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Thread: Chainplate Woes: Opinions Welcome

  1. #1
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    Default Chainplate Woes: Opinions Welcome

    When I rebuilt Bucephalus's deck a couple years ago, and refastened her, I splurged on casting her a new pair of chainplates (aluminum bronze), and I shifted them aft from their original position by 11", to give the mast some much-needed backstaying. The old chainplates had been sawn out of 1/8" half-hard silicon bronze plate, and the holes for the clevis pins had been worn oval over the years. The replacement was due.

    Nothing else on the rig has changed, and the new chainplates are fastened with the same three 1/4" machine-bolts, with the same oak backing pads on the inside of the planking as the originals. Atop the oak backing pads are not just standard washers, but fender washers, to better spread the compression of the nuts.

    However, each year since, the new chainplates have "started": a hairline crack appears in the paint around the chainplate, typically after a hard sail, and the lower edge of the countersunk machine-bolts' heads lift a hair's breadth, showing that the chainplates have been pulled upward in line with the shrouds. Each time, these past two winters, I have removed the chainplates, saturated the holes in the planking with epoxy to harden the wood, replaced/rebedded them, and tightened down the bolts to where the planks (eastern white cedar) are slightly compressed and the chainplates are slightly flexed at each bolt --in other words, as tight as I can take them before bending the metal or crushing the wood.

    At no time in the 30 years previous had the original chainplates started (that I ever noticed). This would tell me that either shifting the shrouds aft 11" has added strain on them --which seems odd since the change has actually widened the stance of the shrouds (although the gaff is now more likely to rest against the shrouds when we're running free)-- or there is something about the new chainplates that makes them less stable --perhaps something to do with them being more flexible than the originals?

    Yesterday, after a rock-'em-sock-'em sail --west wind 5-15, gusting 25 (west winds are like that, here)-- I checked the chainplates and found that yes, the %$#@ing things have once again started. Hairline cracks in the paint and, on each side, a pea-sized blob of bedding compound that had squeezed out. It was a lulu of a sail, but by no means the most exciting she has ever seen --and the original chainplates had never moved.

    So clearly something needs to change.

    My options are, as I see them:

    1) More 1/4" bolts. Each chainplate currently has three, one through each of the top three plank strakes, the top one through the sheer clamp as well. They are fortuitously spaced so that I could add two more bolts per chainplate: one at the bottom edge of each of the top two strakes.

    2) Change to larger, 5/16" bolts. I would need to lightly re-shape the heads of 5/16" machine bolts, making them an "oval head", --because I don't want to remove any more metal from the chainplates by deepening the countersinking already there-- and the heads would thus stand a little proud, but that's a pretty minor detail.

    Which would provide better resistance to the strain of the rig? More bolts, or bigger bolts? My intuition says more bolts would be better, but if my intuition were trustworthy, the cussed chainplates wouldn't be moving around in the first place.

    (I recognize that what might also need to happen in conjunction with 1 or 2 are new, slightly (1/16" - 3/32") thicker chainplates that resist the bending of the tightened bolts, thus distributing the force of the bolts across a greater area of the planking (where otherwise they currently tend to flex at each bolt). In tension the chainplates are more than strong enough, but if they are too flexible under the bolts, that may be just as critical as tensile strength.)

    I have received one reputable opinion that I should go with larger bolts, but I'm looking for confirmation or refutation of that opinion.

    What are people's thoughts?

    Alex

  2. #2
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    Default Re: Chainplate Woes: Opinions Welcome

    Is the spacing on the bolts similar? By moving the chainplates aft, I'm thinking you have more stress because of how the shrouds are now balancing the forestay more.

    While not an engineer, I'd think another bolt or 2 would be better than bigger bolts - as they spread out the load more.

    How big a boat is she? I forget - even though I've enjoyed pics of her.
    "If it ain't broke, you're not trying." - Red Green

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    Default Re: Chainplate Woes: Opinions Welcome

    Both

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    Default Re: Chainplate Woes: Opinions Welcome

    I had something similar happen except on my mast, so I counterbored out the hole well oversized and cast a hard epoxy bearing in situ. Rebored and bolted the fittings in . That worked.

