requried section out later. Even if you use some less expensive material for the temporary part. This will make the whole structure more rigid, what you are trying to lift at the moment is structurally unstable in my opinion. Its current shape forms a natural weak point at the location of maximum bending moment for lifting.
    Its not to say you wont need to take care with lifts, but at least reduce the risk of it buckling along the way. Im guessing there is still a bit to do before you can glass the outer shell, so anything you can do to reduce the chance of damage along the way will pay off before too long.

    I agree with getting some help for your lift. Even though you need to use lifting gear, at certain points many hands may come in very handy if things get a bit unsteady. If you are alone and things start to go wrong, there may be little option to let it fall and jump out of the way. Just dont give them the beer until jobs done!

    You have come this far, don't give up now :)
     
  2. rasorinc
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    rasorinc Senior Member

    good drawing HOYT. cannot give you points--"have to spread it around" Stan
     
  3. hoytedow
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    hoytedow Wood Butcher

    Agreed.
     
  4. hoytedow
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    hoytedow Wood Butcher

    Tie the strongback to every bulkhead in its reach so that no sag can occur.
     
  5. viking north
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    viking north VINLAND

    Get youself an I beam that will reach 2/3 the length of your hull. Set up the I beam longitudionally and position your cranes on either end (say 3 or 4 ft. in from the ends ). Lift the I beam about 1 foot above your hull. Set up about 6 rachet cargo straps equidistant along and at the extreme ends. Place the straps around the hull and around the I beam. Tighten them all equally. Raise the hull out of the mould using your cranes. It will work like "A walk in the park" -- Geo

    A yacht is not determined by the vessel but by the care and love of her owner.
     
  6. AndrewK
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    AndrewK Senior Member

    Catbuilder; sorry to see you are having trouble getting the first half hull out.

    First thing to check is that the hull is not being held down by glue onto the battens? its surprising how much a hold the epoxy has even on packaging tape. Tap every batten with a hammer & or lift the foam from underneath at each glue line.
    Temporarily shift one of your lifts to the centre to confirm that the centre of the hull is not being held down.
    Has the laminate cured sufficiently? cut off a piece of laminate and slowly heat up in a pot of water to see at what temperature it starts to soften and become pliable.


    If the above two are right then the only reason why your hull relative to mine lacks stiffness is that I fully installed all my bulkheads but one, even for this one I made the bonding flange.
    This is normal practice for this construction approach, all B/H's as a minimum plus any longitudinal's like shelves, bunk top and floor landings you can install now not only serve the purpose of stiffening the hull but also lock in the true shape as well as being easier to do at this stage.
    I am surprised that Kurt's approach is to have temporary B/H's. This is wasting materials and time.
    I know we have discussed this before and you are using plywood for yours rather than foam as I did but I still dont see why you would not install them. The connecting ones, mast & rear beam, saloon entry beams/ bulkheads are going to have joins in them as you are making them from 4 x 8 sheets, so making a join at the hull centre line or the inboard hull side is not an issue.


    Cheers
    Andrew
     
  7. AndrewK
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    AndrewK Senior Member

    I can see the thinking behind the lifting beam approach it will help get the hull out. But what then, how are you going to join the hulls, at what stage do you install the real bulkheads, are you doing the external laminate before joining the two halves?
     
  8. viking north
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    viking north VINLAND

    Andrew are you building to a set of designed plans? If so they should include the steps one should follow to maintain proper hull shape and integrity.This might include some sort of female mould type of set up just to hold the two halfs in reasonable position to facilitate wedging them accurately together for mechanical fastening prior to glassing the join from inside and installing the permanent bulkheads. Once this is completed roll her over and get at the outer skin. I have never built in the half hull method so I'm possibly not the best person to advise here but Andrew seems to have a good handle on what to do from previous experience. Having said that If you use my method for extraction I can't see you runnung into any problems with undue stress on the hull. ---Geo.

    A yacht is not defined by the vessel but by the care and love of her owner
     
  9. rwatson
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    rwatson Senior Member

    Those bulkheads would be a big weight proportionally for sure.

    Getting 'lumber' may compound the lifting problem unless it is built to make a stiff light web.
    Putting a whole lot of 2 x 4 along the top wont make a very stiff backbone to lift with, and spreading them underneath will just makes for a whole lot of extra lifting points to co-ordinate.

    try getting lengths of form ply, and building an I Beam with lengths of pine nailed along the edges, or you can buy those 'HiSpan' floor joists ready made from the local hardware shop.

    The IBeam will be really usefull for the other hull parts later, as well.
     

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  10. rberrey
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    rberrey Senior Member

    Cat, what the above poster (rwatson) has drawn is the the top of the line lifting beam for your situation. With your hull having a void in the middle this will pick up balanced, bow to stern, and not fold your hull up side to side. I think I may have 2or 3 old DD lanyards that would do for rigging for you, it may be this weekend before I can look. rick
     
  11. rwatson
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    rwatson Senior Member

    oops - a lot of this was covered earlier, I somehow missed all the other similar recomendations.

    Looks like the numbers are in favour of the method
     
  12. cthippo
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    cthippo Senior Member

    I don't think spreaders are necessary. If you look at the pictures in his first post he has additional straps wrapped around the hull at the temporary bulkheads to keep the hull from spreading. These are providing more force than just lifting at the bulkheads would, so I think it's fair to say compression from lifting is not going to be a problem. While spreaders wouldn't hurt, I don't they they would help any in the present situation and would just add yet more weight.
     
  13. viking north
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    viking north VINLAND

    longitudional lifting beam

    I agree CT, his problem is longitudional not transverse. The temp bulkheads will handle the transverse stresses no problem. Watson mentioned building an I beam or purchasing a floor web. I think the floor web is an excellent idea and it doesn't have to be wood it can be one of those commercial steel floor/roof webs. These are relatively light and super strong. I have one as the main roof beam in my work shop, 38ft. long and have an overhead travelling crane set up on the lower angles. They are available down to 12in. deep and should be easy to locate at any metal or building scrapyard.
     

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  14. War Whoop
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    War Whoop Senior Member

    Cat what is the plan? when you get this one off ,set it on the other half then glass the outside skin to keep everything honest?
     

  15. rwatson
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    rwatson Senior Member

    ..... or perhaps even from a local pre-fab garage supplier/builder
     
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