Bedding a bulkhead

Discussion in 'Boatbuilding' started by rpdwyer, Feb 2, 2016.

  1. rpdwyer
    Joined: Dec 2013
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    rpdwyer Junior Member

    Hello all.

    The motor mount stringers in my 97 Larson Cabrio 310 have been removed in preparation for replacement. They ran from the transom to a bulkhead that is the width of the boat a sits right up against fuel tank. The bulkhead (2 pieces of marine ply) had separated at the top so I cut it out as well.

    Given that the bulkhead sits within an inch of the fuel tank, I will not be able to glass the new bulkhead to the hull from that side (removing the tank would involve cutting the aft cabin to pieces...a project I don't intend to take on a 20 year old boat).

    What can be done short of glassing the bulkhead to the hull that will still make it water proof?

    My plan is to fabricate my bulkhead from 2 sheets of marine ply using a west epoxy resin between the 2 pieces and coating the bottom edge thoroughly. If I then bed the bulkhead to the hull in a thick bead of epoxy (or something else?) will that prevent water intrusion in the absence of tabbing it to the hull in glass?

    I can obviously glass the bulkhead to the hull on one side...just not the tank side.

    Thanks,

    --Rick
     
  2. hoytedow
    Joined: Sep 2009
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    hoytedow Carbon Based Life Form

    With an inch of clearance between the tank and the bulkhead, you should be able to push pre-wetted glass down with a dowel down to the inner side of the hull.
     
  3. rpdwyer
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    rpdwyer Junior Member

    Thanks for the reply.

    All the videos I've watched show really methodical rolling to get the glass to really sit flat and completely wet...so was not thinking that I could attack the problem that way.

    Aside from that method though, is it possible to make the bottom edge water proof just using a resin...or even a big fat bead of 5200? The bulkhead would be glassed to the stringers of course.
     
  4. gonzo
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    gonzo Senior Member

    You can glass in a small frame of say 1x1 or 2x1. Then glue and screw the bulkhead to it and glass on the outside.
     
  5. philSweet
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    philSweet Senior Member

    I'm confused about what you are worried about. Most of the blind side of the bulkhead should be sealed with epoxy (several coats) before installation. Areas to be bogged or tabbed should be carefully masked off and left raw (plus a couple inches extra). If the fuel tank is spaced up off the bottom as it should be, I'd make a full sized perimeter template like Gonzo described (molded dimension such that you can work behind it) and use it to build up a flange on the blind side to locate the bulkhead. It doesn't have to be continuous, but it does have to be dead on as far as being a transverse plane. This will save you massive frustration locating the bulkhead when everything is slimy. Be sure to allow for drainage if applicable.

    Once cured, strip the temp mold from the flange. Then coat the masked-off areas of the blind side, bog, and fasten the bulkhead in place to the flange. This all needs to be done in one go working wet on green. Everything not completely sealed should be staring at you. Now you can tab it in and seal the remaining areas of the engine bay side.
     
  6. rpdwyer
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    rpdwyer Junior Member

    Agreed...it would be sealed ahead of time.

    The part I'm not clear on is not the front face or the blind face, but the bottom the bulkhead where it meets the hull. I attached a crude drawing with both an overhead and side view. Red represents the bulkhead, black the fuel tank, blue stringers still in place, yellow the stringers removed so far.

    I don't follow you on the frame portion. I was going to reconstruct the bulkhead out of two pieces of marine ply sandwiched together with epoxy. I could then coat both sides with epoxy aside from the areas I need to tab the stringers to. The bulkhead would be one large section outlined in red in the bottom view. My concern is making the bottom edge of the bulkhead water proof as water collects on both sides. So when it's in place and tabbed and glassed to the stringers, it's water proof everywhere except where the bottom edge meets the hull on the tank side in the top picture. Where it meets the hull on the engine mount side I could tab it in without a problem.

    Gonzo, are you saying to use 1X1 material glassed to the hull represented by the green lines, completely glassed so it's water proof...and use it to attach my bulkhead in red at the base of it?