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    Default Re: Chainplate Woes: Opinions Welcome

    How big a boat is she? I forget - even though I've enjoyed pics of her.
    She's 19' LOA, 16' LWL. This is her about a week ago, under conditions where the chainplates were not at all at risk:



    Both
    Yeah, I'm thinking that's a possibility, too. I'd like to try one at a time, though.

    I counterbored out the hole well oversized and cast a hard epoxy bearing in situ. Rebored and bolted the fittings in.
    I've definitely considered that. I may need to explore it further. How big were your bolts, and how big an area did you counterbore for the epoxy "bushing"? Did you bore all the way through, or just partial thickness?

    Alex

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    Default Re: Chainplate Woes: Opinions Welcome

    I'd go with the extra bolts. You definatly have more strain having moved the shrouds aft. Under the old placement, the mast took most of the forwar thrust and that bowing would slightly depower the rig. Now it does not get that relief. Anyway, the step and partners and mast all thank you.

  7. #7
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    Default Re: Chainplate Woes: Opinions Welcome

    I agree with Ian.
    IMAGINES VEL NON FUERINT

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    Default Re: Chainplate Woes: Opinions Welcome

    Are you sure there's no movement in the planking?

    Rick

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    Default Re: Chainplate Woes: Opinions Welcome

    God she is a nice boat.
    I'm thinking the holes need over boring, filling with WEST 404, re boring . I bet the bolt holes in the planking have gone oblong.
    What John said in # 4.

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    Default Re: Chainplate Woes: Opinions Welcome

    Even if I added an extra bolt I'd still create epoxy bearings with microfibres( forgotten the numbers of the additives) Sets like rock and you've dramatically increased the bearing surface of the bolt by doing that.
    How much and how far through? I wouldn't exceed the width of the plate so it remained hidden. what ?... inch ... inch and a quarter or half.. something like that. I wouldn't go right through.

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    Default Re: Chainplate Woes: Opinions Welcome

    the cast bronze chain plates are flexing? that does not sound right... ?

    what happens if you paint over the started /cracked paint, does it crack again... do they continue to move?

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    Default Re: Chainplate Woes: Opinions Welcome

    More and bigger bolts thru 1 1/4 inch diameter epoxy plugs to spread the weight. I can't imagine any excuse for not doing all three, and while I was at it I'd probably bed the plates in thickened epoxy.

  13. #13
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    Default Re: Chainplate Woes: Opinions Welcome

    Three concerns:

    I'd not heard of aluminum bronze for this. Your deflection may be from that.

    On Meg we bolted through an inner strip of bronze that served as a giant long washer. I think that helps rigidify the whole.

    I would not attempt an epoxy cure here.

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    Default Re: Chainplate Woes: Opinions Welcome

    I like the thought of creating epoxy bushings. In doing that I don't think you'd need to go up a size with the bolts. Can you use the old chainplate as a backer under the nuts? That will minimize or eliminate any tendency of the bolts to distort the new chainplates.

    Jeff

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    Default Re: Chainplate Woes: Opinions Welcome

    Under the old placement, the mast took most of the forward thrust and that bowing would slightly depower the rig. Now it does not get that relief. Anyway, the step and partners and mast all thank you.
    Before I moved the chainplates aft, running before a stiff breeze the forestay turnbuckle would be slumped over sometimes almost parallel to the horizon --no exaggeration. That much flex in mast and partners was pretty scary. There's a reason I moved the chainplates aft! I don't want to add a second set of shrouds --and on a 19' boat I shouldn't need to-- but even if I have to, I'm glad to have the backstay action. (She goes markedly better to windward for it, too, with the improved forestay tension.)

    I bet the bolt holes in the planking have gone oblong.
    They had after the first season with the new chainplates, but as I said in the OP, I've hardened the holes with epoxy since they first moved. The first time, that involved re-filling the bolt holes with thickened (microfiber) epoxy and re-drilling --including a sort of counter*sunk* area at the outer surface of the plank that I imagined might work as a washer to prevent the holes from becoming oblong. This past winter, I was surprised that the holes *weren't* oblong, but the previous treatment has done pretty well. They don't have the epoxy "bushing" some of you are advocating, but they aren't too seriously distorted.

    Sets like rock and you've dramatically increased the bearing surface of the bolt by doing that.
    That's what I'm seeing. Even if I continue to use 1/4" bolts, if I then "cast" a 3/4" diameter epoxy bushing, I've increased the shear-load bearing surface threefold.

