"The chainplate will buff out and probably be okay" - Page 2 - SailNet Community
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post #11 of 23 Old 11-25-2012
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Re: "The chainplate will buff out and probably be okay"

What your looking at is the Bob Perry design that even by scantling history has withstood the test of time ... probably more of these 'knees' have circumnavigated (Valiant, Tayana, Tashiba, Baba, Valiant) than any other design combined.
This design is quite robust, and comes with a inbuilt safety factor of approximately 4:1, well above the normal 3:1 found on most 'blue water boats'.

The only real flaws of this design is the internal wood which carries the moisture that since ~1975 (now 35 years after the fact) is now for the past few years has been showing severe vulnerability. The other vulnerability is that this arrangement is (or can be) a 'friction connection' and needs routine maintenance torquing ... if properly torqued, those bolts will NOT be in shear but rather only supply the 'clamping (compressive) force' which enables the joint/join surfaces to carry all the loads through stress transmitted 'normal' (a vector component in friction) loads; induced friction between the mating surfaces is what makes this 'joint' work .... and without a lot of 'stress risers' as is found in MOST other 'flat plate' designs, quite impressive for a then ~26 year old 'kid' designer if in fact that is what Bob Perry intended.

Such a repair is quit easy and straight forward for those not timid with composite FRG work. To make this assembly 'work' and remove all the vulnerabilities of moisture intrusion ... simply reconstruct without wood, and change the studs into removable shoulder bolts, and be sure to use a torque wrench when assembling. Certainly MUCH better than using an external chainplate design where the loading on those 'bolts/studs' are 'sequential' due to the elasticity of the stainless and where the bolts are vulnerable to maximum shear loads... sequentially, and resultant 'bearing saddle stress' (a stress riser) during the 'elastic stretch' under max. load of such a long chainplate. The 'secret' of this design is that the studs are welded to a strap which is encased (drilled into) in wood behind the fiberglass web; replace the wood with non-permeable material and weld the nuts to the 'strap' so the bolts can be removed and routinely inspected and 99% of the vulnerabilities of this design are gone ... permanently.


If that chainplate through deck and/or the screwed down teak decks were extremely well maintained, you'd not be seeing such 'grief' .... and the only 'maintenance' would be to routinely change out those chainplates at approx. 1 million load cycles (about 1+ circumnavigation) for fatigue considerations.
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post #12 of 23 Old 11-25-2012
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Re: "The chainplate will buff out and probably be okay"

The exterior chain-plates of the Morgan OI is the best approach. Stainless steel needs oxygen and sunlight to remain sound.

Never allow dissimilar metals to touch, or at least never in any place difficult/impossible to inspect. Even stainless components in contact should all have same number:


The hairline crack story was apposite.
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post #13 of 23 Old 11-25-2012
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Re: "The chainplate will buff out and probably be okay"

Cheerev,

External chainplates aren't that much better than internal. While its true that the side that is exposed is usually fine, the side against the hull is as prone to corrosion as an internal, except that because it is almost always wet, corrosion can be even worse.

Certainly they have different problems, but they do still have them.

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post #14 of 23 Old 11-25-2012
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Re: "The chainplate will buff out and probably be okay"

Bob Perry personally talked me out of looking at Tayana 37's largely due to chainplate issues.
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post #15 of 23 Old 11-26-2012
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Re: "The chainplate will buff out and probably be okay"

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Originally Posted by xymotic View Post
Bob Perry personally talked me out of looking at Tayana 37's largely due to chainplate issues.
PFFFFFFFTHHH!!! Don't listen to Bob Perry! What, you think he knows something just because he designed them!?! If he's so smart then where is HIS youtube video? Go with the guy on the youtube video, he sounds like he knows what he's talking about. All you need is some of this to buff it out.


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post #16 of 23 Old 11-27-2012
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Re: "The chainplate will buff out and probably be okay"

"I didn't see the need for stainless"

Translated - the next owner will have a little project in a year or two.
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post #17 of 23 Old 07-11-2015
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Re: "The chainplate will buff out and probably be okay"

Rich,

I concur with what you say on this post. I'm planning to make the CP knee rebuild you are describing while I'm here in Guatemala but have limited materials available (e.g. no G-10 or shoulder bolts appear to be available in the country). I may have to modify to use epoxy-treated Teak and hex head bolts instead. My plan is to cut a 2 1/4 inch wide area of FG behind the existing chainplate. Replace the existing wood and bolts with new teak or G10 block (1" x 3" x 13") and a backing strip with SS nuts. The edges of the FG at the cut will be feathered back for approximately 1 inch and a new 1/4 inch layup of FG placed using stranded FG mat and epoxy resin. Due of the limited access space from the existing cabinetry, I was hoping than a FG overlay of approximately 3 inches along each side of the existing knee would be structurally adequate for this buildup.

