Chainplate replacement - Rotten and water soaked Knees under my chainplates

Discussion in 'Sailboats' started by alaskanviking, Apr 30, 2012.

  1. alaskanviking
    Joined: Apr 2012
    Posts: 1
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: San Diego

    alaskanviking New Member

    Chainplate replacement - Rotten and water soaked Knees under my chainplates
    Been Hard at work removing two of my chainplates in a 1984 Passport 42' pilothouse cutter. worst thing i have had to do to my boat yet.
    old chainplates are fiberglassed in place in a kinda H shape. had to cut out the lovely teak backs, of the salon cabinets to access them. two plates on starboard were right at cabinet bulkheads. very depressing. used a Rigid Multitool to cut out the fiberglass covering. found that you must use the metal and wood saws, as the wood only blades turn into scraper blades far to quickly. while cutting out the chainplates i hit 3 pockets of water on the starboard plate, and 5 on the port. nasty black ooze drained out. when i finally got the plates off the port side the knee was all black on the bottom. tried to drill drainage holes but the wood was way to spongy or rotten. even the wood that initially looked ok on both sides was spongy, i could almost push the drill bit in without pulling the trigger. i then cut one of the fiberglass sides of the knee on both sides and they pretty much just fell out.

    obviously these were not installed correctly and fiberglassed on every side, one of the knees did not even go all the way to the deck and had a void of about 2". the wood looks like fir, but its so bad i cant be sure. its a simple job to replace them, but i need to know what wood i can use to do so. should i go get some blocks of fir, Ironwood(IPE), Mahogany, or would Oak be a better choice? what about replacing it all with a block of G10 Fiberglass epoxied into place? i have a decent piece of oak that may be about the right size. not sure which way to proceed here.

    i believe that area is solid and not cored, i will be able to find out when i remove the rubrail and get the single bolt out. currently i am planning on continuing with g10 fiberglass plates glassed together for the knee, and internal titanium chainplates throu bolted to exterior sister plates. i believe that this may be my best solution while balancing corrosion and water resistance, cost, and sustainability/ease of repair.

    i thought about composite chain plates, i really like the idea of no leaks and glassing directly to the hull and deck, however i believe that with design work and having someone else come in to install them it would be too cost prohibitive. i like the idea of sustainability, once i head out cruising. there is always a chance of something breaking, and composite chainplates and titanium have not been in use extensively to 100% garuantee anything (althou i suspect its close). i like the idea that in the slim chance that something did break/crack/bend/get damaged/ect i could have bolt on spares or limp to the next port and have a temporary stainless or bronze chainplate made from drawings or cad file. or even have something shipped out if need be. while i would keep some mas or west system epoxy on board and i could technically make a repair if need be, i could no longer garauntee that the plate would be ok. to change it out id have to regrind everything out, which would be even more fun with the carbon. i am definately tempted, but that slight risk holds me back. the same would go with encapsilating titanium into place. while it is unlikely, anything is possible, and so i think i would prefer to bolt on.

    Thanks

    Robert
     
  2. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 122, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    New knees should be plywood. You will have to laminate several layers of ply to get a knee that's thick enough. Tab them in well and encapsulate them as well. Use epoxy instead of polyester resin. No matter what leaks through, no water should find a way into anything except the bilge.
     
  3. PAR
    Joined: Nov 2003
    Posts: 18,023
    Likes: 353, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I think G-10 is a better choice, combined with epoxy, no further issues once everything is embalmed properly.
     
  4. michael pierzga
    Joined: Dec 2008
    Posts: 4,865
    Likes: 114, Points: 0, Legacy Rep: 1180
    Location: spain

    michael pierzga Senior Member

    Yah Alaskan... rotten knees... I feel your pain.

    Just spent two weeks cutting the Chainplates out of a big metal boat. Brutal job.

    contorted stance, Nose to grinder, sparks flying , plus a Couple hundred trips up and down the ladder left my knees blown. Can barely walk. I suggest you fit a pair of braces. 5 bucks, not bad.
     

    Attached Files:

    • knee.jpg
      knee.jpg
      File size:
      5.9 KB
      Views:
      2,099
  5. peterchech
    Joined: Aug 2010
    Posts: 241
    Likes: 9, Points: 0, Legacy Rep: 127
    Location: new jersey

    peterchech Senior Member

    Def don't replace the plywood with plywood, it will rot again eventually. You might want to stop the leaking too, while you're at it. Re-bed whatever needs rebedding, it's an old boat and this kind of thing must be done every few decades or so.

    PAR suggests G-10. The skip of the phrf boat I race on makes his own panels and uses them for reinforcement of the boat (primarily a floor "grid"). He uses a few layers of heavy mat glass, soaks it in thin epoxy, then "laminates" the layup between two plywood panels with a heavy weight on top of them to give compression (waxpaper is used for the release).

    I'm not sure whether foam core can be used for knees, but for structural purposes he laminates his panels onto structural foam (I think airex but not sure) then cuts to shape and glasses them in. It is very effective, on a small (26') boat at least. I'm not saying it's the ideal method, but it does work for him for flat panels.
     
  6. PAR
    Joined: Nov 2003
    Posts: 18,023
    Likes: 353, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Peter, the skipper you mention doesn't know epoxy, though the use of G-10 is wise. He's using mat, which is just a waste of resin when doing an epoxy laminate. He can save a bit of money (in resin) by using directional fabrics only in his grid laminate. It'll be lighter, stronger and probably slightly cheaper when you count everything up.

    Structural foam can be employed, but you need the right foam and a well executed laminate schedule. It's not something the novice should try, without having the spec's setup professionally, to insure it'll hold up to anticipated loading.
     
