I don't understand the point about better performance downwind ... assuming that we always use spinnaker downwind?

the difference is at the most 3 in. at the fractional ... or are we referring to a straighter mast with minimum bend?

I am estimating that the angle range of the mast -- controlled by the movement of the mast step set -- is ~2 degrees.

Vic - I believe the downwind comment is related to where the effective center of effort is based on the position of the mast and forces on the boat.

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.

As indicated in the previous post, shroud tension influences rake, mast prebend and headstay sag. Likewise for fixed backstay tension, but in light to moderate conditions it's best to adjust the turnbuckles so the block rises to the top of the split backstay allowing you to depower using the adjustable backstay.

Maximum rake with the mast step forward gives you just a touch of weather helm in light to moderate conditions, providing some lift from the rudder. It does depend on having enough weight on the rail to maintain a reasonable angle of heel. If your crew is underweight and the boat is heeling over too much you don't need more weather helm.

You don't want so much weather helm that you are dragging your rudder through the water upwind. It may be the modified rudder, but I also wouldn't want to have 'no hands' like the picture in the rules thread.

Regardless of where the mast step is set, I think an excessively saggy headstay also hurts pointing. Since shroud tension is so much higher than headstay tension I'd theorize that adding a few turns isn't going to reduce mast rake. As wind speed increases and you begin to heel more, you can depower by adding turns to the shrouds. Since you don't need more weather helm why not maintain the same mast rake by adding turns to the headstay?

Not quite. Headstay sag will result in increased draft in the sail. This helps provide the power that the boat needs. A saggy headstay actually pulls the mast forward. Think about the geometry. If I add sag to a line, the points at the end must come closer together, resulting in the top of the mast coming forward. Pointing ability results from the angle that the leading edge of the sail makes to the apparent wind. If the halyard tension is loose, the draft is further back, resulting in a higher pointing angle in light air. This is why many of the top J-24's have ugly scallops along the leading edge. As the breeze comes up and the sail stretches a bit, we tighten the luff of the sail via halyard tension. We also tighten the shrouds, tightening the headstay, and flattening the sail since the headstay sag is less. BTW, the large amount of sag I was referring to in my earlier post is approximately 6 inches in the 24, and I am approximating between 8 to 10 is about right in the 30.

Light air - Deep Draft in Genoa, max headstay sag, loose genoa halyard tension
Medium air - Less draft in Genoa via tighter headstay and less headstay sag, start to tighten halyard. Don't forget that headstay tension also is a result of main sheet tension. As the breeze comes up and you tighten the main leech, you are also tightening the headstay.
Heavy air, tighten shrouds if not done already, tighter headstay, tighter halyard tension.

I understand the geometry but disagree with the conclusion. Are you assuming the headstay is a straight line and pressure from the Genoa causes the headstay to sag and pull the mast forward?

At max headstay length the sag is built in. Grab the upper shroud at 1300 lbs and pull. Pretty tight. Now try the same thing with the loose and wobbly headstay. You can add quite a few turns before it will be tight.