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Thread: Backstay Chainplate Discussions [pg 152 in Manual]

  1. #46
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    I was looking at the A286 on ebay and noticed his back stay,
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    #97 "Absum!"

  2. #47
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    Talking imho (son of loco)

    On the single backstay chainplate, imco, just the way the system is designed it has to loosen up. The chainplate is pulled forward (as well as Up) above the deck. Below, the bolts are being pulled by the plate in those two directions. The plate is attached to One side of the knee. And the knee is wood. There has to be Some movement in this scenario. How can it be avoided when the boat is sailed?*

    The original thin backstay plate could easily twist in its attempt to straighten itself to the loads on it. Result being a loose stay. The same may be happening to an upgrade that somewhat duplicates the original system.

    In 300 series s.s, a problem occurs when you get a small active area attacked by a large passivated area. When salt water gets into a tight place like thru a rubbered hole in the deck or under a washer behind a nut, oxygen is kept away from the steel so it can't maintain its passivation.

    These steels have a small amount of carbon in them that will combine with the chromium and make carbide. In cloride water an electrical reaction occurs between the combined areas and the uncombined areas. Pitting and crevise corrosion is what's happening when the boundarys break down.

    It's impossible to know what steel you have and how it has been worked. Bending drilling heating welding may change properties. 304 has more carbon it it than 316. Carbon is the problem in salt water, so in 316 it is held to a specific minimum and molybdenum is added to keep those grain boundarys together. Guess it holds the chromium in check? That's why highly polished 316L (passivated) is not as sensitive to partial encapsulation, or oxygen starving. (It's still not trustworthy, imco.) The trouble with pit and crevise corrosion is that it may not be caught or noticed befor the metal just lets go.

    __________________________________________________ _____________________________
    *imco, An external backstay chainplate on the transom is the best way to go. With the top bent at the proper angle and four 1/2" bolts (bronze), there is no stronger connection. It is also my opinion that there is no force that is going to bend the stern of the Ariel in way of the backstay. One might, as a test, spring a batten between the coamings, fore and aft in the rear hatch, and see while sailing if there is any change in the bow of the batten. It's a huge hole in the rear deck there. But I'll bet NO change in dimension will be observed!
    Last edited by ebb; 08-07-2006 at 08:17 AM.

  3. #48
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    Very interesting idea Bill. I'll have to think about that one, but I do agree with Ebb in that flexing of the stern would not seem to me to be consistent with the movement of the cover plate forward to the degree that the screw holes are out of alignment. (cover plate holes now forward of fiberglass holes that once aligned). If the transom (and aft rail) was flexing forward and the knee with it, the whole assembly would move in unison, would it not?

    For four years there was no movement of the backstay chainplate cover plate, but now there is. If the transom were deflecting, then this should have been a problem all along. It wasn’t. I am interested to see when I remove the chainplate if the bolt holes have elongated to any degree in the wooden knee. This would allow some slippage, although the chainplate seems solid as a rock when I put pressure on it at the dock.

    As far a Ebb's crevice corrosion issue, that is correct Ebb; crevice corrosion will be worse for 304 than for 316 stainless. That is why I plan to replace my 304 backstay plate with 316 eventually. You can find a nice discussion of this phenomena and a lot of other cool stuff in “Dan Spurr's Boat Book.”

    After I pull the chainplate, I'll let you know if there has been any crevice corrosion, but based on how the chainplate looks and after only four years, I strongly doubt it. Leaking from above through the polysulfide caulk is the least of your worries when your chainplate is in the floating aquarium of your lazarette locker. The salt water splashing up from below will probably do a lot more damage than the fresh water leaking in from above in the long dormant winter months.

    The issue here is that not everyone wants a chainplate or two on the outside of their transom. Carl Alberg and Pearson decided to put the backstay chainplate on a knee in the lazarette locker with a heck of a long unsupported neck that bends forward. As strange as that seems to us all, it also seems to have worked just fine in most cases for forty plus years. Refining that system in a retrofit with modest moifications is a reasonable thing to do without having to re-engineer the transom or slap external chainplates on the transom. Also, despite the fact that we are all bronze lovers at heart, the yards and rigging shops these day are making their chainplates from stainless steel and not bronze.

