I would like to know how downrigger fishermen deal with the blowback or swayback of the cannonball when trolling. How do you know at what depth you are actually fishing?

.... watch the sonar. For Blackmouth, I keep my ball one to two feet off the bottom at all times. I run at a depth of choice by feel.

Lower the ball slowly as you get close to the bottom. When the ball hits, give it a few seconds and slowly lower the ball again.. The blowback should keep you in the "sweet area" with a 48" release at 3 feet above the ball. When the ball hits the second time, read the sonar depth and troll along this contour.

As long as the blowback isn't extreme, you can just knock off 10% and call it good.

You can "calibrate" your particular rig against your fishfinder by running somewhere where the bottom is known sand or mud, and just lower the ball until it starts to drag. Check your downrigger counter, then compare with your fishfinder depth.

It all will depend on your speed and weight of the ball. As a general rule, a 45 degree angle on your cable will put your ball about 10' shallower than what your footage counter says at approx. 100ft. The current also will play a part as will the amount of drag of your outfit. I usually run 10 - 15 ft more line out on my cable than what the bottom reads. As stated above, expriment with your gear and you'll get a pretty good idea really quick. Just remember that the ball on the side you turn towards will sink deeper as you turn so be ready to bring it up if you bounce. Good luck and let us know how you do!