Chainplate

Discussion in 'Sailboats' started by paulm1, Oct 2, 2007.

  1. paulm1
    Joined: May 2005
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    paulm1 Junior Member

    Hi All,
    I'm hoping for a little insight. I need to have a new chainplate fabricated to replace one that broke. I pulled the other side to use as a template. I don’t like the way it was constructed. The construction is three piece. The long tang (~3.5’) is bolted to the bulkhead in the cabin. The tang then goes thru the deck and is welded to the bottom of the “beauty plate” that is screwed to the top of the deck. The third piece that accepts both cap shroud and lower turnbuckles is welded to the top of the “beauty plate” at 90˚ to the long tang. Is having the chainplate integrity dependent on two welds asking for trouble? Am I being overly cautious? If not, does any one have any ideas on how one might redesign?

    Thanks
    PaulM
    Pearson 303
     
  2. alan white
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    alan white Senior Member

    You're right to question the design. Whoever designed it didn't consider crevice corrosion.
    The tang should be continuous. The 'beauty plate', if you still want one, which only means two more holes in the deck, should be a seperate piece (harder to make with that slot, of course). Otherwise fill the cover plate holes with epoxy.
    It may be a wider piece of stainless needs to be substituted in order to space the two attachment points far enough apart. The turnbuckle toggles ought to have room to avoid touching each other. If not, consider the wider tang or a fat link to raise one toggle enough to give it room.
    Where the tang exits the hull, the slot should be oversize to allow a cushion of sealant all around. While you're at it, make sure the deck slot is solid and no coring or is exposed. Plywood backing should be sealed with epoxy. Bare coring (I've seen this) should be dug out all around and filled with solid epoxy

    Alan
     
  3. paulm1
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    paulm1 Junior Member

    Alan, thanks for your reply. I have considered using a single tang, but this has two issues. In the original design the cap shroud and lower are inline with the mast and the turnbuckle toggles are free to adjust themselves to the angle of the rigging at the clevis pin of the lower fork. If I use a single tang the chainplate will be 90˚ to its original orientation which means that the lower will now need to be behind the cap shroud an inch or so (I don't believe this is a real problem). The second issue and of more concern is the there will be an angular force on the lower fork of the turnbuckle (especially the lower shroud) due to the rigging. I thought of having an extra long tang and bolt on a second attachment point for the lower shroud. This would keep the original geometry and could be made to the correct angle for the rigging.

    PaulM
     
  4. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    I assume that the bulkhead/internal surface that the tang is bolted to is fore and aft? This is what would require the 90 degree change in orientation of the chainplate axis if the lower and cap shrouds were mounted on the same athawartships plate topsides. The reason for this construction is to allow the realtively "wide" base of the topside piece to resist the transverse compression and overturning load caused by the angle of the lower (assuming the cap shroud is very close to vertical). If the new plate is orientated fore and aft while being the same thickness as before, it will most likely be too weak to resist the athawartships bending moment caused by the lower.

    From a structural point of view, orienting the new topside plate alongships will requires a significantly thicker plate that should be fingered for the individual fingered, bent to the proper angle, then annealed to carry the load. Or you could have individual chain plates for each shroud, but this will complicate sealing through the deck. Additionally, a much larger and thicker fashion plate, well fitted to the buried tang(s) or fully welded to it(them), should be used to carry the athawartships peel and bending load into the deck beam.
     
  5. alan white
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    alan white Senior Member

    Fore'n'aft bulkhead, then. I would cut a new slot in the deck, at 90 degrees to original (like a Tee with stem inboard from the original), and this will move the shrouds inward a tiny bit (unless you can get to the outboard side of the bulkhead), but negligably.
    Then a wider piece of flat stock can be welded (to form a tee-section) to the upper chainplate, with all welding belowdecks. This allows more bolts to be used, always good, and now the tang is oriented properly.
    There's lot's of compression between the inner side of the tang and the deck inboard up to the (usu.) cabin unless the bulkhead is angled inward at the top, but in any case, some means of tranferring the lateral compression load might be added, as now the load is concentrated on the thin edge of the tang, and not its face.


    A.
    Good idea to do both sides, which I assume you're doing.

    A.
     
  6. paulm1
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    paulm1 Junior Member

    Alan, jehardiman
    Thanks for your replies. You both are correct in that the bulkhead is fore/aft. I have added some pictures as attachments from the internet. They are not great, but may give a better orientation. And I will do both sides.

    Alan,
    I had not thought of doing this. The welded surface area would be much greater. I would only be able to make a tee section with the stem outboard and would be the upper 1' of the chainplate terminating at the top of the deck. The stem would continue to except the turnbuckles. This would move the rigging slightly outboard.

     

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  7. alan white
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    alan white Senior Member

    "If you are welding the tee section, where are the extra bolts?"
    Assuming your original chainplate had staggered bolt holes where it was bolted to the bulkhead, the new T-section chainplate would presumedly straddle the old holes and be bolted thru the bulkhead on both sides of the tee's stem. So I meant mounting bolt holes, and the opportunity to use more bolts if desired because the SS plate they are drilled thru is wider.
    If you need to know SS thickness, think about cross-section. Tensile strength of stainless is between 100k psi all the way up to 250 psi for 17ph and such. Figuring on 100 psi, a cross section of the piece involved might measure 1/4" x 1" , or 3/16" x 1 1/2", or 1/4 of a square inch, or 25k psi. Rigging loads for an offshore boat might be as high as 10k lbs (as an example---- this is all rule-of-thumb stuff you can look up for your boat based on a host of factors). But say your boat might stress a chainplate to 10k lbs, then 1/4 of a square inch of stainless steel in cross section (like 1/2" x 1/2") would be a 2 1/2 times safety factor. In general, I always would size a tang or attachment cross section at twice or three times the tensile strength of the wire used (as long as the wire was properly sized to begin with).

    A.
     
  8. paulm1
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    paulm1 Junior Member

    Alan,
    I understand. I was going to start the "Tee" after the bolts. There are seven holes inline down the center on the bottom third of the chainplate. That would give the the "Tee" ~1' of welded surface area. I could pass the stem of the tee through a fashion plate for the attachment of the turnbuckles and weld the end of the top of the "Tee" to the bottom of the fashion plate.

    Paul
     
  9. alan white
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    alan white Senior Member

    Yep, Paul, that would be very strong, though it will require a T-shaped hole.
    Maybe you can use the original lower chainplate. My concern would always be that no unpolished welds got buried under the fashion piece, for reasons of preventing a repeat of what happened. I personally would end the lower chainplate below the deck, even taper it a bit to eliminate stress concentration at the very top of the lower plate. Then I would polish at least the upper few inches or so, especially where the stainless passes thru the deck. I would use the fashion piece only cosmetically, to cover the original slot. It's more work to make the covering plate fit the deck perfectly when welded solid.
    It really doesn't even need screws. Adhesive alone will keep it in place.
    As long as the lower chainplate reaches almost to the deck underside, the lateral compression will transfer to the bulkhead directly. It's nice you can get into the space outboard of the bulkhead.

    Alan
     

  10. paulm1
    Joined: May 2005
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    paulm1 Junior Member

    Alan,
    Many thanks for your suggestions. I took some measurements when I was at the boat last. Looks like it will work. I will take your suggestion not to weld the fashion plates to the chainplate. Much easier constrution. Although I will still have a "Tee" shaped deck hole unless I can mount from underneath.

    Paul
     
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