    The question remains: What factors, absent corrosion of the chainplate, would cause the chainplate to pull forward and or up in the direction that backstay forces would tend to pull given the fact that the chainplate is bolted to what appears to be a solid knee. Perhaps all that is happening here is that despite the width of the unsupported “neck” of this backstay chainplate, there is enough pressure to gradually bend the neck forward. That is a scary thought, but not inconceivable, in as much as Bill's chainplate pulls forward when he tunes his rig for racing. if that is all that is happening and the flexing is not enough to cause metal fatigue, then perhaps there is no problem. It's till a bit spooky though to see that little chainplate cover pop up under sail.
    Last edited by Scott Galloway; 08-07-2006 at 02:58 AM.
    Scott

  4. #49
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    In 2002, some work was done to my standing rigging by a professional rigger. At that time the rigger installed a 3/16 inch backstay chainplate made of #304 stainless steel. This chainplate was thicker than the original, but one of the bolt holes left a bolt partially exposed at the top of the knee, and another bolt hole was drilled in the wrong place, and had been re-drilled, leaving an unfilled hole.

    In 2002 I removed and replaced that chainplate and had a new chainplate, which was identical, except that the boltholes were where I wanted them. I also glassed a small extension at the top of the knee. The fifth (topmost) bolt passed through this knee. I cut the professionally installed chainplate into two pieces intending to use the lower portion as a backing plate, but this proved infeasible. I kept those two pieces.

    Do to circumstances described in an earlier post above, I removed this “new” backstay chainplate on August 13, 2006. With the exception of a small amount of what appears to be crevice corrosion adjacent to the cover plate (See Photo), and two areas near the bottom bolt hole and the bolt hole third from the bottom, no corrosion was evident with the exception of superficial rust that removed easily with bronze wool. The pits near these two boltholes may have not been caused by corrosion at all, but may instead have been preexisting imperfections in the metal at those two locations. The stainless steel looked like it had been etched at those locations. No rust was evident. In all of the above cases, the corrosion was minor and did not result in any significant weakening of the chainplate.
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    Last edited by Scott Galloway; 08-13-2006 at 10:26 PM.
    Scott

  5. #50
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    The chainplate installed professionally in 2002 was installed, but never used. The rig was under tension then, but I did not sail the boat before replacing that chainplate. The chainplate that I had built to replace it was installed by me and used under sail on approximately 150 days between 2002 and August 13, 2006, when I removed it for inspection. Photos C and D show that the chainplate has a definite bend in it laterally. This bend was caused at the time the chainplate was bolted into place, since the chainplate slot in the deck (taffrail) is slightly to the starboard side of the knee. The chainplate must be inserted through the slot in the taffrail and then pulled back against the knee in the bolting process.

    Photos D and E demonstrate that this bend is identical to the bend that was in the chainplate that was installed but never used under sail. It is also evident from photo D that the two chainplates have identical outlines when laid flat with the professionally installed plate on top of the plate that I installed and used over a four-year period (150+ sails). Since these two plates are identical in outline and in lateral bend, there has been no observable distortion either laterally or longitudinally.
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    Scott

  6. #51
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    Photo F shows the entire chainplate with its cover plate in place. The angle of the transom and the angle of the longitudinal (Aft to forward) bend in the plate make the Ariel backstay chainplate operate essentially as a lever, where the backstay forces are operating on the handle of that lever, pulling it forward and up. I loosened the nuts on the bolts, and experimented by placing forces on the top of the backstay chain plate. The evidence that I found on my boat (hull #330) is that this lever will take advantage of any disparity between bolt size and bolt hole size in either the chainplate or the knee. Although I found no evidence that the bolts had enlarged the drilled holes in the wooden knee, I had no trouble wobbling the bolt in most of the holes. This was also the case in the chainplates themselves. Observation led me to believe that the holes in both the chainplate and in the knee were drilled slightly larger than the bolt diameter, as could be expected.
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    Scott

  7. #52
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    Photos G and H show the degree to which the bolt will work in the chainplate hole. A similar degree of motion was possible in the boltholes through the knee. I believe that under sail with higher wind loads (30+ mph) this potential for movement permitted the backstay to rise and pull forward sufficiently to push against the forward edge of the chainplate slot in the deck causing some minor gelcoat cracking in the deck, and also against the cover plate, which raised the front edge of that cover plate, and causing the cover plate screws to elongate their holes in the fiberglass deck.

