If you use line counters here is a way to calculate how deep you are.

You have to keep your speed consistent in order to apply this to fish that are suspended though.

1. Keep boat at consistent speed.
2. For bottom bouncers or inline sinkers and bananas, keep line at 60 deg angle. Adjust your weight and spinners until you achieve this. In order to determine angle, start 0 at line hanging straight down to bottom. Measure 60 degrees from there to your line off the side of the boat.
3. Now let out twice and long a line as the depth you want to hit. This works well for depths up to 50 feet. At about 35' depth though, you have to account for a bit more length because of line drag.

If you do not have counters try it this way.

1. Follow steps 1 and 2 above. You will have to travel over an area with a constant bottom depth for this method in order to find your depth to handle turn ratio.
3. Now let out line until you reach bottom and can feel the bump every 10 seconds or so.
4. Mark your depth so you know what depth you have reached.
5. Reel your lure up until you reach the surface and count the number of handle turns.
6. Now you have a relationship between your depth and the number of turns on your reel. Lets say the depth is 45 feet and your reel turned 30 times. This means 1 turn equals 1.5 feet.
7. Now you can let line out by reeling backwards to get to your required depth, or let line out until you reach bottom and reel up until you get to your required depth.
8. This also works well for depths up to 50 feet. At about 35' depth though, you have to account for a bit more length because of line drag.

We've used this second method for years to troll for northerns in MI and walleye in Erie.

Good luck,
Rickerd