As an ex-racer, I've sailed on a number of boats that were tuned to have side bend in the upper mast as the breeze came up. If the upper shroud(s) are on the "easy" side of the tension range, as the weather
side loads up (and maybe stretches just a wee bit), the lee side can get a little slack, just as described. The lowers should be set up tighter, so there's no bend to leeward of the lower span. The effect is to slightly depower the top of the main, while keeping maximum drive in the bottom. Keeps the boat on its feet and the lee rail out of the water
While this is not common on cruising boats, it may just be that the initial set-up of the uppers wasn't as tight as before, or there may have been a bit more stretch than expected for new wire - with the same result. (Or maybe your rigger is a racer
at heart and thinks you'll be faster in a breeze that way.) Since you've already checked for problems with the chain plates and other damage, it's likely the set-up. It should at least be the first thing you address.
Good first step is to make sure the mast is straight up and down. Measure the difference in length of the main halyard
to the rail. It should be the same on both the port and the starboard sides. (Plus or minus an inch is enough precision for that check.) Assuming the mast is straight in the boat:
Take up two turns of the turnbuckles on both sides
and check again. If that's enough, fine. If not, take another two turns and .... As mentioned above, the lee upper will be a bit soft as the load goes up. If you push on the windward upper, it will be bar-tight. The lower will have noticeable give when you push on it. But on a cruising boat, it shouldn't be seen to flop around.
On a cruising boat it's not desirable to have the uppers as loose as you are describing, but not a disaster. The problem is that your sails
aren't cut for that amount of mast bend and it may not be good for them. You don't want premature wear on your sails
- mostly because you don't want to have to replace them prematurely.
And if you see visible slack in the lowers, or if you can move a lower by hand, that's dangerous and should be corrected immediately. Get the lowers done first, since too much bend in the lower span may put the mast out of column and you might lose the rig. Tighten the turnbuckles, as above, until there's no slack on the leeward side in up to 15 knots breeze. Then do the uppers.
Don't overdo the take-up on the turnbuckles. The goal is to get enough tension, not too much. You're not trying to pull the mast through the bottom of the boat! If your starting point doesn't have floppy shrouds at the dock
, the adjustment that's needed isn't going to be a whole lot. Two turns of the turnbuckle is probably going to be about 1/8 inch.
If that doesn't take care of things, you need to get a good rigger on it. Or you can PM me if you need more details.