    Observation tells me that the knee is solid, no corrosion of the chainplate or bolts is evident, and no knee failure is evident. I do not believe that the transom is pulling forward. I believe that the lever arm of the chainplate is merely pulling on the bolts and that is causing some slippage due to sloppiness of the boths in their drileld holes. Of course this is just a theory. I invite other perspectives and ideas.

    In attempt to remedy the problem, I removed the chainplate, filled all cover plate screw holes with epoxy, and I will re-drill them. I coated all boltholes in the knee with West System epoxy and then I added a thin coat of the same epoxy thickened with colloidal silica. I will file the holes to permit a tight fit of the chainplate bolts using a round file. I also did this with my shroud chainplates when I installed them in 2004. Finally, I sanded a slight forward slope from top to bottom in the forward edge of the fiberglass deck chainplate slot so that if there is still some motion of the plate in the slot, it will not put pressure on the fiberglass taffrail and cause more minor gelcoat cracking. I was thinking about replacing the #304 chainplate with one made from #316 stainless steel, but it does not appear that this will be necessary in the short term. Removal and inspection of the chainplate took about one hour. Reinstallation should take a second hour including re-bedding the cover plate. #304 stainless is stronger than #316, but more susceptible to corrosion.

    Any thoughts?
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    Last edited by Scott Galloway; 08-13-2006 at 10:40 PM.
    Scott

  8. #53
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    Scott, thanks for the very thorough inspection and photos. I like the "beauty" cover for your chainplate - I've been wondering what I'm going to do about that and wishing I had one made too.

    I'm wondering about the setup you describe below:
    Quote Originally Posted by Scott Galloway
    ...Photos C and D show that the chainplate has a definite bend in it laterally. This bend was caused at the time the chainplate was bolted into place, since the chainplate slot in the deck (taffrail) is slightly to the starboard side of the knee. The chainplate must be inserted through the slot in the taffrail and then pulled back against the knee in the bolting process.
    Just a couple of thoughts for discussion. I would think enlarging the slot in the taff rail to allow the chainplate to freely exit without being forced to bend would accomplish at least three things:
    1. Reduce unneeded lateral stresses on the knee and on the chainplate itself.
    2. Allow the chainpate to be bolted snugly to the knee, which would eliminate any "play" the bolt holes may have, and I would think you don't want.
    3. Eliminate unneeded stress on the taff rail.

    Are bigger bolts out of the question for your chainplate? I'd think "play" is a bad thing.
    Mike
    Totoro (Sea Sprite 23 #626)

  9. #54
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    simple alternative backstay plates

    An anternative improvement to the Pearson backstay on the Ariel would be to have a chainplate on both sides of the knee. IE TWO chainplates of exactly the same pattern.

    The plates would be flat with no kink in them.

    You could toggle between the plates for the turnbuckle.

    The stress on the plates would be more even and you probably could keep the hole(s) through the sternrail caulked.

    You probablly could use the same miserable BBQ tong material since you have now doubled the specs and evened the forces.
    Last edited by ebb; 08-14-2006 at 07:35 AM.

  10. #55
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    An anternative improvement to the Pearson backstay on the Ariel would be to have a chainplates on both sides of the knee. IE TWO chainplates.
    That is exactly what I have on #226. Prior owner's 'beter idea' that seems quite a bit stronger then the original.


    s/v 'Faith'

    1964 Ariel #226
    Link to our travels on Sailfar.net

  11. #56
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    Nothing new under the sun, ehhhh, Craig?

