Bulkhead replacement tabbing

Discussion in 'Sailboats' started by boudica, May 27, 2024.

  1. boudica
    Joined: May 2024
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    Location: Salt Lake City, UT

    boudica New Member

    Hello

    I am replacing the main bulkhead in a 1975 Ranger 26 because it was rotted through. I have cut the new piece based on the old piece, except that the few inches near the hull was rotted through and I can't get an exact shape.
    I have found mixed information about the space between the bulkhead and the hull. I read to leave an 1/8" gap?
    Alternatively, a friend is trying to convince me that I should glue it into place as adhesives are stronger than tabbing and there are enough structural pieces it is being bolted to that it can't flex much anyway. He is a mechanical engineer and before that worked in fiberglass manufacturing of structural pieces (not boat related), but that still isn't boatbuilding.
    Can someone confirm that I need to do tabbing? And can someone explain what purpose it is serving structurally?
    I included a picture showing the ceiling, cabinet and settee connections it will have as well as where the old tabbing is.
    Thanks!
     

    Attached Files:

  2. Tops
    Joined: Aug 2021
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    Location: Minnesota

    Tops Senior Member

    Welcome to the forum Boudica
    Getting the new bulkhead shape closer should not be too difficult. Put the old one back in the original position and use 'tick sticks' (thin pieces of wood, cardboard can work in a pinch) and tape or hot glue gun them into place to make points to fair a copy of the curve. Picture is tick sticks placed to meet hull to make v-berth top panels without the existing by using cardboard as a substitute.

    spot_s21_template 1.jpg

    As far as the detail for the bulkhead attachment, is it known if the bulkhead was acting like a partition or a structural member? The quantity of old holes makes it seem pretty important. Are the bigger holes for a chain plate?
     
  3. kapnD
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    kapnD Senior Member

    That doesn’t look like it was structurally tabbed in at all.
    The wall covering doesn’t look like fiberglass to me, and I see no evidence of fiberglass on the old piece.
    If it were holding a chain plate, it’s a miracle that it lasted long enough to rot!
     
  4. fallguy
    Joined: Dec 2016
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    fallguy Boat Builder

    I’m not going to go into specifics and enter debate. The reason for tabbing is to spread the load. If it wasn’t tabbed before; don’t bother now is a general case.

    Consider a bond of a 3/4” piece of lumber 4” long. The bond area is 3 square inches. A load of X (say 30 pounds) is 10 psi. Now, extend the bond surface out 3” each side, the bond surface is now 3x12 plus 3 plus 3x12 or 75 square inches. So the 30 pound force is over that space and the psi loading is less than 0.5 versus 10 above.

    Now extend that case to an ocean load and multiply it by a factor of 20. The 10 psi load is 200 which exceeds most typical foam core compression loads. The one with tabbing is 600/75 or 8psi.

    So, unless your ME friend does the load analysis and knows the hull strength, he hasn’t earned his tea.

    A margin is desired. This is to avoid a hardspot that will cascade to the hull outer surface and result in a ridge in the hull. The margin can be filled with a foam pad or it can just be glued, but glues are also hard. You just want to avoid pressfitting the part or it will show.
     
  5. boudica
    Joined: May 2024
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    Location: Salt Lake City, UT

    boudica New Member

    I appreciate the insights and friendly welcome.

    The fabric is hiding the tabbing, it was indeed tabbed in. Though I wonder how long it has been detached from the fiberglass, the wood was so rotted it crumbled everywhere the fiberglass was attached. That is why there doesn't appear to be any fiberglass on the old bulkhead, it disintegrated.

    It does hold a chainplate, it is located directly under the mast. It is a Gary Mull design and there is supposedly a structural metal beam spanning above the bulkheads under the mast. The boat does not have compression posts here or anywhere that I know of.
    On the subject of said beam, it is rusting and the rust is leaking through the bolts when it rains. I am going to work on the leaks above to end the leaking to both the beam and the chainplate. The question is, do I need to worry about corrosion of the beam that affects it's integrity? What do you suspect it is made of? Cast iron? Stainless? It is a 1975 Ranger 26, I haven't been able to find details on that.

    The boat is new to me. It sat neglected for at least two years as the water levels went so low no boats were able to be in the marina. The leaks got bad in that time, not sure it's maintenance before that.

    The included pictures shows in blue where the bolts are, and the yellow lines is where the tabbing was. I removed most of it, I was considering glueing the new bulkhead to the tabbing tha tis left. It is very stiff, where the rest was fairly thin and flexible. Though, maybe that is a boat sin?