    How much and how far through? I wouldn't exceed the width of the plate so it remained hidden. what ?... inch ... inch and a quarter or half.. something like that. I wouldn't go right through.
    Right. The chainplates are 1" wide, and the planking 5/8" thick, which sounds like I'd probably go with a 3/4" diameter bushing 1/2" thick.

    the cast bronze chain plates are flexing? that does not sound right... ?
    I know; it's weird. The originals, cut out of bronze plate, are significantly less flexible than the cast ones. I don't like it either.

    what happens if you paint over the started /cracked paint, does it crack again... do they continue to move?
    I don't know, but that's exactly what I've wondered, too. So far, I've re-bedded the chainplates each season (twice now) after their movement, trying to fix the problem, and since I don't re-paint in the middle of the season, each season has begun with fresh paint over freshly-bedded chainplates and has ended with a new hairline crack. But yes, the question remains: have they moved as far as they're going to move, and it's time to call it good and leave them now that they've settled into place? I've owned her since day one, and I don't remember cracks around her chainplates the first spring I repainted her --but that was 32 years ago. I suspect that it is as Ian has diagnosed, and that the backstay work has resulted in greater strain now than the chainplates endured originally.

    I will say that once the cracks have formed, typically at the beginning of the season, they don't get bigger. Thus far, at any rate, the chainplates move, and then stop. (Knock wood!)

    More and bigger bolts thru 1 1/4 inch diameter epoxy plugs to spread the weight. I can't imagine any excuse for not doing all three, and while I was at it I'd probably bed the plates in thickened epoxy.
    I'm thinking about it. I doubt I'd bed the plates just because from all I've read epoxy bedding doesn't really contribute to resistance to pure shear load like this. But the "use everything including the kitchen sink" approach does have some appeal. I'm still trying to get a better consensus on the More vs. Bigger question at the moment, though.

    Thank you all. I'm learning a lot.

    Alex

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    Default Re: Chainplate Woes: Opinions Welcome

    Aluminum Bronze is quite hard and is therefore highly resistant to stretching. The fact that it has been cast makes the metallurgic grain of the material less strong than a chainplate that has been made out of rolled plate as while the material is hard, the grain is not running paralel to the length of the plate. The fact that the old chainplates stretched but did not break is and indication that using the same material again would be a better choice. However you might consider doubling the plate from two pieces of 3/16 silicone bronze plate and using round head bolts that are smaller in diameter than the ones you just used and staggered along the length on both sides of the centerline. This will keep the strain from being all in one place by spreading the bolt pattern. You might even consider increasing the length of the chain plates in order to have another bolt or two. The plates can be tapered in width towards the lower end. This is a consideration if the chains are long pieces that are in line with the load. No two of the fastening bolts should align with one and other.
    Jay
    Last edited by Jay Greer; 06-21-2018 at 09:15 PM.

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    Default Re: Chainplate Woes: Opinions Welcome

    Folks posted while I was writing!

    I'd not heard of aluminum bronze for this. Your deflection may be from that.
    It's a very high tensile strength bronze alloy. Don't use it near your compass --it has iron in it, and *does* attract a rare earth magnet-- but it's pretty amazing stuff for strength and corrosion resistance. I have no idea how it compares in flexibility to, for instance, manganese bronze.

    Can you use the old chainplate as a backer under the nuts?
    Hm. Good idea, but none of the holes match, so if I go that route I might as well use a new piece of bronze, as Ian suggests.

    Alex

  18. #18
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    Default Re: Chainplate Woes: Opinions Welcome

    The fact that it has been cast...
    Okay, now we're talking science that I can get my head around. That makes sense.

    you might consider doubling the plate from two pieces of 3/16 silicone bronze plate and using round head bolts that are smaller in diameter than the ones you just used and staggered along the length on both sides of the centerline. This will keep the strain from being all in one place by spreading the bolt pattern.
    Not sure I could double the plate, as you suggest. Interesting idea, though.

    It also sounds --from the suggestion of going with smaller bolts in a staggered pattern-- as though you would advocate for "More bolts" (spread the load) rather than "Bigger bolts" (increase resistance to a concentrated load).

    You might even consider increasing the length of the chain plates in order to have another bolt.
    Fortunately, that's what I did. Which is why I have the option of adding two new bolts in addition to the original three per side.

