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

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

  1. CatBuilder

    CatBuilder Previous Member

    I am trying to move my hull today and it's not going as well as I would have hoped. The hull is sagging in the middle.

    I haven't lifted it more than 4-5 inches off the mold because the very center (where there are no topsides or deck) is not fully leaving the mold, even though the ends are. It's still just barely touching at one station.

    Here are the pictures:

    [​IMG]
    Stern lifted up to this height.

    [​IMG]
    Bow lifted up to this height.

    [​IMG]
    Just aft of the center of the boat is still touching, but ever so lightly I can lift it with one hand, easily to bring it up off the mold. Also, see this big temporary bulkhead in the picture? See how it's not attached to anything? I cut it back as best I could to get more weight out of the middle, but it's still sagging about the same. I cut all the bulkheads in the middle back.

    [​IMG]
    Sag is to the left in this picture, between the two cranes.

    [​IMG]
    Here, you can see the sag to the right, just under the crane (and going forward toward the other crane). Notice how it looks compared to the side with the bilge (left side) where you can see everything is lifting fine. It's where there is no hull or deck (right side) that's giving me trouble, as I suspected.


    [​IMG]
    These are the lift points, because further amidships from here, there is nothing to grab a hold of and lift - because there is a huge part of the hull missing here.


    How do I get rid of that sag and support the middle??


    What do I do? How do I stop the sag and keep the boat from breaking in half? :confused:
     

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  2. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    YIKES !!!!!!!!!!!!!!!! you need spreader bars. STOP... Youre crushing the hull with the straps. You must use spreader bars.
     
  3. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Spreader bars
     

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  4. rasorinc
    Joined: Nov 2007
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    Location: OREGON

    rasorinc Senior Member

    No strongback and only 2 pickup points....................not 4. Why do you bother to ask for help? get it level and screw in a strongback on the bottom with the vertical facing the floor-- add 2 more pickup points
     
  5. CatBuilder

    CatBuilder Previous Member

    Michael, I'm definitely stopping, but I'm not crushing the hull at all.

    I have each strap on a bulkhead and the bulkheads are taking the load nicely. There are 9 temporary bulkheads installed for this lift. There is no change in the shape of the hull, except at the very middle. It is folding up such that the bow and stern are raised, but the middle is sagging.
     
  6. CatBuilder

    CatBuilder Previous Member

    I took some of what was said and incorporated it.

    I am confused as to what the strongback is, if it is not what I had put on here. I thought it was something running fore and aft. Do I need something else? Your description of one in the other thread, while clear to some, I'm sure, wasn't clear to me. I don't understand what you mean by a strongback. I have one on the ground below the mold.

    Should I be lifting the boat by a second strongback above it? :confused:

    I found on a trial lift that I didn't need double straps. These straps are 5" wide and didn't cause any damage to the foam. They lifted things up just fine. It's just that the hull was bending up in the middle.

    Can you please explain what you wanted me to do in more detail? It's not that I didn't listen. I only partially understood.

    I've never moved anything big like this before, so I'm a beginner. Well, I did move my last hulls, but they broke into pieces. :(
     
  7. CatBuilder

    CatBuilder Previous Member

    Rasor: I didn't see your last post in my other thread. Is this the strongback?

    "If your weight is correct go with the L 2-2x4s well nailed together giving you a 5" flat piece and a 3-1/2" vertical. "

    Do I have to make this full length? If so, how do I join the 8' sections?
     
  8. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    I haven't been following your build progress. Its difficult to understand how much structure and stiffness you have in the hull shell. Obviously not much . obviously the hull is very light.

    I would use a longitudinal Ladder frame , 2 spreader bar, two crane , arrangement to lift off the mold. Timber 2x6 or steel box sections should do the trick...your hull must be very light.

    . Once lifted, then what ?
     
