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