    They aren't wide enough to stagger the pattern, though. Which is a bummer.

    No two of the fastening bolts should align with one and other.
    How critical is that, with the strain being across the grain of the planking rather than in-line, where the strain could split it?

    And I forgot to address:

    Are you sure there's no movement in the planking?
    Yes, thank Fortune. At least judging by the un-marred plank seams around the chainplates, everything looks fine. And by blind luck, I was smart enough to make the new chainplates long enough that the bolt pattern is now across three planks where it was originally across two. Originally there was one bolt in the sheer strake and two in the #2 strake, in the top and bottom quarters of the plank; the current pattern is one in each of the top three strakes, each in the top quarter of the plank; my current leaning toward "More bolts" would put two bolts in each of the top two strakes, in the top and bottom quarter of each plank, and the fifth bolt in the top quarter of the third strake.

    Alex
    Last edited by Pitsligo; 06-21-2018 at 09:38 PM.

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    Default Re: Chainplate Woes: Opinions Welcome

    Hi Alex,
    I'd have to go with Ian and Jay on the fastening scheme. You will get more clamp force from multiple, staggered fasteners than you will from fewer, larger fasteners and it will distribute that force over a wider area. I can't speak to the alloy, but I understand fastening systems fairly well.

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    Default Re: Chainplate Woes: Opinions Welcome

    I would like to see close up pictures of the crack the surrounding area and the interior chain plate area.

    When you moved the chain plates did you reorient the angle of the plates to match the direction of pull from the shrouds?

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    Default Re: Chainplate Woes: Opinions Welcome

    Quote Originally Posted by navydog View Post
    I would like to see close up pictures of the crack the surrounding area and the interior chain plate area.

    When you moved the chain plates did you reorient the angle of the plates to match the direction of pull from the shrouds?
    Excellent question.

    I absolutely believe that more fasteners is better than just bigger ones. As said, strip of bronze instead of the washers would also help spread the load.
    "If it ain't broke, you're not trying." - Red Green

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    Default Re: Chainplate Woes: Opinions Welcome

    When you moved the chain plates did you reorient the angle of the plates to match the direction of pull from the shrouds?[/QUOTE]

    My first thought as well....

    Damn nice looking boat.

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    Default Re: Chainplate Woes: Opinions Welcome

    We always reckoned chainplates should span a minimum of three planks. They should be aligned to the line of the shroud.

    If the plates are external, round head bolts are better that countersunk bolts for a bolt in sheer. The bolts should not be threaded the whole way up, just the plain shank in the shroudplate and the planking – the threaded part in the backing timber and washers etc.

    Aluminium bronze is an excellent material in its normal rolled strip form - very strong, hard and corrosion resistant. Don't know about in the cast form – probably as Jay says, not so strong.

    The shrouds will be under quite a bit more load because they now have a backstay function as well as an athwartships shroud function – as evidenced by the tauter forestay etc. Though you have increased the shroud angle, which theoretically improves the load, they probably were not actually taking any significant fore-&-aft load previously – and now they are.

    I don't think the shroudplates are stretching (or not enough to be relevant) – I think you are indeed getting a small amount of movement of the shroudplate against the planking, which is indicates that they are under too much load for their length and the fastenings.

    With evidence of movement like you have, I would replace the shroudplates with longer ones, made from rolled bronze strip rather than cast – aluminium bronze, silicon bronze, phosphur bronze should all be OK, sized correctly - with one larger size bolts than now, spanning at least three planks – minimum 4 fastenings.

    While epoxy-ing the holes - or making epoxy bushes etc - is good, properly designed and fastened shroudplates should be OK without – as they were before the days of epoxy. A good backing piece, at least the same thickness as the planking, preferably a hard timber like locust, shaped to fit the curve of the planking is pretty essential. Then large pattern washers should be fine.

    With a rig like yours, I usually do a calculation considering the aft shrouds as a backstay as well as a shroud – and it's surprising how large the loads can get. It would be worth doing a calculation to get a handle on the likely loads.

