This page has information on lure depths when fished on monofilament, braided, stainless steel and monel lines. This is based upon field testing in a variety of conditions.
Factors Affecting Lure Depth
The speed of the water is the biggest single factor effecting depth. Water speed is the sum of the boat speed and the current velocity and vector. In the Chesapeake Bay currents can vary from less than .5 knots to as much as 5 or 6 knots depending on conditions and location.
Even a slight increase in speed affects lure depth drastically. Most anglers that fish in the Chesapeake Bay agree that boat speeds of 1.5 to 4 knots seem to work best when trolling for striped bass and trout. Bluefish prefer 3.5 to 5 knots. Other species prefer even faster presentations. Tuna can be caught successfully at 16 knots and wahoo have been caught at speeds in excess of 20 knots.
The simplest way to make a lure run deeper is to add weight. This can be accomplished in a number of ways. Inline sinkers can be added in front of the lure. Another way is to use a 3 way and run tandem lures. One can be heavy and the other smaller. This allows the angler to present the correct size lure at the correct depth. If you troll a single weight at a given speed and line length, the depth will be approximately the same when you troll tandem lures which total the original weight if all other factors are unchanged.
Some lures have a lip which makes them dive. Others have a unique shape that inhibits the sinking of the lure.
Line Distance, Type and Diameter
Line diameter and material are big factors. The highest running lines shear the water less efficiently. The weight of the line can also help it run deeper. The spectrum of lines from shallow to deep are: mono, braided, multistranded wire, single stranded wire. Thinner lines run deeper. Line diameter is a compromise between the ability to run deep and the strength needed for the type of fishing. As line length is increased, the drag becomes significant. After a certain distance of line is deployed, the gain in depth decreases dramatically as line length increases.
Mono is the most common line used in trolling. It has advantages such as knot strength and durability but does not run as deep as other lines.
Braided line is popular when trolling for striped bass, trout and bluefish. Braids run deeper than mono but not as deep as wire line.
Single Strand Stainless Wire
This is inexpensive and runs deep. It kinks and breaks unexpectedly. Single strand stainless steel wire is used while trolling to catch tuna and wahoo.
Single Strand Monel Wire
Monel wire is expensive to use and requires some getting used to. For low speed trolling some anglers prefer this as a deep line. Monel runs as deep as stainless and deeper than other lines at low speeds.
Multi Strand Stainless Wire
Multistrand wire runs deeper than mono or braided and is very resistant to abrasion. It is often used around structure such as bridge pilings.
In the field lure depth prediction is simply impossible. The factors are complicated and ever changing. As the boat travels over the water, the current velocity and direction is constantly changing. As a boater travels against a current the boat speed and current oppose, making the lures rise. When the boat reverses direction the 2 forces combine and lures run deeper. This action is ever changing.
Even a small increase in speed can makes a big difference in lure depth. One way to get a feel for things is to note boat speed and let a test weight out in a shallow area with a hard bottom. If the current is significant, you can easily see differences in how much line it takes to hold bottom when reversing direction. This only works at low speeds, perhaps 2-4 knots.