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

    Teddy, I don't have a clue if I need all the bulkheads. :(

    This is an especially difficult part of the build for me because this is where my wood/epoxy build failed... the hulls broke apart when we moved them. There is a lot of fear here because if this hull breaks, I will not be able to start again. That means no income (from charters). I have to do this very, very carefully.

    Yes, I think the stronback will work for keeping things spaced at the proper distances - might as well try...
     
  2. JRD
    Joined: May 2010
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    JRD Senior Member

    Thinking about it, the area of missing deck where your cross beam comes in is massively reducing the bending strength at the point of maximum bending moment. Agree with the others about a strong back beam to spread the load. Ladder frame with 6" x 2" spaced say 2' apart, Ensure the 6" is in the vertical plane to give you a deep beam. Put spacers between them at same intervals as your bulkheads.
    Drill holes at 2' spacing thru each bulkhead and tie lanyards from the bulhead to the ladder frame (these can be quite short). Once you take light tension on the cranes, adjust the lanyards to ensure the weight is distributed evenly on the hull. Your glue fillets from bulkheads to the hull should be more than strong enough if epoxy & glue powder.

    If you have enough height use 45 degeree angle strops from the sides of your ladder frame up to the hooks on your cranes. If the included angle is greater increase your strop size and ensure there is reinforcing where you connect them so the frame cannot collapse at that point or try and get out of parallel.

    Some one may have already described this better but a scab (doubler) plate is basically a plate glued or screwed to your existing surface that is used to connect fittings or lifting points. It spreads the load over a greater surface area and gives you a thicker section in which to put screws in without adding weight to the final structure.
    Good luck
     
  3. cthippo
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    cthippo Senior Member

    OK, the attached (poor) drawing shows how a ladder beam works. Basically it's 2 long (20 or 24', whatever you can get, with 6 inch 2x4 blocks between them in the form of a ladder. This gets attached to your cranes and then I would use as many straps as I could get my hands on between the ladder frame and the hull.

    [​IMG]
     

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  4. Tim B
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    Tim B Senior Member

    I'm amazed that you're proceeding with your build with no longitudinal stiffening so far. You need at least a keel and a deck crown section running fore and aft, and for a slender hull, stiffeners on each side to take lateral loads. If it's flexing this much now, what's it going to do in the water?

    Stop and think,

    Tim B.
     
  5. cthippo
    Joined: Sep 2010
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    cthippo Senior Member

    He's building it with foam core construction so by the time it gets to the water it will have another layer of glass outside the existing foam, plus a deck to hold it together.

    It's just getting it that far...
     
  6. rberrey
    Joined: Oct 2010
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    rberrey Senior Member

    Cat , you may also move your pick points or( lift points) in toward the middle. You should have app 25% of the load on the bow and stern side of your pick point, 50% in the middle of the pick points. Your sag maybe because your load is not balanced, the strongback will help, use 16' 2x4s or 2x6s vert as stated in other post nailed to your flat 2x4s making a L, stager the joints of the strong backs. If the hull sags again and your strong backs dont then nail a 2x4 across the top of the strongbacks where it is sagging, run a strap under the sag area over the strongbacks on top of the 2x4 that is nailed from strongback to strongback.Try adjusting your pick points to balance your load and pick it up a little, if it is going to sag due to material weakness then it will bend on both ends and the middle ith balanced pick points. rick
     
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  7. CatBuilder

    CatBuilder Previous Member

    Very interesting!

    Thank you, Cthippo.

    Just want to ask everyone: Would Cthippo's method be equivalent to Rasorinc's method? As in... would there be any advantages to either?

    Looking at Cthippo's picture, it seems I could slide one of these ladder beams he drew up under all those straps I already have in place around each bulkhead. That would make for 9 lift points and a nice cradle as compared to fitting the "L" shaped strongback and proceeding with my 2 (or 4, using two straps) lift points.

    Which method would be better keeping in mind I have to take the hull and flip it over, too? (as well as roll it outside and spin it around)

    And Cthippo: My boat is 45' from end to end. I'd need to join several smaller beam sections together... is that usually done with the ladder beams?

    Tim B: This is going to be one very VERY stiff catamaran. The core is 25mm, when the spec calls for 19mm. Stiffness isn't one of my problems. Moving partially complete hulls is. The keel goes on after I get this half hull out of there and make another one - to join this one to. Cthippo is right about the type and method of build I'm doing.
     
  8. p_smith
    Joined: May 2010
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    p_smith Junior Member

    i like ct's idea,

    or maybe use a standard floor truss, or ibeam truss

    then use as many straps as possible

    if, of course you cant get more engine hoists and spread them around the hull.
     

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  9. rberrey
    Joined: Oct 2010
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    rberrey Senior Member

    Cat, using a spreader bar or truss would be better for you , put the strongbacks on anyway. Flipping it over will also be easer, you'll just roll your straps to one side. You can adjust your pick points and put your vertical 2x on and lift it now. rick
     
  10. CatBuilder

    CatBuilder Previous Member

    JRD's lanyards sound good to.

    Basically everyone's idea is good, except mine: Screw a couple 2x4's to it! ;)

    So which idea will work best if I am to pull the hull, then set it on the ground, then use the shop cranes to lift it up on edge and flip it over into a "turtle" configuration?
     
  11. CatBuilder

    CatBuilder Previous Member

    Ok, that makes sense. So 2x4 strongbacks AND Cthippo's beam? His beam is the spreader bar you're talking about??

    Yeah, I am really looking to have a solution that allows me to flip it over easily, like you are saying, by adjusting my straps to one side.
     
  12. rasorinc
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    rasorinc Senior Member

    A scab is a form of gusset they are 2 names equal. A strongback is a poor man's beam. center it on where you saw the most flex
    It should be long enough to pass under the 2 inside straps 12" Just but the 2x4s together and drive a diaginal nail from
    the top of 1 into the center of the other then use the scab to tie it all together. screw it into the bulkheads like you have done to existing 2x4 or just add the vertical 2x4 to the flat one. add 2 more straps or non flex stretch rope
     
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  13. hoytedow
    Joined: Sep 2009
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    hoytedow Wood Butcher

    See these illustrations for clarification. Somebody correct me if I am wrong.:
     

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  14. CatBuilder

    CatBuilder Previous Member

    Looks like you're right, Hoyt. :)

    Were people (aside from me) mis-naming these things?

    I'm going to start tomorrow by building the strongback the right way (as the L), then add Cthippo's beam if I am still bending.

    Does that sound right?
     

  15. rberrey
    Joined: Oct 2010
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    rberrey Senior Member

    Cat , cthippos beam is my spreader beam, his picture is of a truss. Build or buy a truss, price one before building, you need to start putting a price on your time if you are to finish on your schedule. A pipe may be better for you to handle alone , I cant draw pictures like ctippo but I'll try to explain how to rig it. What ever you use pipe or truss should be over 50% of the hull length , with a pipe you can put I bolts though, two on top for your hoist hook up , as many on the bottem for you want, the I bolts on the ends of the pipe (bottem) can have straps going to the hulls at a 45 . Maybe someone can draw this out for you . rick
     
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