    Cheers -- George
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    Default Re: Chainplate Woes: Opinions Welcome

    Well, there you have it: chapter and verse from a well known designer of boats of this size. Nothing to add to that except Thanks, George!
    IMAGINES VEL NON FUERINT

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    Default Re: Chainplate Woes: Opinions Welcome

    You will get more clamp force from multiple, staggered fasteners than you will from fewer, larger fasteners and it will distribute that force over a wider area.
    That's my thought, too, but hearing it from others is reassuring.

    The only possible trouble is that they won't be staggered, since the chainplate is only an inch wide. Even so, I'm thinking More rather than Bigger, and if More doesn't do it, change them up to Bigger the next time.

    I would like to see close up pictures of the crack the surrounding area and the interior chain plate area.
    I'll try to remember to take my camera down to the boat today when I go in to wind her clock.

    When you moved the chain plates did you reorient the angle of the plates to match the direction of pull from the shrouds?
    Yes! That was one of the details I really sweated, and actually got it pretty much dead on.

    As said, strip of bronze instead of the washers would also help spread the load.
    I'll see what I can dig up in the bronze sheet bin.

    We always reckoned chainplates should span a minimum of three planks. They should be aligned to the line of the shroud.
    The originals spanned two. I lengthened the new ones to span three.

    If the plates are external, round head bolts are better that countersunk bolts for a bolt in sheer. The bolts should not be threaded the whole way up, just the plain shank in the shroudplate and the planking – the threaded part in the backing timber and washers etc.
    I followed the original builder's lead and went with countersunk bolts, but yes, in retrospect I'd probably use carriage bolts. Fortunately, TopNotch fasteners has actual machine-bolts: machine screws that aren't threaded for their entire length, just the last bit.

    Though you have increased the shroud angle, which theoretically improves the load, they probably were not actually taking any significant fore-&-aft load previously – and now they are.
    That fits. They were taking *zero* backstay load before, being set exactly athwartships from the aft face of the mast. Now they're doing two jobs.

    I would replace the shroudplates with longer ones, made from rolled bronze strip rather than cast
    I guess using rolled instead of cast is now the back-up plan, if adding bolts doesn't do the job. I suppose I need to find a source for 1/8" x 1" bronze stock.

    While epoxy-ing the holes - or making epoxy bushes etc - is good, properly designed and fastened shroudplates should be OK without – as they were before the days of epoxy.
    That was my thought as I was lying in bed this morning.

    A good backing piece, at least the same thickness as the planking, preferably a hard timber like locust, shaped to fit the curve of the planking is pretty essential. Then large pattern washers should be fine.
    Yup, that's what I've got: fitted white oak backing plates with fender washers, and regular washers between the fender washers and the nuts. I copied what she had originally.

    With a rig like yours, I usually do a calculation considering the aft shrouds as a backstay as well as a shroud – and it's surprising how large the loads can get. It would be worth doing a calculation to get a handle on the likely loads.
    That's the next step, if two more bolts per side doesn't fix the problem.

    Thank you all. This is great information and explanation.

    Alex

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    Default Re: Chainplate Woes: Opinions Welcome

    Not seeing a picture of what the area of the chainplate looks like really makes it hard to speculate. Usually, when I design a rig I go between a frame bay with a short "stanchion" frame that ties into the sheer clamp and sets the chain plate off of the planking a bit. This further ties the planking together and imparts more strength to the attachment. Here then a wider chain plate can be fashioned from plate the double sheet further spreads the load. The width also permits better staggering of the fastenings. I do find it rather amazing that such a small boat can generate enough load to create cracking of the bronze material. Another factor is if the plate is bent to match the curve of the hull or the bolts are taken up enough to place undue face loading on the bronze.
    Jay

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    Default Re: Chainplate Woes: Opinions Welcome

    All, if you squint, this boat wants to have running back stays...I know that is not a happy conclusion, but most any boat of this type of design would be sporting runners...
    A local design here in the Bay uses a backing 'frame' that resolves to a tight fit up under the clamp, so that the shear load of the chainplate bolts is spread up to this structure, as well as to the planking... also, we should remember, chain plates have highly variable loads at any give moment, so I would expect minute motions, paint is not really a flexible resilient product, so not surprising to note 'starting', for chain plates, I often build discrete cover plates, that we can periodicly lift up and rebed with your best choice sealant, to try and deal with the inevitable motion... Great looking boat! Cheers. Steve

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    Default Re: Chainplate Woes: Opinions Welcome

    Typically Friendship sloops do not have running backs, they were working boats developed for lobstering single handed and the rig is plenty beefy not to need running backs, unlike most racing yachts.

    the cast chain plates flexing sounds very strange to me, most I have seen are 1/4 inch thick even for a 16' sailing dory and would never flex... could the wood be flexing? or can you see the metal bend?

    if the emtal is bending I would consider different plates.