  9. CatBuilder

    CatBuilder Previous Member

    Yes, my hull was maybe 300lbs (136kg). It was very light before I started adding hundreds of pounds of temporary bulkheads. I added 9 full size bulkheads, tabbed in with bog tabs that are about 4" (10cm) long, 3 per bulkhead.

    The hull currently is only 1" (25mm) Corecell foam with a skin of 34oz (1150g) triaxial inside. I need to take the hull out of this mold, turn it over and glass the outside with a layer of 34oz (1150g) triaxial. I planned to just put it on some cinder/cement blocks for this, then stand it up, out of the way, while I make the next half.

    How do I make the longitudinal ladder or timber 2x6 the length of the boat, but still able to take the compression loads of the hull?

    Also, how do I make this structure so it can take the load when I flip the hull over?

    When it is flipped, the longitudinal piece will be in compression instead of tension, I would imagine.

    I really need some help. Lots of it...
     
  10. rasorinc
    Joined: Nov 2007
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    Location: OREGON

    rasorinc Senior Member

    A strongback is a flat(horizontal) 2x4 with a vertical 2x4 or 6 nailed into the side of the flat 2x4 the 1,5" part. The vertical is what gives it the strength to prevent sag.
    Can you get 2 guys under the center sag area to use thier shoulders/arms to keep it straight while you lift it off to the ground? Join the 2x4s together with end nails and put a 36" scab 2x4 over the joint and nail all tight--12 nails on each side of the joint threw the scab piece. Use # 16s or 16 duplex nails for ease of pulling out. You can put this on the bottom, vertical facing the floor strap over it tightly USE 4 STRAPS OR SCREW TO THE BOTTON IF ANYTHING TO SCREW TO. sTAGGER YOUR JOINTS ON THE FLAT 2X4 AND THE VERTICAL 2X4s. nail the flat and vertical with 16s at 6" on center.
     
  11. michael pierzga
    Joined: Dec 2008
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    Location: spain

    michael pierzga Senior Member

    Unless the ends of your hull are very heavy, The ladder frame only needs to be the length from crane to crane. From the picture it looks as if you are crushing the hull.

    I would lift off the mold... then rest on the shop floor, then turn over.

    Since Im not there, its difficult to judge space and movement. Go slow...drink coffee. Lifting boats is easy...
     
  12. michael pierzga
    Joined: Dec 2008
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    Location: spain

    michael pierzga Senior Member

    box frame
     

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  13. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

    For starters you could try with weights (like two buckets full of water) in both ends of the hull to balance and see if it helps enough.. If it doesnt you need that strongback as rasonrinc said.. Aluminium ladder camed to mind too when Micheal mentioned ladder frame, that is if you find one (or two piece) long enough..
     
  14. CatBuilder

    CatBuilder Previous Member

    Thanks. This is making a lot more sense to me now.

    I'm sorry to have so many questions, but when you say "scab", is that like a gusset? A piece that's there to do a butt joint?

    I'm also not sure what end nails are. Something to hold the butt joint in addition to the scab/gusset?

    Staggering the joints make good sense. Thanks.

    So I would attache this strongback to the bulkheads, like those dodgy 2x4's are now? The current 2x4's are screwed in with 3 screws per mating with the bulkheads. I would use some extra 2x4 blocks to really make sure the bottom of the strongback doesn't move relative to the bulkheads this time, just to be safe.

    How about placement? Do I put it down the center? Over to that one side that's sagging? Have to make two of them? (I hope not)

    Thanks again. I'm going to buy some lumber for this and will start in on it.

    I may go with the 2x6 for the vertical since this thing did develop quite a bend. My temporary bulkheads probably weigh more than my hull, so they are contributing to the problem, I think.
     

  15. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

    Basicly it's geometric.. bcs the lifting point aren't placed in balanced places you got to do two things. To keep the lifting point at fixed distance (now they try get closer each other) and secondly support the mid section..
    Anyway.. a lot of framing as you said.. Are you sure you need all?
     
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