One of the crucial considerations in trolling is presenting the bait at the proper depth. Many lures come with a depth rating, but knowing the precise depth your baits are actually running can be challenging. Calculators and protractors may not be standard gear on an offshore fishing trip, but the calculation becomes easier when you think of the rod, line, and bait as forming a large triangle with the boat. As such, anglers who aren't afraid to channel their inner mathematician can use a bit of high school geometry to estimate the depth of their bait.
Measure the distance from the back of the boat to the bait while the lure is on the surface. One easy way is to pre-measure and pre-mark the fishing line every ten feet with a permanent marker, then count the number of marks that pass when you let out the bait. This will give you the length (L), which will form the base of the triangle in your calculations.
Place the rod in the rod holder in the position it will be while trolling. Measure the vertical distance from the rod tip to the surface of the water. This distance will become your height variable, H.
Increase the boat's speed until you reach your desired trolling speed.
Estimate the angle (A) made by the fishing line intersecting the surface of the water. A protractor may be useful for estimating the angle but may prove difficult to use while the boat is in motion.
Locate the tangent of angle A, either by using the tangent function on a calculator or by looking up the value in a tangent table.
Multiple the tangent of the angle A by L, then subtract H. This will give you an estimate of how deep your bait is.
When taking the measurement for length L, hold the rod tip as close to the surface of the water as possible; this will provide a more accurate measurement.
This geometry-based method will provide a very rough estimate of depth, but the actual depth will be affected by a number of environmental factors, such as the size of the bait, trolling speed, and the resistance of the fishing line passing through the water.
Resistance of the fishing line is largely determined by how far back a bait is being trolled, and a higher resistance will tend to cause the bait to run shallower than the estimate would predict.