    Could the bedding compound be the problem?
    counter sunk screws/bolt heads tighten as the part under them tries to move, so a tiny bit of movement, especially with a lubricant such as a layer of bedding compound between the plates and the hull, may be OK...


    Keep an eye on them and see if they continue to move... my concern would be cracking/breaking a chainplate... if they really are flexing.
    Last edited by Daniel Noyes; 06-22-2018 at 12:00 PM.

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    Default Re: Chainplate Woes: Opinions Welcome

    This boat design depends on the main sheet to load the head stay when working to weather. The main usually has a topping lift to aid with using the boom and main sheet to keep the jib stay tight. This is a boat that does not need complication. In fact, I would have left those single shrouds in their original position. Just looking at the forward face of the mast, in profile, shows a slight bow in the stick. This further slacks the jib stay as the height of the mast head lowers with the bend. This means that the shroud is really tuned banjo string tight! This can lead to cracked frames and other structural problems like hogging.
    Jay
    Last edited by Jay Greer; 06-22-2018 at 12:21 PM.

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    Default Re: Chainplate Woes: Opinions Welcome

    What George said is sound (and valuable) advice.

    But, I will add the caveat that countersunk fasteners are not typically used in shear applications. They will add an additional "compression" or wedging load to the assembly that would not be present with standard cap screws and nuts. I forged silicon bronze carriage bolts for my application which has the shrouds set aft to work as backstays. 200 sg. ft mainsail, 4-3/8" bolts, three planks, bedded in Sika Flex with a wide white oak partial frame, also bonded with Sika. No movement - 20 years, and this little boat has worked to weather in some very strong SF bay wind, well over 40 knots.

    In addition, when crunching the numbers for the additional "backstay load" on the shrouds, they quickly became alarming and I increased the wire size, the pin and turnbuckle size to accommodate a comfortable safety factor. At first I thought they would look too large and out of place, but they just disappeared.

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    Default Re: Chainplate Woes: Opinions Welcome

    I do find it rather amazing that such a small boat can generate enough load to create cracking of the bronze material.
    Ah. You misunderstand: it is not the bronze that is cracked, it is that by moving, the chainplate has left a hairline crack in the paint where it has shifted. As has been pointed out, paint is not especially elastic.

    All, if you squint, this boat wants to have running back stays...
    I certainly considered them. And that may be the result if I can't make the current arrangement sound.

    Typically Friendship sloops do not have running backs, they were working boats developed for lobstering single handed and the rig is plenty beefy not to need running backs, unlike most racing yachts.
    Amen. And in truth, she has proven her strength in serious wind before I shifted the chainplates --on one memorable (!) sail, the USCG reported gusts to 59kt, and we were out in open water by the Duck Islands (long story, and *NOT* my idea of a good time)-- but most sloops have their aft shrouds a little farther aft than B's were, providing that backstaying, so I'm happy to have changed things up a bit.

    the cast chain plates flexing sounds very strange to me
    The chainplates are solid enough on the boat, it's when I have them off and in my hands that I can make them flex more than the original plate chainplates. They're 1/8" bronze, so even "more than the originals" isn't much. I just found even that small difference curious.

    Could the bedding compound be the problem?
    I have wondered *exactly* the same thing! Is the bedding compound serving as a lubricant? I just don't know. Next time, I might bed it in something that is an adhesive as well as bedding. Probably 3M 4000 UV, since that's what I keep "on the shelf".

    my concern would be cracking/breaking a chainplate...
    No, I seriously doubt that would happen. I'm far more worried about tearing them loose.

    The main usually has a topping lift to aid with using the boom and main sheet to keep the jib stay tight. This is a boat that does not need complication. In fact, I would have left those single shrouds in their original position.
    Actually, B does have topping lifts, combined with her lazyjacks.
    I agree that she does not need complication.
    As much as the forestay slacked off when running, before I shifted her chainplates, I'm pretty happy to have some backstay action. There were a couple occasions when conditions were right that her mast started "pumping", too. Not good. Also, when I was removing her old deck, the foredeck fastenings showed signs that they had been worked pretty hard, and I wouldn't be surprised if that came from taking the compression load of the mast.

