How to repair damaged bulkhead section adjacent to stringer

Discussion in 'Fiberglass and Composite Boat Building' started by Jason Rodgers, Jan 23, 2025.

  1. Jason Rodgers
    Joined: Mar 2021
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    Jason Rodgers Junior Member

    Owner hung a 20HP outboard on a lazarette bulkhead while offshore, resulting in cracking of the bulkhead ply edge and delamination of the tabbing where it joined the hull adjacent to a longitudinal stringer. Cracked ply section has been cut away and the resulting plywood void needs to be addressed prior to re-tabbing of the bulkhead.

    How would you guys re-establish the removed bulkhead section to enable tabbing?
    Concern is the void repair material dislodging, and getting the right balance of strength and flex.
    Boat is 40 foot mono completing ocean crossings. Original layup details and photos below.


    Layup hull stringers bulkhead.png

    Bulkhead repair 1.jpg

    Lazarette bulkhead 2.jpg

    Lazarette bulkhead sideview.jpg

    Lazarette bulkhead abover.jpg
     
  2. Rumars
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    Rumars Senior Member

    Template the missing corner, add allowance for a 8:1 scarf, cut the scarf on repair piece and existing bulkhead and glue in with thickened epoxy. Fillet and tab to hull using biax, with a generous stepped overlap downwards on the existing tabbing.
     
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  3. fallguy
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    fallguy Boat Builder

    IMG_2760.png Only to add to Rumars great post something patronizing and obvious. You cannot repair to paint, so I am wondering how your repair perfectly hits the paint… Bond strength of paint is poor. Make sure to precoat old plywood before scarfing with epoxy putty as those end grains like to suck resins from putties.

    I would extend the tabbing layers down onto the existing tabbing some as Rumars said; so also odd to see paint there now.

    I’m pretty sure I see glass over white paint. This should be removed. The bond strength of the paint is poor vs epoxy to wood. You will probably find the glass lifts off the painted area easily with the paint on it and wherever you bonded to wood; things are better. My apologies if I am correct.

    Also, I am not a fan of the light woven glass used. I would prefer to see something more significant. That woven is a candidate to fail. The better way to do the repair is to make the tabbing go over the repair section.

    I will accept a correction from Rumars, but I would not have bonded glass on paint.

    No paint should be in the repair zones..start over a bit. It looks like the putty work is good.

    Sand all that woven off and start over with the glasswork..get some 1708 or biax tapes..there is probably little reason to glass the repair area directly, as it can be done with the tabbing if you are crafty

    My post is rambling some, sick with flu and tired.
     
    Last edited: Jan 24, 2025
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  4. wet feet
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    wet feet Senior Member

    i have a strong suspicion that the white coating is flowcoat and if the surface waxiness was removed,it ought to give a bond as good as that between the original laminate and the original gelcoat because they are very ,very similar materials.While I agree that newly exposed plywood can absorb quite a lot of resin,I wouldn't use a putty on it,a simple sealer coat will be more than sufficient and in most cases a liberal application of resin without any primer will be the boatyard's solution.More important will be to extend the new glass a sensible distance onto the old part of the bulkhead.About the only thing I would do differently is to extend the new glass to the upper corner of the stringer and since it was the only area to be damaged by the loads that the bulkhead saw,I might add a little more glass to the upper 200mm of the tabbing.
     
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  5. fallguy
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    fallguy Boat Builder

    It looks awful shiny to me is all…like no sanding.

    The mismatch of bond strength and substrate makes using epoxy a little silly. Of course, this is backward thinking.. using epoxy correctly is best
     
  6. gonzo
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    gonzo Senior Member

    That laminate is dry and full of voids. Also, there is no fillet at the joint. If this is a structural bulkhead, my advice is to grind it back and start again.
     
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  7. fallguy
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    fallguy Boat Builder

    At first when I looked last night, I thought perhaps it was just the peelply making the laminate look poorly done, but it does indeed look like the glass is not down on the edges as Gonzo mentions.

