Does anyone know if there a free trolling Cart or App for how much line to let out for crank bait depth
Precision Trolling is the way to go. They used to have a book, which is no longer in print, but they have electronic app now. You can purchase the dive curve for an individual bait for a few bucks, or can buy the whole app for like $100 or so. Definately worth it whether you are buying the whole app or just a few pages. Depends on how many different baits you have to troll with.
Precision trolling is good. Though it is not free.