    Just looking at the forward face of the mast, in profile, shows a slight bow in the stick. This further slacks the jib stay as the height of the mast head lowers with the bend. This means that the shroud is really tuned banjo string tight! This can lead to cracked frames and other structural problems like hogging.
    Actually, if you put a string on it (as a straightedge), the mast curves aft. In her first twenty years, the (solid spruce) mast hooked forward and twisted 15 to starboard at the truck (ah, the joys of using a green tree!). In '06 I pulled all the hardware off and flipped the mast so that what had been the forward face was now aft, as well as re-aligning all the masthead hardware to be on center.

    So no, that forestay stays pretty consistant now, and it isn't very tight, as forestays go (judging by other forestays I've known).

    countersunk fasteners are not typically used in shear applications.
    Yeah, as I mentioned above, had I to do it over again, I'd use carriage bolts. As it was, I used the same fasteners as were original. If I end up replacing the chainplates, I won't make that mistake again.

    In addition, when crunching the numbers for the additional "backstay load" on the shrouds, they quickly became alarming and I increased the wire size, the pin and turnbuckle size to accommodate a comfortable safety factor. At first I thought they would look too large and out of place, but they just disappeared.
    It does sound as though I would do well to break out the calculator and crunch some numbers. And it sounds as though you had the same aesthetic concerns as I do, so your assurance there is welcome.

    I'm learning tons, folks. Thank you.

    Alex

  32. #32
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    Default Re: Chainplate Woes: Opinions Welcome

    Quote Originally Posted by Jay Greer View Post
    This boat design depends on the main sheet to load the head stay when working to weather. The main usually has a topping lift to aid with using the boom and main sheet to keep the jib stay tight. This is a boat that does not need complication. In fact, I would have left those single shrouds in their original position. Just looking at the forward face of the mast, in profile, shows a slight bow in the stick. This further slacks the jib stay as the height of the mast head lowers with the bend. This means that the shroud is really tuned banjo string tight! This can lead to cracked frames and other structural problems like hogging.
    Jay

    oh, hadn't thought of that...


    yeah, are the shrouds just barely taught? is the lee shroud slack when going to windward? you dont' want to over tighten this rig.

  33. #33
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    Default Re: Chainplate Woes: Opinions Welcome

    are the shrouds just barely taught? is the lee shroud slack when going to windward? you dont' want to over tighten this rig.
    Yep: shrouds are just barely taut, and the lee shroud gets a little floppy when going to windward. I grew up on gaff rig boats, cocking my snoot at the high-tension rigs of modern boats. Dyed in the wool: I like low-tension rigs. You wouldn't be able to play tunes on B's rigging!

    I just enjoyed a non-WBF conversation where it was pointed out that the bolts aren't failing, so going with larger bolts won't do anything to help. It's the planking that's failing. I need to distribute the strain over a larger area of planking, so that the points where the bolts interact with the wood are each taking less strain --in other words, just as many of you have been saying, I need More Bolts, not Bigger Bolts.

    Going up to 5/16" bolts would increase the shear-bearing area by 3/16", but won't disperse any strain. Adding two more 1/4" bolts will increase the shear-bearing area by 8/16" *and* (more importantly) disperse the strain over 66% more area.

    So I think I have my answer.

    Alex

  34. #34
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    Default Re: Chainplate Woes: Opinions Welcome

    Bigger bolts will help too. Blunter blade trying to cut through your planks. More bolts, bigger bolts, epoxy bushings. Just do it all and go sailing.

  35. #35
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    Default Re: Chainplate Woes: Opinions Welcome

    If you build 3/4" dia. epoxy bushings at each bolt you've added three times the effective bearing surface per bolt. For the three bolts per chainplate that gives a total of nine times the bearing surface that you have now. If you only add two additional bolts per chainplate and don't do the epoxy bushings then you will be adding two additional effective areas for a total of five per chainplate. Nine is more than five. If you add bolts and bush them all you get a grand total of fifteen.

    An additional benefit of building epoxy bushings is you will be able to tighten the nuts all you want without compressing the plank.

    Jeff

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