Right now the class has mostly gotten away from adjustable diamond stays. They are still installed on a lot of boats (mine included), but the only time I use them are to put the sail up. The extra prebend allows me to push the sail up a bit easier (no halyard).
Technically it can be used to adjust the flex point of the mast
, but there is already so much control with the downhaul and mast
rotation that it's pretty much superfluous. The major real advantage is in very light air when you can use the prebend to open the lower 1/3 of the sail without effecting the top 2/3. But honestly no one I know uses it this way. It's just too fine an adjustment to matter much, so sailmakers have just designed for a set prebend in all conditions.
On a crewed boat like a Tornado it may have a place, but there is already to much to do on the A-Cat to mess around with this type of fine tuning.
I had to look it up. Apparently it was the NA I was thinking of, there was no J-35 Worlds as far as I found. I am thinking of the early 80's back when Smoke, and Heartbeat (I think it was) as well as Big J, were the hot boats in the fleet. I didn't start sailing on them until much later, so all I have are the stories I have been told.