I hope you might be able to provide some input regarding my tentative plans, or pass-on more specifics regarding experiences you may have had rebuilding the chainplate knees on a TY37.

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Bob Hoagland
s/v Inca Rose (1983 TY37 Mark II, Hull #383)
now on the hard in Rio Dulce, Guatemala
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post #18 of 23 Old 07-12-2015
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Re: "The chainplate will buff out and probably be okay"

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Rich,

I concur with what you say on this post. I'm planning to make the CP knee rebuild you are describing while I'm here in Guatemala but have limited materials available (e.g. no G-10 or shoulder bolts appear to be available in the country). I may have to modify to use epoxy-treated Teak and hex head bolts instead. My plan is to cut a 2 1/4 inch wide area of FG behind the existing chainplate. Replace the existing wood and bolts with new teak or G10 block (1" x 3" x 13") and a backing strip with SS nuts. The edges of the FG at the cut will be feathered back for approximately 1 inch and a new 1/4 inch layup of FG placed using stranded FG mat and epoxy resin. Due of the limited access space from the existing cabinetry, I was hoping than a FG overlay of approximately 3 inches along each side of the existing knee would be structurally adequate for this buildup.

I hope you might be able to provide some input regarding my tentative plans, or pass-on more specifics regarding experiences you may have had rebuilding the chainplate knees on a TY37.

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Bob Hoagland
s/v Inca Rose (1983 TY37 Mark II, Hull #383)
now on the hard in Rio Dulce, Guatemala
I cut out the existing fiberglass cap that was encasing the old wood as RichH describes. I used plastic from a case of beer to line the cap, filled with epoxy filler, inside was the backing plate and a solid piece of hard wood and the new bolts, which were welded to the backing plate. Just before the epoxy filler went off, peeled off the the old cap. The resulting new form was then glassed in. A few pictures for you here:
Attached Thumbnails
chainplates & Langkawi 034.jpg   chainplates & Langkawi 040.jpg   chainplates & Langkawi 075.jpg   chainplates & Langkawi 091.jpg   chainplates & Langkawi 101.jpg  

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post #19 of 23 Old 07-12-2015
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Re: "The chainplate will buff out and probably be okay"

^More pictures of laying up the glass here:
Attached Thumbnails
chainplates & Langkawi 147.jpg   chainplates & Langkawi 148.jpg   chainplates & Langkawi 149.jpg   chainplates & Langkawi 150.jpg   chainplates & Langkawi 154.jpg  



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post #20 of 23 Old 07-12-2015
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Re: "The chainplate will buff out and probably be okay"

This is how I 'renovated' my Ty37 chainplate knees about 12 years ago:

I removed the chainplate and cut into the knee along side of where the plates were attached into the knee to remove the internal 'E-shaped contraption' that holds the non-removeable studs, and most importantly then removed all the wet internal 'softwood' that was apparently used as a form over which the knee was constructed. As stated before, this OEM knee was a damn good idea ..... if the knee was kept sealed from water intrusion. However when the interior of the knee does become wetted and then rots the studs SEQUENTIALLY loosen and the chainplate and studs start to rust AND you get 'saddle stress' cracks (sequentially) across the face of the plate emanating horizontally from each stud borehole in the plate - such saddle stress failures look like mini-happy faces. All this is due to rot and moisture under the plate in the knee. The only real problem with the OEM 'design' is that the studs are permanent and the wood core retains moisture.
Remove that wood, put nuts on that inner plate, put ferrules through a FRG core so the bolts are removable, beef up that knee because you disturbed/cut it --- all is well.