    @wet feet
    ~the reason for using epoxy putty for the bond is the scarf Rumars advises is a vertical surface and clear resins will run out of the margin .. clear resins do not work for that portion of the work, not to mention the OP needs to use putty for fillets as well, I would never use clear resin on any scarf as even on a table, there is always an angle and gravity to flow resins, as to the flowcoat comments; the bond of epoxy to flowcoat will be the weaker of the two or the flowcoat; epoxy to wood is stronger; so you always want to bond glass to wood unless the original glass is there and if so; your comment is valid, but only on prepared flowcoat, not shiny surfaces .. also, that is a piece of wood running for n aft, so not sure that is even flow coat, kinda looks like porch paint to me


    OP , you never try to save finishes when making structural repairs, it is part of the cost of the work, save perhaps replacing a transom while avoidong an external skin, etc
     
  8. wet feet
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    wet feet Senior Member

    Who believes that we may be looking at peelply over the laminate? I do and I think the main function of the bulkhead is to keep the cockpit from slowly sinking down within the hull.It appears to be doing that quite well and were it not for the unfair loading from having a substantial outboard hung from it,it has fared well.I don't see much cause for concern.
     
  9. Jason Rodgers
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    Jason Rodgers Junior Member

    Thanks Rumars. Never done a ply scarf before off the bench. What plane/tool and methods would you advise.
     
  10. Rumars
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    Rumars Senior Member

    I would probably use a laminate trimmer (small router) with a jig clamped in place. Also clamp or hot glue a backing board on the other face of the ply, this way you can push in the router without fear of breaking the edge.
    Other tools would be small planes, and/or sanding blocks, it's just a lot more work.
    The key is to try it out beforehand to insure that everything works as you imagined. Clamp a piece of ply in a vice to simulate the bulkhead and make a few practice scarfs on it.
     
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  11. Jason Rodgers
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    Jason Rodgers Junior Member

    Cheers all for input on job.

    I would never repair or fiberglass glass over paint - the edges of the fiberglass repair (peel ply section) are to be feathered back removing perimeter fiberglass edges on old gel coat (brown colour). The new white paint is Alexseal 501, which will be ground off when re-tabbing the bulkhead, along with the original brown gelcoat, which is metamorphic rock hard.

    The delaminated CSM tabbing was ground out in this area using 36 grit. Fiberglass cloth was laid on opposing angles down over the stringer angles. Aim was to avoid boggin any low spots when re-tabbing of the bulkhead with 600gsm biax, although a few small holes were filled with thickened epoxy while doing the cloth layup.

    Thoughts on how to address the probably compromised ply bonds along the top mid section of the ply. Would it be worth wrapping the bulkhead in biax and perhaps running some uni along the top and into the walls of cockpit ply structure and hull?

    1738106028548.jpg
     
  12. Jason Rodgers
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    Jason Rodgers Junior Member

    Thanks great advice...I've wasted a ton of time by trying to save time by not doing trials.
     
  13. wet feet
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    wet feet Senior Member

    You seem to be heading in the right direction.I don't think you will find it easy to get a perfect fit between the new plywood section and the old bulkhead,but do the best you can.The solution might be something as simple as adding a dab of filler or a small fillet to the edge that attaches to the hull and then applying a wide strip of masking tape to the bulkhead faces and dribbling in un-thickened resin to fill any gaps.Once cured you can laminate over the new piece and onto the main part of the bulkhead.If the original builder used CSM and it held until grossly abused,why not do the same thing?Given the space constraints and the geometry of a small router,you won't be getting very close to the hull surface with the cutter.A rasp will be more likely to succeed,quieter,less messy and much more useful.

    As for potentially compromised bonds along the top edge,in perfect workshop conditions you could possibly wrap a laminate over the top but the fibres wouldn't really want to conform.It would be altogether simpler to add a double-say 80mm deep and made of the same plywood.alternatively,you might consider the extent of the potentially damaged area and decide that is is such a small percentage of the depth of the bulkhead that it hardly matters.
     
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  14. fallguy
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    fallguy Boat Builder

    I recommend using an oscillating tool for the cutting in place. It won’t be perfect, but this is why you use epoxy putty.

    uni is not too good here as the tow direction only works one way

    use biaxial fabrics, probably on the order of 6” wide and 4” wide will be good

    I’d prefer 1708 for ease of use and epoxy will work on it just fine. But 1200 or 1700 are also okay if available, for the 1200, go 3 layers and staggered so like 6”,5”,4” or so

    for ply bonds, the best thing is epoxy or epoxy putty dripped into them and support of thicker fabrics; do you mean the bulkhead? Those should be cut away in the scarf making…
     
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