It is not ususual for the leeward shrouds to go a little slack in a breeze, although 10 knots is a little bit early for that to happen. A little bit of sag (to leeward)in the rig
is not too bad and is the inevitable effect of shrouds stretching in a breeze.
The amont of sag in a rig
will increase over time as shrouds and stays will tend to stretch a little over time and use. That is normal and to be expected. Stretch will can also occur from sources that are not normal and acceptable such as a turnbuckle
backing off or a bad swage that has slipped a little. Assuming that you have checked your rigging
, I suggest that you take up on your shrouds and stays. Assuming your mast is on center, you need to tighten the upper shrouds and the back and forestay, twice as many turns a your lowers and make sure that you turn each pair (port and starboard) teh same amount. I would start with a single turn of each lower and two turns on the uppers and stays. I would sight up the mast bith at the dock and under sail to make sure the mast is straight at the dock and develops and even curve in a breeze.
The effect of too much sag in a rig
is that the mast bends slightly to leeward. This puts more sailcloth into the mainsail, powering up the mainsail, and alos allows the forestay to sag more, powering up the jib
, as the breeze increases. This is the opposite of what you want to happen in a breeeze as more powerful sail shapes will increase heeling and weather helm.