Proper way to tab bulkheads on a fiberglass boat

Discussion in 'Sailboats' started by peterchech, Apr 25, 2012.

  1. peterchech
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    peterchech Senior Member

    If you look at a production boat like a Hunter, many of the structural bulkheads are tabbed in with one layer of glass roving. I don't see what looks like filler at the actual joint. I guess this is to prevent "hard spots", however that works...

    On a better quality boat like a C&C, it looks like they use several layers of maybe 10 oz regular glass. Being opaque, I can't tell whether there is filler at the joint or not.

    Say I wanted to add a structural bulkhead to my boat, or else reinforce an existing one. What is the best/proper way to attach the bulkhead to the hull of a glass production boat?
     
  2. souljour2000
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    souljour2000 Senior Member

    I am not sure... but we will hear from the experts...I will say that I think a small fillet along the tabbed areas with a few layers of glass over it would be needed to reduce any bulkhead movement and seal joints to reduce moisture migration ...sealing the butt ends of the ply with epoxy is critical so it doesnt wick up moisture. Then consider leaving a small gap at where bulkhead, etc. panels meet the hull/coachroof that can be sealed as part of the epoxy fillet...and then tab over with light/heavy glass layers...
     
  3. gonzo
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    gonzo Senior Member

    It depends on the hull construction. Cored hulls don't usually get print through from hard spots. If the laminate is solid, it is common practice to either leave a small gap or have a foam tapered filler in between.
     
  4. peterchech
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    peterchech Senior Member

    What kind of foam would be used as filler? Mixed floation foam? Seems that would expand too much...
     
  5. iceboater
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    iceboater Junior Member

    I use carriage bolt (photo1) of different size to shape fillets from fiberglass bonding paste (photo 2) from Oldopal. I am not familiar with using expansion foam as filler, I think Gonzo means pre cut foam fillets.
    For glass over the fillet, I use any ware from 1800-4500 gr/m2 of glass (sorry, metric) tapered out each layer depending on the design.

    Axel
     

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  6. gonzo
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    gonzo Senior Member

    Yes, I meant precut foam fillets
     
  7. PAR
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    PAR Yacht Designer/Builder

    [​IMG]

    A typical foam shape, used under the edge of a bulkhead, at the interface of the hull shell. Triangular shapes are common as are trapezoidal stringers. Any shape can be cut, though these can save time.

    A filler can be mixed as well, which serves the same purpose and commonly applied as a fillet or bedding under an element.

    As far as proper tabbing, well it depends on the loads. A single layer of fabric isn't uncommon. This doesn't mean it's very durable, just that it's an out of sight place manufactures can save some money.

    Directional fabrics are much better suited to tabbing than anything else. It's all I spec except on small craft where cloth will serve.

    If you could be more specific about your application, more precise answers could be applied. What ever it is, I'll probably tell you to use biax and epoxy, no mat. It's just easier, stronger, uses much less resin and isn't going to fail easily. As to how much biax, well this depends on the boat and the bulkhead.
     
  8. peterchech
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    peterchech Senior Member

    Well I ask for two reasons. I have a 1982 hunter 25 and have repaired cracked bulkheads on it before.I used the same method as I would on a s&g dinghy. An epoxy and hard filler joint, filleted to about 1/2 inch half round, then 2 layers of 10oz glass cloth. I was wondering if this is adequate, or whether the epoxy filler joint would create a hard spot or even a stress concentration point.

    The second reason I ask is because the owner of the Phrf boat I will be crewing on ripped out his interior and installed a "grid" of shallow bulkheads around the keel intersection area. I like this idea alot. I was wondering what the proper way to tab them into my boat would be, since his is an aired cored hull and mine a thin (hunter) solid laminate.

    Why bias tape? Don't most s&g designs call for regular old tape? An idea on the proper layup would ne helpful too...
     
  9. Steve W
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    Steve W Senior Member

    When you use regular 0-90 glass tape only half of the fibers are running across the joint,with double bias tape all of the fibers cross the joint making for a much stronger joint. As Par pointed out how much glass you should use depends on the application,if you are tabbing in a main structural bulkhead such as the ring frame under the mast you would use more glass than a piece of furniture. For some applications just a large fillet will surfice. We do maintainance on a 40ft cold molded race boat that has some bulkheads just filleted in and after 30 years,so far so good.
    Steve.
     
  10. SamSam
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    SamSam Senior Member

    Another way is to slip the appropriate sized tool socket over your finger and drag that along the filet. A really good tool is a ball bearing welded on a rod. With that you can go into a three way corner (like the inside bottom corner in a box) and all the fillets can be done at once and will match perfectly.
     
  11. SamSam
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    SamSam Senior Member

    Another trick for glassing bulkheads with heavier fabrics. We made 30' powerboats AquaMarine Island Hoppers) and I had to tab the bulkheads to the hull. We used Fabmat, which was 24 0z woven roving glued to 1 1/2 oz mat. The 8 & 12" wide strips had to be cut at chines and strakes to conform to the hull and the fabmat was too thick to wet out from one side. So the procedure was to lay it out mat side up on cardboard, wet that out, flip it over and wet out the other side, pick that up, stick it in position and roll out the bubbles.

