Lines are everywhere. You wrote them at school if you were naughty, you navigate by lat and long lines, the Aztecs drew them on the ground and were only visible from the air, trains travel on them, you get my drift. However there are no lines other than the seams on my sails.
At the worlds I could not help notice that the top sailors all had lines on the sails. It looked like they had marked the seams in black and I wondered what was going on. Then I remembered in my yachting days.North Sails produced what they called was a sail's scope, which was a piece of plastic with a grid on it so when you viewed the sails from below you could assess how far aft the draft was and at what depth. We used the seams or a line drawn on the sail to investigate the characteristic and then tweak the rig to create the shape we wanted.
I wondered if the same could be done on an IOM so out came the practice A rig and on went the lines. 3 on the main and two on the jib, basically filling in the seam overlap. I have a 1cm felt pen and a foot ruler to make the line.
Setting the shape of the mainsail was easier with the lines. Now I don't have the set up skills of BG but I found the lines a big help in creating the right shape although I need to see them on the water to see if they provide the guide I need. The lines on the jib told me that the practice jib was past its sell by date as the leach no longer held its shape. That sail is gone and replaced with a slightly less aged one. See picture below.
It maybe fashion but I certainly found that looking a the picture of the top boats at the worlds, those sails with lines on them painted a very clear picture of the sail shape and twist.
In the first picture it is easy to see the shape of the sail. The line provides the contrast and a consitstent position to measure the sail. Of course you can use the seam but the line I feel provides much more definition and can easily be seen on the water.
If you note the upper line on the jib, you can see how the leech is breaking down slightly. The jib has a lot of mileage but there is still life in it. On the main, the mast is set up dead straight and would benefit from slightly less ram.
The open at Eastbourne was postponed for a week as the lake is at the down stream end of water flow in Eastbourne and the apron from which we launch the boats is under water. The forecast for rain was atrocious for the morning..