    Phil, if I follow you, you suggest building up a glass flange that, when cured, my bulkhead would rest up against it? But doesn't that still leave the issue of sealing the bottom the bulkhead from water intrusion?....unless the flange ran the entire width so water could not get by?



    Appreciate any explanation.

    --Rick
     

    Attached Files:

  7. gonzo
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    gonzo Senior Member

    The 1x1 take the place of the forward tabbing. Instead, you have wood to screw the bulkhead into. Also, it will keep it in place while the resin cures.
     
  8. philSweet
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    philSweet Senior Member

    You're going to bog the bottom in with an epoxy putty after you have sealed the edges with several coats of laminating epoxy.

    Begin with a warm bulkhead and apply laminating epoxy to the edge as the temp drops and it will soak up nicely. You can use a quick gelling epoxy for this and be done after an hour of babysitting.

    Now get the bulkhead located and stable. You want to be able to quickly and accurately locate the thing. There should be a nice gap all around the hull, like 1/4". A shelf that sits on the fuel tank might work for height. But you want to be able to hold the thing still and floating in space.

    Next you will mix up some putty. It really helps to have played around with putty recipes some before you do an engine bay, but oh well. Since you are going to tab over the area in places, you don't want to use any weak fillers at all. They would weaken the tabbing if you got a smear where you didn't want it, and you will. So just use a fumed silica thixotrope (Cabosil, Aerosil). This is where the little flange I mentioned really helps you. When you slap the putty down, you have it corralled so it can't disappear down the blind side. If nothing else is available, buy the foam gaskets for window acs that are about 3/4 inch square and GLUE the stuff in place to form a dam. Or use wood strips as Gonzo suggested. But use something. Don't be shy with the putty. Lay it on an inch thick and then bed the bulkhead down into it and secure the bulkhead. Now you can work the putty into a fillet and not worry about it all draining out to the blind side.
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I run into this fairly regularly and I think your problem is you don't have room to tab the bulkhead to the longitudinal stringers and hull shell, on the tank side. Sometimes you just have to cheat, if the budget or complication makes doing it right, impossible.

    Since you can't tab on portions of the one side of the bulkhead, you need to tab the living crap (technical term) out of the available side, id that's all there is. If it was me, I'd use a combinations of the methods previously described. First I'd expose all that I could and make tabbing, as Phil describes, around the lower portions of the inaccessible areas. Some temporary blocking, covered with plastic packaging tape will serve to shape the vertical part of the tabing, which can be popped loose, when the goo cures. Now, you have a bonded flange, the bulkhead can butt up against. When it's time to install, butter up this vertical section of the tabbing, as well as the bulkhead contact points and stand the bulkhead in place, mashing the thickened goo between, for a good bond. The rest is just like anything else, tabbing to the hull shell and other structural elements (stringers, etc.).

    Can you post some pictures?
     
  10. rpdwyer
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    rpdwyer Junior Member

    I hope to get down there this weekend and will take some pics.

    I may just call in a glass guy to do this portion of it and then take it from there.

    All this because the manufacturer couldn't be bothered to seal the limber holes in the stringers....frustrating.

    --Rick
     

  11. LP
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    LP Flying Boatman

    How much clearance do you have between the fuel tank and the hull?

    I've used an approach similar to PAR's successfully for different reasons and if you made a shallow frame of ply, 1x or what ever and covered it with packing tape, you may be able to get in and form the fillet as PAR suggest. When the fillet is set or if you have room with the "frame" in place, you could run tabbing on the hull, up the fillet and possibly a short way up the frame on the packing tape. After you pull the frame and goop and install your bulkhead, you would have some tabbing already in place and could maybe run a second course that picks up the bulkhead and overlaps the tabbing in way of the fillet.

    Even if you only get a preformed fillet on the forward side and have to do all of the tabbing from the back side, it will be stronger and help spread loads better than not having a good fillet on the front side at all.
     
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