WOW. I have been reading the comments on rig tension and I am absolutely amazed. I am sailing with 1300 on the uppers and 1100 on the lowers as my base setting, and the boat is FAST. The J-30 has the identical problems that the J-24 has. It is underpowered, with an undersized keel. The goal for upwind performance has is to power up the rig for light to moderate air by inducing a large amount of head stay sag. You know that the shroud settings are correct when the leeward shrouds are just kissing slackness. At my settings, my leewards rarely slacken, even in 20+ knots. So what to do if the main looks funny? That is controlled by the ratio between the lower and upper shrouds. If you have a large amount of curve in the main luff, you will want a more prebend in the mast, therefore the difference between the upper tensions and the lowers will be greater. If the main has less luff curve, the ratio will be closer, pulling the spar straight. The position of the mast butt is set to provide the static mast bend that the sailmaker recommends, such that application of backstay and main sheet tension do not invert the mainsail.
If you have sailed a fractional rigged boat with running back stays such as a Star boat, you can see the relationship between our shrouds on the J-30 to runners on a Star. The lowers do the same function as the check stays, ie. straighten up the mast and add headstay tension by countering the tension applied by the upper shrouds. This enables the balance between draft depth in the head sail established by headstay sag, vs. flattness of the mainsail. Sailing side by side with a tuning partner and changing rig settings is a good way to establish base shroud settings.