    Isn't it wierd that the A/C has that nice stem casting, bronze shroud plates

    and that dinky backstay strap?
    Almost an afterthought. "Oh, hey Carl, we forgot the backstay this time. Wudowedonow?"

    An UPgrade would be to keep the theme going from the front end of the boat.
    Use some bronze plate. Just an opinion.
    Last edited by ebb; 08-14-2006 at 07:55 AM.

  12. #57
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    In answer to MDB,

    "I would think enlarging the slot in the taffrail to allow the chainplate to freely exit without being forced to bend would accomplish at least three things:
    1. Reduce unneeded lateral stresses on the knee and on the chainplate itself."

    Pearson simply put, the knee or the slot in the wrong place. This is also the case for some of the shroud chainplates. They also had to be slipped through the slot at an angle and then pulled back to the knees or bulkheads. I can just imagine a guy at the Pearson factory with a saber saw taking a wild guess from above at where to cut a chainplate slot to align that slot with the knee.

    So unless you want to cut a new slot or have a humungous hole in the taffrail, which is itself a structural element, The bend (or kink as Ebb calls it) is a reality. This necessarily means that the chainplate may not be snug against the knee at the top most chainplate hole, which is probably not a good thing. Ebb's idea of having backstay plates on both sides of the knee is an interesting idea, and I thought about that, but it is probably overkill for limited coastal cruising and day sailing applications. The governing issue here is how strong is the knee attachment to the transom in relation tot eh strength of the chainplate assembly. I thought about twin chainplates with a teak or synthetic spacer in between them above deck, but decided not to do that. Again in four years of use there has been no observable corrosion of stress deformation of my 3/16 inch #304 stainless steel backstay chainplate.

    "2. Allow the chainplate to be bolted snugly to the knee, which would eliminate any "play" the bolt holes may have, and I would think you don't want."

    Yes it would certainly do that. I believe, however, that the play in the bolt holes is due to the size of the holes. I also noted this with my shroud chainplates, so for the shroud chainplates, I reinforced the bulkheads and knees and then lined all of the bolt holes below with epoxy. Ithen filed them to the perfect diameter in a one-by-one laborious process. This will be even more fun in the difficult to reach spaces of the lazarette.

    The difference between the shroud chainplates and the backstay chainplates is that although the sides of the hull do slope and the forestay knees slope inward, the shroud chainplates are straight and more or less (except die to slot placement errors) rise vertically though the deck. So the forces imposed by the rig on the shroud chainplates are primarily vertical. The backstay chainplate on the other hand, is fabricated so that the top portion rises (slopes) forward after the last bolt hole. This in combination with the aft sloping installation on the knee creates a very powerful lever, and the forces imposed by the rig are both vertical and forward to a very significant extent. The forward slope and the length of the backstay between the top bolthole and the deck insures that the forward component of those forces will be very significant.

    "3. Eliminate unneeded stress on the taffrail."

    Yes it certainly would do that also, but I am not sure that the lateral force of the chainplate against the starboard side of the slot is a problem. It may be, but I have been more concerned with the longitudinal force exerted by the chainplate on the front edge of the slot. That is why I filed underside of the front edge of the slot to create more room for the chainplate to "work". A previous owner also must have tried this, because the slot already extends forward of the cover plate, leaving less taff rail before the fiberglass taffrail drops to the deck level.

    "Are bigger bolts out of the question for your chainplate? I'd think "play" is a bad thing."

    The boltholes were drilled to the correct size, but the correct size still allows play given the forces “at play”. This was also the case with the shroud chainplates. In other words, the next larger size bolt appears to be too big for the hole, and the next lower size drill bit appears to be too small for the current bolts.

    Note to Ebb who wrote: "An upgrade would be to keep the theme going from the front end of the boat. Use some bronze plate. Just an opinion."

