i can see the ridges on the water from the wind off the jib and i can see the direction from the direction finder--when i dont like to look up too much, i bring a yarn like thing onto my shroud so i know from where the wind comes and then make sure no luffing on the leading edges and i am outta there....but i dont put on the sails them selves-- when the wind hits the sail, it spreads out into different directions--some goes down and out of the sail that way. some comes aft and out of the sail, and some hits and swirls. sail trim is control of the direction to which the wind not in use goes. there is always wind not used. that wind is what makes the ridges in the water under the jib. i have never been inclined to race a sailboat--is way too oxymoronic and stressful for me--i like fast cars and slow sailboats--i get the decent shape for cruising--hail--we didnt sail under 8 kts except in calm times!
i can see using them for racing, as you need to see where the unused wind is going and catch it for speed....but i amnot that greedy with it---i will share---besides i am old--fast is for kids and boys...some folks do use them and i cant knock ye--just, please understand we dont all use them, and we dont all use tell tales for the same reasons----is all good---i laughed at phillip for using them in the start of our trip=-he uses them on the shrouds to see from what direction comes the wind---that ws very helpful, but no the same as you guys are talking --yarn or string on sails--different view of wind for different functions.
when yer learning--all is fair--
when i am done with this project and out of sin diego, come for a sail....not sure where , yet, but you are welcome to check it out...will have warm water and breezes .....