Chain Plates 316 or 304

[Richhh]

Ive had one fail ... just a lower so it wasnt too threatening.

Dont ask me, just to to any good riggers shop and ask the questions: how many have you seen fail? how many have you replaced over the years? etc. Brian Toss hails from your area, pose that question to him.

[Boracay]

With all this talk about chainplates failing could anyone give me some information on the size and type of chainplates that are likely to fail, and how long they might last before failure?

[Richhh]

A chainplate will last about a million load cycles before you start to 'see' the evidence of fatigue. Thats about one complete circumnavigation for a well designed plate. Loose rigging, etc. that imparts shock/impact loads lessen this approx. value. Then when the inevitable cracks start to form one must consider how much salt water is allowed to penetrate to start additional failure through the mechanism of crevice corrosion. Since its impossible to monitor the number and force for each cyclical load and the exact spot that the fatigue starts is usually hidden inside of a wad of caulk (not a good idea for stainless to not be exposed to the oxygen in the atmosphere) the usual and conservative way is to pull them about every 5 years and visually inspect them. After 15 years its probably a good idea to pull and inspect every 2-3 years as you 'know' that the metal is getting old and tired .... plus you should be religiously recaulking anyway.
Another means is to keep close tabs on a particular 'owners group' if there is an 'active' one. An owners group will quickly alert other owners of discovered problems that begin to develop .... and then you can sort out where your particular boat is in comparable age and usage, etc.

There's no fixed answer since sailing and the forces acting are soooooo variable. Your eyeballs and a magnifying glass and a queezy feeling in your stomach is the best means to tell when a plate is starting to 'go'. If when you look at a plate, simply note the 'luster' to make sure that a zone or 'fan-shape' of dullness isnt developing. If the plate has a 'bend' in it ... very carefully scrutinize the whole area of the 'bend' for 'impressions' of 'suggestions' of cracks ... then trust your gut. ditto on any surface irregularity such a notches, weld/cutting scars, etc.

Hope this helps.

[Boracay]

Its just that I am starting to build my new 45' yacht to a modified Roberts design and the chainplates don't look right.
At the moment I am looking at 10mm x 45mm x 400mm for the shrouds and stays. All fastened with 10mm bolts.
At this stage it is not a lot of trouble to go much bigger. The extra weight is insignificant on a cruising boat and the additional cost is small.
I'm tempted to go to 10mm x 100mm x 450mm or longer.
12mm stainless and 12mm bolts are the next stage up, but much harder to cut, drill and carry.
Would they be justified?

[Alan Wheeler]

Going wider in plate is of little advantage. Going thicker is the better idea. The 12mm plate and bolt gives a greater surface area on the load carrying area and thus greater strength and less wear. I went 10mm x 45 mm on the 8mm rigging and doubled plates of 10mm x 45mm on the 11mm rigging.
There is little difference with drilling a 12mm plate as against a 10mm. The importance is a good sharp bit, correct cutting speed, cutting fluid, a good drill press with a constant cutting pressure and never NEVER allow the bit to ever vibrate and squeal, As soon as that happens, you will never drill the hole as the SST machine hardens and will take the edge clean of the cutting bit.

[Richhh]

Chris31415 ----

The best answer I can give is: Find out what *safety factor* that the designer originally selected. Dont tell him why but just ask! Another way to ask him is: what was the design stress you selected for the chainplate? - if he answers 25000 then probably OK, if higer than 30000 then 'beef it up' proportionally to be under 25000 psi.
"Hello Mr. designer, I'm putting one of your boats together and my intuition tells me that your chainplates are too weak. Would you mind telling me whats the stress level you designed these for? If greater than 30000 psi, make 'em heavier (doesnt matter if thickness or width), if 25000 psi then probably OK.

For fatigue you need about a 3:1 or 4:1 safety factor so that the maximum developed stress is equal to or less than 30% of Ultimate tensile strength Stainless has an ultimate tensile value near 90000 psi. To design (and without actual proof dynamic testing) for fatigue endurance you need to be UNDER the 'fatigue endurance limit' (less than 25-30000 psi) so that 'by design' the plate will virtually last forever vs. fatigue. The 'endurance limit' is the stress value at which fatigue does not 'usually' occur. I perceive that forensically, most boats plates are designed at about 30-40000 psi .... much above the 'endurance limit' ... and the reason why chainplates fatigue. If they were designed to be under the 'fatigue endurance limit', they would rarely fail.

This is for a STRAIGHT LINE plate ... one that does not have any little 'bends'. If there is a bend near where it comes through the deck ..... then derate the stress by about ANOTHER saftey factor of 4 ..... that's right ... 3 for fatigue, 4 for 'bends' .... thats a safety factor of 7. 90000/7 = 13000 psi. A safety factor of ~6 is what you'll find on a heavy-lift crane, elevated tram, etc. ... 'important stuff' that 'moves' and where people might get killed, etc.
If there must be a bend, make it a *long radius bend*, and NOT a 'kink'.

Straight chainplate with NO bends = less than 30000psi.
Chainplate with ANY little bends, then less than 13000psi.
Dont forget the 'mirror polish'.

Hope this helps
;-)

[Boracay]

I might just go for 12mm plate with 12 mm bolts.
Same on the bow fitting, where the forestay will attach to a welded plate. However the weld should be over 12' long.

[CSY Man]

Another means is to keep close tabs on a particular 'owners group' if there is an 'active' one. An owners group will quickly alert other owners of discovered problems that begin to develop .... and then you can sort out where your particular boat is in comparable age and usage, etc.

Yeah, that is what I did.

Heard of several CSY boats that had chain plates fail.
Then a discussion of crevise corrosion and all that on the CSY boards.

Some riggers suggested chain plates should be changed every 20 years.

With my boat approacing 24 years of age, a few years ago, I bit the bullet and replaced all 9 chain plates.

A nasty job on the CSY 33 (Or on any boat I giess) and it took me 3 months of part time work. Had to wait for weeks at a time to get 2 or 3 plates made as I did not want to de-stay the mast by removing all the plates at once.
Each plate was different and exact copies had to be made.

The new ones should be good for another twenty years, and by that time I am hoping to be 69 and spending more time at anchor than under sail..No need for another round of chain-plating...

[GordMay]

See also the Scantlings Table at:
http://cruisersforum.com/photopost/s...php?photo=1325