So, cutting through the knee next to the OEM plate, I removed the internal strap with the stud heads welded, all the wood, and tapered the cut to a 12:1 face angle
After chiseling, prying and cussing to remove all the sodden softwood, I then ground all the interior faces to 'clean' and added approx. 1/4-3/8" of roving/cloth to all the internal faces - this to add stiffness to the remaining parts of the knee. I built 2 new 'webs' of approx 1/4-3/8" FRG cloth - these 'webs' are perpendicular to the hull face ... the new 'box' fits 'sloppily' between the these 'webs'. My rebuilt chainplate knees are mostly 'hollow' (a fabricated box beam).
The 'box': I constructed an inner 316 SS plate to mimic the internal 'OEM strap with the studs attached', this time with 316 nuts seal welded to the new inner 316 plate. (Imagine the 'box' as having one side/face being the internal plate, the opposite side being where the the chainplate attaches, and flat sides between the two.) I then measured all the 'in place' dimensions, took the interior plate + welded-on nuts made a form (a 'box'): Internal Plate with nuts, stainless steel 316 ferrules though which the new removable 3/8" 316 shoulder BOLTS would pass through to the weled-on nuts, then liberally WAXED the internal bore of the ferrules and nut threads to prevent epoxy adhesion, filled the 'box' with cut and chopped FRG roving and filled the 'box' to a smooth top face (when on a table). When cured, I simply drilled into to internal ferrules to clear any wax and errant epoxy and ran a 'long' tap into the nuts to clear them. This all created a solid (box) of strand filled epoxy + 3/8+" ID ss ferrules + 316 nuts on the internal plate, which I then inserted and epoxied in place and flat/flush to the area where the new chainplates would attach - exactly to where the OEM plates attached; I left the 'far' side of the plate/'box' with some extra room/space for the new bolts to penetrate through.
With the new 'internal structure' epoxied in place, I then overlayed the remnants of the OEM knee (overlapping to well beyond the sides of the OEM knee to the interior hull sides) and applied ~3/8"+ of new heavy FRG cloth/epoxy over the entirety of the old knee. The ferrules were easily seen through the new overlay, so it was easy to bore into the new (not fully cured FRG) to locate those ferrules.
Before the new epoxy/FRG overlay full cured, I covered the new chainplates with plastic wrap (so they didnt stick to the curing epoxy/FRG) and torqued them in place to the internal 'box' and knee structure ... in order to have a 'perfect' flat face (friction surface) on the the new FRG overlay between the new chainplate and the new internal structure *** (see below).

*** When I was the assoc. chief engineer in a 'heavy/mega-lift' / hazardous design bureau I would NEVER EVER allow any structure to be constructed with ANY bolts primarily subject to SHEAR!!!! all such non-welded joints were to be full compression FRICTION JOINTS in which the tensile strength of properly torqued bolts supplied the FORCE to drive the two mating faces together; therefore, friction carried all the load; and if and only if, the bolts stretched the bolts then 'could' support the shear load on the shoulders of any bolt ... but that would be if and only if .... AFTER the frictional force joint failed. You need perfectly FLAT faces to affect a good friction joint. Such a friction joint also 'prestresses' the underlayment and makes it MUCH stronger (in shear, in bending/buckling, etc.). Bob Perry apparently used this very same design concept of a proper 'friction joint' in those Ty37 knees - admirable and quite remarkable.
My new plates were increased by ~25% thickness, that OEM 'kink' where the plates pass through the deck was made into a 'long radius bend' to lessen the obvious and vulnerable stress riser of the old plate (press brake) 'kinked' design. The plates are mirror polished to 'brilliant' - and 'slathered' with Boeshield™. I retorque (to 80%) the bolts yearly, replace them every 5-6 years.

I guess what Im stating here is that the OEM design was excellent stresswise; the only fundamental problem was moisture intrusion + wood core, and the studs werent 'removable'. My calcs of the OEM design indicated that the intrinsic Safety Factor well exceeded 4:1. Since I was 'disturbing' the old structure, I decided to beef the new replacement structure ... 'up a few notches' for 'just in case'.
I didnt want to go to the external chainplate route, as external plates require one to constantly re-reeve jibsheets every time one changes the fairlead car position; plus, external plates require a larger 'kink' (stress riser) in that chainplate. Plus, when I was PHRF racing this boat, I was sheeting and barberhauling the headsail 'inside' the shrouds, to make it 'point' better and external plates would be a 'REAL' PITA when doing so and without adding additional inside jib tracks.

If I were doing this all over today, Id consider to rip out the entirety of those knees and put in a system equivalent to what Beneteau uses - Rods instead of plates with kinks, those rods affixed to trunnions affixed to the internal hull instead of 'bolts', and plate through-decks with leak-proof membranes that accommodate rigging movement (and no side loads on the through-decks). ---- Pure and elegantly simple straight-through tensile stress accommodation - a nice evolution. (Someone at Beneteau probably designed heavy lift cranes, etc. ??)

Sorry but I was too busy and covered head to toe with epoxy to take any pics of these knee refurbs. I still have one left to do (totally hidden behind all that 'cabinetry, shelving and veneer, etc.' in the shower stall) so maybe Ill take some pics then.


FWIW - Assuming that you have replaced those disastrous screwed together Grand Deer rigging toggle bolts, that welded together bowsprit cranse collar is probably more vulnerable to failure than any other part of the rigging. I have a 'better' design ... in the shape of the greek letter 'phi' ()where all the principal stress is carried by a continuous plate between the headstay and bobstay connection. The welding on the OEM cranse collar is very vulnerable to cracking - check it often.

The infamous Grand Deer toggle bolts:

Last edited by RichH; 07-12-2015 at 11:39 AM.
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