Chainplate Knee Replacement and Material

Discussion in 'Sailboats' started by SVQuestofPaget, Jan 11, 2022.

  1. SVQuestofPaget
    Joined: Apr 2021
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    Location: Boston, MA

    SVQuestofPaget New Member

    Hey Folks - Longtime reader and first time poster here - I'm not sure if this question should be posted here or in Boat design, but here goes! I've been restoring a 39ft Frers custom sloop, his first commission after leaving S&S in 1969. Hull design is very similar to an S&S Swan 40, though lighter displacement at 16,300lbs, and with 670sqft of sail area. The boat and sistership were built by Frans Maas in just three months to be ready for the Admiral's cup series, and many of the period-acceptable construction methods that were employed at the time have now become issues.

    I'm currently considering the chainplates, which consist of a 5mm stainless gusset and 8mm welded lug, glassed into the hull via three long welded tabs. The area of the hull has had no visible moisture damage, though water has leaked down the lug (which penetrates through the deck) for decades. The deck rotted in this area from water intrusion, though the hull shelf appears quite solid. My concern is the inability to inspect where the chainplate pierces the deck and shelf, and that they're over 50 years old and have never been serviced or replaced. A few incisions in the shelf adjacent to the lugs have revealed minor staining of the metal, with some stubborn areas that don't appear to clean up with scrubbing - crevice corrosion? Fatigue? Can't say without removing, and I don't want to trust them as they are.

    In anticipation of long distance sailing and bringing peace of mind, I'd like to address the old chainplate lugs in one of two ways. The first option would be to cut the existing lug at the hull flange, and drill the 5mm plate to receive new chainplate lugs. I don't look forward to drilling 1/2" holes through the stainless gusset, but it would be the simplest way to install new lugs - the gusset and glassed tabbing is dry w/ no delamination or rust evident.

    The more thorough option is to remove the stainless frames entirely and glass in new composite knees in their place, through which a standard stainless chainplate lug would be through bolted. In this case, I've drawn the knee to be longer and wider for greater surface area against the hull. What to use as a base material for the knee? G10, Plywood w/ epoxy anulus for each bolt, Coosa? According to the blueprints, the hull is solid glass with extra laminations in the way of the chainplates.

    I'm not an engineer or designer, but prefer to work along the adage of "nothing too strong ever broke". I'm looking for input from engineers or designers on the forum as to the validity of either of these solutions, as well as preference on base/core material for the composite knee. I'm also very happy to pay an engineer or designer for their time and assistance if anyone wants to message me directly.

    Thank you in advance!
    -Eric

    Quest of Paget, Login • Instagram https://www.instagram.com/sv_questofpaget/

    New Chainplate Design, A.jpg Chainplate Original A.jpg Chainplate Lug 2.jpg
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Too strong can be an issue by creating stress risers. A too rigid structure member may cause other parts to fail. Why don't you send German an email and ask about it. He designed it after all.
     
  3. SVQuestofPaget
    Joined: Apr 2021
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    Location: Boston, MA

    SVQuestofPaget New Member

    Thank you! That's quite true - I don't want to make the chainplate knees too stiff in relation to the rest of the hull. Interestingly, Frans Maas didn't build the chainplates quite the way Frers designed, at least not according to the construction plans that I have. I should have noted that I have attempted to reach Frers as well.
     
  4. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    I just had a look at your Instagram photos - Quest is simply gorgeous (I have a soft spot for all S & S designs, and their relatives), and you are doing a wonderful job with the restoration.
    I had a look for a photo or two of the chainplates taken from down below, but couldn't see any - if it is possible to photograph them easily, could you post some photos of them for reference please?
     
  5. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Where are the photos of the boat?
     
  6. SVQuestofPaget
    Joined: Apr 2021
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    Location: Boston, MA

    SVQuestofPaget New Member

    Thank you - It's been a lot of work so far, still plenty to go, but at least the deck and cabin are structurally solid now. A big improvement over hopping from deck beam to deck beam!

    This photo shows the aft lower chainplate, starboard side. It's the only section that doesn't have subsequent layers of interior paint applied to it as it's usually hidden by mahogany ceilings. You can make out the middle and bottom tabs glassed into place, and the upper tab is hidden by the trim, but is just below the hull flange. I've also included a scan of the original construction drawing drawn in '69.

    One thing I like about this design is that any water coming down the lug drips off the end where it is welded to the gusset, instead of down into the glass. Perhaps I should be, but I'm not actually concerned much with the structure within the glass.
    -Eric
    IMG_3300.JPG 2022-01-12 09-50 page 1.jpeg
     

  7. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    If you're not concerned about the structure in the glass, simply cut of the lug and weld a new one to the knee. A good welder can do it in situ, even with that beam there.
    Second option would be to take it out and have a new one made. Either stainless, bronze or titanium, as you wish. Bronze and titanium don't have crevice corrosion problems.
    Both options will require a deck repair with a generous epoxy plug, sufficient clearance for sealant and a top cover plate.
    Third option is to replace with a composite chainplate. G10 is convenient to use as a core, saves some lamination time over ply or foam. The holes and uni bonding from your drawing are not necessary, just sufficient tabbing. The advantage is the absence of hull penetrations, the deck is epoxied to the chainplate.

    Do you know what the original chainplate arrangement was as drawn by Frers? Just out of curiosity, since the present one worked fine for 50 years.
     
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