    Because I am interested in understanding, @fallguy am I correct in saying the purpose of tabbing vs other means is that tabbing distributes the load more, leaving no heavy pressure spots on the hull? I think our tea-less ME thought the only purpose of the bulkhead was for strength to the chainplates. But actually, it is also for distributing the hull pressures. Is that close?

    I will leave a gap, and use the tick sticks to get a better shape. And it sounds like I will stick to tabbing.
    I am going to throw in more detailed pics to paint a more just picture of the project.

    PXL_20240527_211056077-EDIT.jpg PXL_20240527_211040764-EDIT.jpg
     
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  6. fallguy
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    fallguy Boat Builder

    I did not mean to offend your ME friend. But an ME ought to recognize a structural connections a bit.

    Tabbing does spread the loads. And it prevents certain lateral forces from damaging a thin 12mm to 20mm bond.

    The fact it was rotten threw him off, because when things are bad, anything is better! But I’m rebuilding a church basement and the minimum doesn’t make it nice or even right. Doing things right matters. Your boat has quite a bit of space between the BH longitudinally (seeing none), so do it right.

    The best way to fix is to remove all the paint and tab it back in with a couple layers of 1708 tapes. I recommend a 6” tape and a 4” tape at least, but if the old tabbing is thicker you could go 3 layers 6-4-4. All tape edges are always staggered and longest pieces applied first. In glasswork, we do not go for shingle effect. Longest first results in less air entrainment and less sand thru.

    As far as the metal, impossible for me to assess or even consider commenting on the information and my lack of direct knowledge. If there is a structural metal beam with corrosion; best inspect it while you can. I’d hatr for you to repair the BH and then need to remove it for a beam repair. Steel corrosion can be addressed by marine welding guys. It is always dangerous to weld around fiberglass; keep a water hose at the ready, a fire extinguisher is useless if resins get going.
     
  7. wet feet
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    wet feet Senior Member

    If I'm reading this right,that is the main structural bulkhead in the boat and needs to be totally reliable.It appears that the steel beam may bridge the central passage and transmit the compression load from the mast step to the rest of the hull via the bulkhead.Back in the early 1980's I was building 25 foot cruiser/racers that had a similar bulkhead,but with the port half extending to the boat centreline and we were using 5 plies of 200gsm mat on each side to bond that bulkhead to the hull and the first ply only went a couple of inches along the faces,with subsequent plies having an inch overlap on the preceding ply.The chainplates were attached with 5 3/8" bolts to spread the load.so you can get an idea of the loads that were imposed.With any doubt about the condition of the steel beam it would be good for peace of mind to determine the condition and replace if found dubious.It isn't hard to find a tiny camera that could be inserted inside a box section or you might find a tame surveyor with an ultrasonic thickness gauge.For the future you need to find how the moisture that caused the damage got in and make sure it can't happen again.Which may mean removing bolts from deck hardware and re-bedding combined with sealing all surfaces of the new woodwork.Also make sure that if wet goods are stowed in lockers beneath seats that there is a route for water droplets to escape into the bilge,where a pump can deal with it.Gary Mull designed boats that sail well and when you get afloat it will become apparent.
     
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  8. kapnD
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    kapnD Senior Member

    It appears that the demolition of the bulkhead has stopped at the floor line.
    The worst of the rot is probably below that.
     
  9. fallguy
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    fallguy Boat Builder

    An excellent point..
     
  10. boudica
    Joined: May 2024
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    boudica New Member

    Thanks for chiming in kapnD. I am a novice so I may very well be missing something. The bulkhead sat on the fiberglass of the settee, is there a separate piece of bulkhead below the settee?
     
  11. fallguy
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    fallguy Boat Builder

    The floor does not support the loads. There is actually a structural element supporting the bulkhead. Whether stringers or transverse member; something supports it.
     

  12. wet feet
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    wet feet Senior Member

    One of the image files in an earlier post shows an access hatch beneath a Shopvac,The answer lies beneath it and while you are looking around down there,check for properly planned bilge drainage through limber holes that aren't blocked with debris.Actually,with a boat of this age it would be a good idea to use the Shopvac to get the bilge as clean as possible so you can take a really good look round and remove any dust or debris that might block a bilge pump.It goes without saying that you should get the bilge spotless before re-launching.If you don't have a webcam with lights on the face of it,I suggest getting hold of one as they are realy great for inspecting confined spaces.I have a friend who laminated some reinforcing ribs via a small hatch while watching the operation on his laptop.
     
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