    Yes I agree with you a complete set of silicon bronze chainplates would be a wondrous thing. I however after checking in with two riggers and three boat yards, elected to use #16 stainless for my shroud plates because,

    1. They have it in stock, and I had a mast down waiting for completion of the chainplate project
    2. Stainless steel is what the professionals are mostly using use these days, or so it appears at least locally.
    3. #316 stainless is pretty good stuff
    4.It appears to be less expensive than silicon bronze
    5. As I recall both #316 and #304 are stronger than silicon bronze...but I haven't verified this recently with a reliable source, so don't rely on this statement.)
    6. The bronze shroud chainplates and bolts that I removed were pretty much gone. So bronze ain't perfect. Of course, who knows what sort of bronze that was... and it was probably original (forty years old). Most of the bolt diameter had transformed to colloidal copper (like a red penny), and this was also the case with the plates. I didn’t cut the plates in half, but I could not strip through the copper. Some of the bronze bolts snapped in two when I was removing them. (See photos of these bolts elsewhere on this forum.)

    I am still contemplating ordering new backstay chainplate not because my current one is damaged or corroded, but because my current #304 is a different alloy than the #316 bolts, washers and nuts, and therefore in the wet, salty environment of the lazarette locker, the system seems to shed a fair amount of rust as evidenced by the streaks leading downward from the chainplate, nuts and washers. Most of these streaks were actually coming from the #316 nuts and washers. The rust is unsightly and troublesome, even if pretty much hidden in the locker...and by the way the rust stopped above the splash zone. In other words there was little rust at or above the top hole, where the salt water from the well does not normally reach when under sail.
    Last edited by Scott Galloway; 08-14-2006 at 12:55 PM.
    Scott

  13. #58
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    Talking

    Quote Originally Posted by Scott Galloway
    In answer to MDB,

    The bronze shroud chainplates and bolts that I removed were pretty much gone. So bronze ain't perfect. Of course, who knows what sort of bronze that was., and it ws probably forty years old. Most of the bolt diameter had transformed to colloidal copper (like a red penny), and this was also the case with the plates. I didn’t cut the plates in half, but I could not strip through the copper. Some of the bronze bolts snapped in two when I was removing them. (See photos of these bolts elsewhere on this forum.)
    Yes, but think of the wonderful protection they offered as anodes for the backstay....






    s/v 'Faith'

    1964 Ariel #226
    Link to our travels on Sailfar.net

  14. #59
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    Scott,
    Consider using silicon bronze bolts for the 316L plates.
    Avoids stress corrosion that will occur even with 316 nuts and washers. imho

  15. #60
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    My guess is that you would tear the knee off the transom and tear a big hunk out of the transom before a chainplate assembly consisting of a 3/16 inch thick #304 chainplate and 3/8 inch bolts, locknuts and washers would fail. Now, it may pay to remove and inspect any chainplate periodically, but in four years of sailing there was no significant corrosion on my backstay chainplate, despite the stresses that it endures and the salty, wet environment in which most of it is housed. I finished cleaning the chainplate today, filed by epoxy lined bolt holes to fit 3/8 inch bolts with a round file, bought some new 3/8 inch bolts nuts and washers for good measure, slathered all of the above with Lanocoat, and put it all back together. There had been no deformation of the #304 stainless steel chainplate in four years of sailing, and almost no corrosion.

    I did take my four-year old #304 stainless steel chainplate to the boatyard to get an estimate for a replacement chainplate to be made of 3/16 inch #316 stainless. After hearing the estimate, and conferring with the yard people, I decided to reinstall my #304 chainplate. The estimate for a new one made of #316 stainless was approximately $250.00, including material and labor. The yard concurred with me that the #304 chainplate that I already have was in good condition, and not in need of replacement, either now or in the foreseeable future. It would have been in their economic interest to tell me otherwise.

    It would seem to me from what little I know about galvanic action on dissimilar metals that a #304 stainless chainplate and #316 stainless bolts would be more compatible than #304 stainless chainplates and silicon bronze bolts, or #316 stainless chainplates and silicon bronze bolts...or am I missing something here?

    Of course in the best of all possible worlds we would be using silicon bronze plates and bolts or #16 plates and bolts.

    Here is another question for you guys: If as C Amos says, depending on the metals we use, our backstay chainplates in the frequently saltwater-filled lazarette may be serving as “anodes for the backstay”, would it make sense to put a zinc on the backstay chainplate?
    Scott

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