Tabbing analysis for chainplate loads

Discussion in 'Boat Design' started by zstine, Dec 17, 2015.

  1. zstine
    Joined: Sep 2013
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    zstine Junior Member

    Hi, I have an old Tartan 41. The main bulkhead has tabbing that's 3/16 to 1/4 inch thick and extends about 15 inches from the hull on BOTH sides of the plywood. The 2 chainplates (5 bolts each) are then bolted through both layers of glass for a total 3/8" to 1/2" of thickness of polyester chopped strand mat. The plywood has totally rotted to dust and I'm trying to determine if the tabbing alone is structurally adequate to carry my rig loads (it has been...). I want to repair by using 'the bent nail' method to remove wood near bolt holes and fill with epoxy. This will maintain the bolts' compression loads and the rig loads will be taken by the tabbing. Any advice would be great! oh, the cap shrouds are 3/8" and 7/16" on the lowers for a total breaking/design load of (14.5k + 21.5k) 36,000 lbs.
    Thanks,
    Zach
     
  2. gonzo
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    gonzo Senior Member

    No, the bulkheads are structural. You will need to replace them to maintain the integrity of the hull
     
  3. zstine
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    zstine Junior Member

    Thank you gonzo. Yes, I understand the bulkhead is structural. the only part of it that's rotten is the port side in the immediate area of the chainplates. An area that the manufacture sandwiched in heavy tabbing on both fwd/aft sides. That probably exacerbated the rot as it stopped air movement. The loading in that area is primarily the chaiplates/rig loads.. in the order of 36,000lbs. I'm not aware of any other loads that would not be considered negligible compared to that high load. Can you please elaborate as to what loads are being resisted that I may be missing? Can somebody address the analysis of 10x bolts loaded in a FRP matrix?
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    That is straight fwd analysis. However you will need to supply a diagram of the existing arrangement and the sizes of each member and where known the materials and properties of said material.
     
  5. zstine
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    zstine Junior Member

    hey, I drew up a quick diagram of what it looks like. There's 5x, 1/2" dia bolts per plate. I don't have bolt spacing or other detailed dims without going to the boat. I thinking if the chainplates ripped out, the fracture line would be at least 12 inches long. with two 3/16 thick tabs, that's an area of 4.5 sq inches. loaded at 36,000lbs (based on ultimate tensile of rig wire) would be 8ksi tensile load. I don't know the allowable. I'm also guessing this is not the best analysis.? help!
     

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  6. zstine
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    zstine Junior Member

    can someone help with the above analysis? I just assumed a length for a fracture line, but I'm sure there's a better way. Also, what is the strength of chopped mat polyester hand laid FRP?
    thanks,
    zach
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    astine

    Thanks for your 'diagram'.
    But with respect that is not what is needed. That is a child's sketch. You need to provide a proper engineering style diagram showing the location of each bolt hole, its size, its relationship to others and the distance from each to the hull sides and deck etc.
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I'm fairly familiar with this particular boat and the tabbing will likely be 3 layers (probably 4) of 24 ounce roving, which measures out to about his measured (assumed) 3/16" - 1/4" dimension. It's highly unlikely the tabbing is all CSM, though it's possible a combo fabric was used, it more likely the chop was a bond interface, which was common of the era (assuming a pre fire Tartan 41). You might want to contact S&S about this issue, as they'll still have the plans for this puppy. If no joy, try Tartan themselves, as they can be specific about the tabbing. S&S did a fair bit of upgrades and modifications to this design, you might want to have a look into these as well, particularly the rudder and keel.

    I'm not sure what "repair" method you're looking to perform, but given you've gotten, what 43 or so years out of the original laminate, you might just considering restoration, rather than reengineering.
     
  9. zstine
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    zstine Junior Member

    Thanks PAR. I attached a photo which shows the surface of there area being chopped, but there could be roving under. I have only owned the boat 2 years. The mast is down for re-rig and I don't have the time/budget for major bulkhead work if it can be avoided. The nuts on the port side chainplates were loose b/c they lost pre-load due to crushing of the rotten wood. I have sailed hard to wind in 35 knts like that and there was no sign of loosening of rig tension or elongation of the bolt holes. The PO has some weird repair done that he would not discuss, which you can see some of. This array of screws goes all the way down the tabbing to the bottom of the hull... not sure if that's related. BTW, this is a 1975, post keel mods. it has the deep draft. not sure when the fire was.

    Ad hoc... thanks, you are right... I will get detailed measurements when on the boat next.
     

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  10. gonzo
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    gonzo Senior Member

    It is not too hard to replace a bulkhead. You can scarf a new piece in and fiberglass over the seam.
     
  11. PAR
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    PAR Yacht Designer/Builder

    It's likely the PO had seen some delamination and used the screws and finish washers to pull the skin back down to the core. Not the wisest idea. I agree, replacing the bulkhead isn't all that bad, once you've cleared everything away to get at it. If the tabbing is still in good shape, it can be incorporated into the new laminate, saving some material costs.
     
  12. philSweet
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    philSweet Senior Member

    The main risk appears to be the laminate buckling, and your annulus technique is pretty crude in that it doesn't actually remove any warpage that has occurred. I think it is an inadequate repair, but better than nothing. It might serve for one more year if you want to plan a better repair for later. However, there's no saving that tabbing if you do the annulus repair. When it comes time to do it right, it will need to be cut out. As it is, a clever Gus can save enough flange that you don't need to access the hull for a foot on either side of the bulkhead.

    So I'm thinking you very carefully scribe and cut the glass back to about 7 inches from the hull, then drill a hole through a chainsaw bar and run a bolt through the hole for a depth gauge. Clamp boards on the outside of the skins so the skins work as a guide.
     
  13. PAR
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    PAR Yacht Designer/Builder

    I disagree about saving the tabbing. Much of it can be saved, if it's still intact. I think it's simply sheared from the bulkhead, so you'll still have to cut one side to gain access to the remains of the bulkhead, but the other side can stay in place and serve as an alignment tool for a new bulkhead. Of course, more laminate will need to go in, but the previous tabbing can be bonded to the new bulkhead and additional fabric used to tie everything together.
     
  14. waikikin
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    waikikin Senior Member

    I agree,

    I would take a similar approach in leaving tabbing in place on one side. scarfing in a new area of bulkhead glued against that tabbing & reglassing in on the access side. You can document the repair with pics so when you sell it there'll be no explanation required past, well "I put it back just how it was built" & then do a better job on sealing the chainplates.
    There's no great expense materials wise, maybe the yard time & external labour might be
    Jeff.
     

  15. philSweet
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    philSweet Senior Member

    I think I may not have been very clear. What I was trying to say was if you did the annulus repair, and later wanted to replace the bulkhead, the annulus repair would make it impossible to save the skins, at least in that area. Whereas if you go ahead and replace the bulkhead now, you can potentially save a lot of the skins, and importantly, the skins under the plates, which give you a template and a known good structure.
     
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