How Do You Scarf 12mm Ply Bulkheads?

Discussion in 'Boatbuilding' started by CatBuilder, Mar 4, 2011.

  1. CatBuilder

    CatBuilder Previous Member

    Well, things are moving along. I am getting ready to glass a half hull after another bout of cyanide poisoning from cutting Corecell, having it overheat and reentering the building without my space suit on. Thought the boat shed was clear from cyanide after some serious cutting, so I took my respirator off and went to work on some strip planking. Ended up having to stay overnight in the hospital due to elevated cardiac enzymes and acidosis from the cyanide. Man, this stuff is deadly! I still don't understand why everyone warns you about epoxy when it's foam that will kill you.

    Anyway, I am making half hulls like this:

    [​IMG]

    Attached to each of these half hulls will be 1/2 of a 12mm plywood bulkhead. I'm putting the bulkheads in right away, before joining the hull halves together to help support the hull halves while I am moving them around.

    I am doing 12mm plywood instead of 3mm/balsa/3mm or glass/foam/glass for several reasons: 1) only a 500lbs difference on the entire boat between foam/glass and 12mm Okoume, 2) $3500 less to do 12mm ply that foam/glass, 3) Time to simply cut plywood to shapes and fit will take many weeks off my project compared to laminating bulkheads.

    Questions:

    1)How do I go about making my half bulkheads into full ones once the two half hulls are connected? As in... how do I scarf these and join them so they come out right? I can't imagine putting scarfs in before installing the bulkheads would work, right? How would you do this?

    2) Some of the bulkheads continue across from one hull, across the bridgedeck to the other hull. These bulkheads are what holds the boat together and take mast loads. The bulkhead with the mast load is actually a truss built with a ply skin and lumber interior. How do I connect these longer bulkheads? Do I have to leave them off and put temporary bulkheads in their place to support the hull? Do I just forget about them entirely until it's time to install them when aligning the hulls and raising the bridgedeck?

    How is this stuff normally done?:confused:
     

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  2. alan white
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    alan white Senior Member

    Scarfing would be a lot of work. Tape the joint both sides with enough layers to equal the strength of the ply.
     
  3. CatBuilder

    CatBuilder Previous Member

    That is what I'd like to do, and I asked the designer, but the designer is saying I need to scarf them.

    Can't I butt them all and then tape them, including the main strength bulkheads?

    Can anyone suggest the strength of butting and taping vs scarfing and why my designer insists on scarfing these?
     
  4. hoytedow
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    hoytedow Wood Butcher

    The longer and wider scarfed glue line is stronger. Maybe your designer would approve of a rabbetted joint. "You can't beat a rabbett." according to dskira, and I tend to believe him.
     

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  5. hoytedow
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    hoytedow Wood Butcher

    I'm glad you didn't die.
     
  6. CatBuilder

    CatBuilder Previous Member

    That's something I don't understand.

    Why is the scarf joint (say a big 12:1 scarf) stronger than putting, say, two feet of 34o triaxial over a flat butt joint and epoxying it to the wood?

    I mean, the composite bulkheads for this boat are made from 34oz triax over foam. I'd have to think the butt joint would be stronger if glassed over.... but then... I'm not a designer.

    Anyone?

    PS: I'll look into the rabbet.
     
  7. CatBuilder

    CatBuilder Previous Member

    Thanks, Hoyt.

    I just hope a lot of people read these posts and understand that you can literally die (very quickly) from the cyanide put off by Corecell if overheated.

    I'l be happy when I am done working with the stuff.
     
  8. hoytedow
    Joined: Sep 2009
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    hoytedow Wood Butcher

    Look at post#4 again.
     
  9. CatBuilder

    CatBuilder Previous Member

    Oh wait. I think I just understood why scarf joints are stronger. It has more to do with the ply laminates than anything else.

    A scarf glues every ply of the sheet together, right?

    Any other joint only glues some of the plies in the sheet together. The glassed Rabbet glues 4 of the plies directly together, while the glassed butt joint glues only 2 of the plies together.

    Is that what my designer is concerned about? That there will be an inter-laminate failure within the plywood since only a few of the plies are actually bonded in the joint?

    Or... to put it another way, the stress on the plywood (bulkhead type sheering stress) will only be carried by 4 plies or 2 plies rather than all plies?
     
  10. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Scarfing is really easy. Stack the plywood on top of each other and set the edges back from each other 3 1/2" which will give you a 7:1 scarf. Use a planer to flatten the steps until you have a flat wedge. Reverse the upper sheet and it will have the same bevel for a scarf.
     
  11. CatBuilder

    CatBuilder Previous Member

    Yes, I am good at doing scarfs after doing hundreds of them on 3mm plywood for my failed wooden hull build of this boat.

    So is the process like this?

    1) Take full sheet of plywood and cut out full size bulkhead.

    2) Cut bulkhead in half down centerline of boat, above keel joint.

    3) Stack half the bulkheads up and do my scarfs (I do them great with a sander/buffer). Stack the other half up and do the opposite scarf.

    4) Install bulkheads into half hulls.

    5) Pray everything aligns to the 1mm tolerance I need to make sure the scarfs come out right.

    This seems very likely to not work because of that 1mm tolerance, IMO. I can't imagine having my bulkheads from one half hull line up with the other half hull to within 1mm. What do I do about that?

    Also, what do I do about the bulkheads that span the entire catamaran, from the port hull, all the way across to the starboard hull? Do I just leave them off for now?
     
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  12. hoytedow
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    hoytedow Wood Butcher

    It would be better to cut them slightly oversize by 2mm and have to sand off the excess than to make them too small.
     
  13. CatBuilder

    CatBuilder Previous Member

    Oh, I'll definitely make the bulkhead halves a few inches larger athwartships than they need to be... no question about it.

    The 1mm alignment issue is in a different plane though.

    I'm talking about getting the faces of the plywood to line up exactly perfectly, so the scarf joint can work properly. Each layer of plywood is 1mm or so thick, so the half bulkheads have to line up to within a 1mm tolerance, fore and aft to get the scarf joint to work out.

    That worries me.
     
  14. hoytedow
    Joined: Sep 2009
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    hoytedow Wood Butcher

    Dry fit and mark or jig scarfed "blanks" with center scarfs before cutting hull shape from masonite or other template material, then transfer template shape onto pre-scarfed bulkhead material.
     

  15. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    When you glue them, put plastic sheeting on the glue joint and then sandwich the scarf with two plywood strips on either side. Screw through and it will align the scarf.
     
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