Hello! Sorry that I took awhile to comment on this thread! I made that big claim and then left it to you guys to fill in the details; not on purpose, but that's how it happened.
Anyway, since the concurrent breaks of 2 starboard main chainplates, while crossing the Atlantic last summer, I've had numerous opportunities to discuss this with people.
What seems to make the most sense, besides the suspect underlying condition of the chainplates themselves, is that without having tuned the rig while under way after the stepping of that mast, the mast in question was most likely vibrating / wrenching forward and backward in relation to the boat.
This "fits" with what I saw of the broken chainplates; i.e., they broke at the point where the chainplates were sticking up from the ribs of the hull where they were thru bolted to.
These "tabs" were perpendicular to the boat. Lets say they were 4 inches wide; well the plane of those 4 inches was perpendicular to the plane of the hull. Such that the portion of the chainplates sticking up, above the ribs, sticking up and through the deck was like a tab of metal.
Those tabs broke, just below the deck. And it appeared as though they broke from being bent, forward and backward, repeatedly, until the bending caused the infamous metal fatigue which will break any metal.
Now, many people will claim that using titanium or perhaps bronze chainplates, and ideally bran new ones, wouldn't have sufferred this breakage. But, shy of actually testing new ones vs. old ones, with and without the tuning which should keep that mast vibration / wrenching from occurring, I can not agree that it is OK to not fully tune your rig if you have new / titanium / bronze chainplates.
Without properly tuning the rig, you are abusing the chainplates / metal, with an eventually disasterous result, regardless of the intitial strength of the metal.
What will be effected by the quality / age of the chainplates, is how long they will last given this abuse. But, it is abuse, nevertheless.
For me, I would choose to sail across the Atlantic with old chainplates, as long as the rig was properly tuned, as opposed to with new chainplates without properly tuning the rig.
Now, what is this "dynamic tuning"? Well, as was described to me, by more than one source, is basically what a few of you did describe here already.
After installing your mast, and adjusting all the turnbuckles to provide a similar / tight tension of the stays / cables at the dock, per your specific boats' rigs' pounds of tension requirements, you are then supposed to take her out under full sail, and adjust the downwind turnbuckles again.
Yes, this would include all the shrouds / stays, i.e., the backstays, etc., all of them.
Now, as for the exact amount of tension to set these to, I am pretty sure this varies by boat and rig.
Many of you apparently already know this, so this isn't anything new to you! For you, fair winds, and have a great day!
But, for the poor saps like me, especially those who are stepping their mast(s), I highly suggest asking your rigger about the final / dynamic / "tuning under sail", and, or, read that book suggested earlier! Because, as was quoted, 2 riggers who were hired by us for our main mast stepping process in Fort Lauderdale never mentioned a thing about this, and it didn't take all that long for the repurcussions of not doing this to become my worst night mare.
Without having read that book, I would greatly appreciate, as many others might, any specific inflormation about how to dynamically tune your rig, per these experts, or any other information about these specifics.
Because all I have heard is to "tune the downwind stays / shrouds" while underway".
And that still leaves much to the imagination about the details.... :-)