  7. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 122, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    I mentioned plywood because there really is no excuse for rotten chainplate bulkheads so any replacement will likely never create an issue. The plywood is solid right through and while foam can be a good replacemewnt material, a novice might consider the simplicity of plywood with an epoxy/glass sheathing. The idea that wood always rots is fallacy. This fear of wood rot in general is very common. Mostly due to badly built examples which if built properly from the start, would never have rotted in the first place.
     
  8. PAR
    Joined: Nov 2003
    Posts: 18,023
    Likes: 353, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Agreed Alan and I'll add most issues are from neglect not the choice of knee material. Plywood is simple and inexpensive, though G-10 is inert and can become part of the laminate. I don't recommend foam without serious laminating experience.
     
  9. Sir Rondo Norma
    Joined: Jun 2013
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Port Hardy, BC

    Sir Rondo Norma Junior Member

    Resurrection

    If I may resurrect this thread, rather than starting a new one, I would like to ask your opinions on opening up the chainplate knee from above and using an auger bit and chisel, removing the plywood core. Then glassing in the cavity to make a solid fibreglass knee. Fill to the top, then gelcoat.
    Wouldn't that be a stronger, simpler way to go?
     
  10. PAR
    Joined: Nov 2003
    Posts: 18,023
    Likes: 353, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Welcome to the forum.

    Yeah, that's a way, but making fairly precise, yet blind plunge cuts with a router, seems tricky at best. Also grinding the inside of the knee surfaces, clean enough for a good bond, would also seem problematic. My thinking is if you're going to hack out 90% of the knee, why bother trying to save the last 10%, when replacement will not be much more difficult and you can insure the bonds are good, possibly improving tabbing or other issues in the process.
     
  11. Sir Rondo Norma
    Joined: Jun 2013
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Port Hardy, BC

    Sir Rondo Norma Junior Member

    Hi Par and thanks for the reply. Part of the reason (admittedly the largest part) that I want to do it from the top side is that we live aboard. Although, I've been tossing different ideas around and I'm starting to lean back towards my original thought of drilling laterally from the inside of the boat, into the knees and using penetrating epoxy and then filling the holes with thickened epoxy.
    There are only 2 knees that need to be done. The backstay knee and one of the running backstay knees.
    Thanks again for the reply and the welcome.

    Cheers,
     
  12. PAR
    Joined: Nov 2003
    Posts: 18,023
    Likes: 353, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    A penetrating epoxy wouldn't be a good choice on a highly loaded part. These epoxy formulations are quite weak, usually fairly elastic too, so skip this step.

    The real problem with "filling" the hollowed out knee shell (once gutted) is getting a good bond to the sheathing and tabbing. I'm not sure of the internal dimensions of your knees, but I can't envision an easy way to do this, unless you open up one of the 3 exposed faces. The least invasive would be the "sided" face. Grind this or cut this off the knee, leaving the tabbing and sides (molded dimension) of the knee intact. You can then insert a small flap wheel with an aggressive grit into the hollowed out knee and prep the surface for laminate and core (or whatever). It would be a simple deal to rebuild the sided face with some cloth and putty. You could do the same from above, through the deck, but again, you're working blind and in my thinking, I'd much rather replace the skinny inboard face of a knee, then matching the deck in finish, color and texture.
     
  13. Sir Rondo Norma
    Joined: Jun 2013
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Port Hardy, BC

    Sir Rondo Norma Junior Member

    Oh I see what you're saying. I didn't realize there would be any elasticity. Very minute, no doubt but still... Given the restrictions inherent in the space I'll be working in, here's what I am thinking now...
    Drill into the inboard edge of the knee with a 3/4" auger bit. They are 2" thick with, presumably, a 3/4" core of plywood. If I space the holes approximately 1" apart, I'd be leaving about 1/4" of material between holes.
    After the holes are drilled all the way up the edge of the knee, I would then use a reciprocating saw to take out the remaining material and effectively gut the rest of the knee.
    Then I would fabricate a new core, probably 3/4" plywood embalmed in epoxy.
    Insert it into the cavity and starting from the bottom of the knee, glass up the incision on the edge of the knee, pumping epoxy into the knee as I work toward the top.
    Re-drill the chainplate holes and reinstall chainplates.

    I doubt that would be too, too hard to accomplish and seems like it would be as strong as it was out of the factory, as long as I can get a good bond to the new core.
     
  14. PAR
    Joined: Nov 2003
    Posts: 18,023
    Likes: 353, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Actually, the elasticity of penetrating epoxy is quite high, much like chewing gum left on a bed post for a night.

    You latest approach seams better suited. I think you'll find a thicker core, but so long as you get back to good solid material (core or laminate) it doesn't much mater.
     

  15. Sir Rondo Norma
    Joined: Jun 2013
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Port Hardy, BC

    Sir Rondo Norma Junior Member

    That much elasticity? Wow, I never would have imagined that. Thanks very much for your advice and feedback. I have started to drill the holes but I'm not in a hurry to get right into it. It will be one of those projects that gets done bit by bit. I know it's going to be quite a while before I will be done.
    Something new cropped up just today. I decided to move a bus bar out of the bilge to a location above the sole in the head. I found that the previous owner had screwed into the hull and used quite a long screw. So long that when the screw came out, water came in. He screwed right through the hull! So that is my new project.
    For the time being, I just coated the screw with sikaflex and re-installed it. Not sure what to do yet. There is no lift around here that can lift my boat.
    Will MarineTex set up underwater?
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.