    By the time you got a 5' section all wetted out, the mat on one side had begun to dissolve and the WR on the other had begun to un weave itself. With all the cuts to make it conform to the hull shape added in, laminating at all was tough, neatly was not possible.

    All this was in open hulls in Florida sunshine.

    After a few boats I started putting the Fabmat in place dry. I would then fold down the vertical halve on the bulkhead, wet out the mat and ply bulkhead and stick it back in place. Then I would fold up the portion on the hull and do the same. Then I would wet out all the WR side and roll out the bubbles. It worked perfectly and 'looked marvelous'.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Fabmat and stitchmat products are really a waste of resin (lots of it) in an epoxy laminate. In polyester structures a necessity, but not epoxy.

    The real reason biax and other directional fabrics are better suited to tabbing is simple physics. Conventional cloth is woven, with the fibers going over and under each other. Under load these fibers have to flatten out, which kinks and weakens them, long before the max elongation of the resin or fibers are reached. With directional fabrics (like biax) the fibers lay on top of each other, essentially being continuous straight lengths. Under load these fibers immediately begin to participate in resisting elongation, so the resin and fibers work in concert without delay, kinks or flattening out first.

    Small taped seam craft can employ regular cloth on their seams because it's more then strong enough to reach the peel strength of the surrounding wood it's bonded to. This is especially true on the outside corners of a taped seam. On larger craft, loads can exceed what cloth can provide, so direction fabrics are necessary.

    Generally, bulkheads are highly loaded in all but the smallest of boats. The use of biax is the best way, to use the least amount of resin and fabric, to achieve the desired strength and stiffness.

    Fillets prevent stress risers and hard points, again from simple physical laws. A stress riser is created when you have a sudden and dramatic increase in density along a panel or element. If you have a plywood planking panel and a bulkhead perpendicularly joined to it, loads traveling down this panel reach the bulkhead where there is a sudden and distinct rise in panel stiffness and density. the edge of the bulkhead makes a hard point as the load transmits through it, which can cantilever on the adjoining planking panel.

    If you can ease this transition area with a ramp, such as produced by a fillet, the load transmission area, is spread out over a much wider area and the density increases more slowly. This helps dissipate the load into the adjoining panel, without a hard point, releasing strain on the joint, the planking panel and the bulkhead.

    As to your tabbing concerns, two layers of 10 ounce doesn't seem like much. I scale tabbing to the loads anticipated. On a boat the size of yours, it would surely be biax, not cloth. Also if in doubt about the creation or elimination of hard points, error on the too big side with fillets.
     
  13. Steve W
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    Steve W Senior Member

    SamSam, such a tool can be purchased from pattern shop suppliers such as Kindt-Collins or Freeman Supply with different size balls on each end, they are chrome plated and are typically used for working leather or wax fillets into a corner. Actually there are a lot of things used in the pattern making industry you guys may find interesting such as the leather fillets and carvable polyester filler and castable urethanes.

    Steve.
     
  14. SamSam
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    SamSam Senior Member

    That's where I got the idea from, seeing it advertised somewhere for what you describe. I'm usually just too cheap for my own good, but I had a few different sized ball bearings, a few pieces of steel rod, a welder and in two minutes I had two of those things which work great.

    You are right, skill sets and tools from other endeavors like patternmaking, auto body repair etc are directly applicable to things other than themselves, like moldmaking and boatbuilding. Even Doctors, watch one working on bones and their arsenal is hammers, saws, drills, steel plates, nuts and bolts. They are sometimes just fancy carpenter/mechanics. When they get done, a little needle and thread, some careful crosstitching like on Grandma's quilt and they're done.
     

  15. SamSam
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    SamSam Senior Member

    Yes, I wasn't commenting on what material is best where, but the process I used to deal with thick materials, difficult to wet out, cut into tricky shapes in cramped spaces and how much better and neater the whole process was compared to the way that was sort of a standard for things like that. This was back in the '70s, epoxy was never used, bias and unidirectional cloth was unheard of. Our arsenal was polyester, 1 1/2 oz mat, 24 oz woven roving for most things and Fabmat (because it held together) for things like tabbing bulkheads, glassing in the shaftlogs etc. Resin and glass was cheap, in 30' powerboats extra weight here and there was not a consideration. The only time I can remember weight being mentioned was when two laminators breezing through on their way to Key West were hired to laminate a hull. Instead of the usual amount of resin going into the hull, when these guys did it it took a whole extra barrel. The owner went ballistic because he thought they stole it. When he realized it was in the laminate, it was no big deal other than 'I guess that boat will just have to haul an extra barrel of resin around forever.'

    After I left there and eventually started making canoes, that's when weight consideration became the priority and laminate materials became an act of 'design and engineering' rather than piling it on.

    Doing that work in Florida though was a huge step for me in organizing how I worked and in doing neater work. If you want to laminate polyester in the hot sun it works best to have all your ducks in a row, everything laid out and prepared because time is very limited and when it kicks it happens in seconds and the time is over for working it, and you still have to clean your tools or lose them too. Forced by heat and humidity to wearing cutoffs and open short sleeved shirts, it quickly becomes apparent that the neater and cleaner your original work, the less grinding and cutting is required to finish the job. As it was, for two years I was never able to wear long pants or long sleeves for the itching of fiberglass. I believe the prank 'itching powder' advertised for sale in comics is only fiberglass dust.
     
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