There is an aphorism in the engineering world: “In theory, there is no difference between theory and practice; but in practice, there is.“
It is fairly well accepted in the “science” of trolling lures that
- The depth of a sinking lure is very dependent on trolling speed.
- The depth of a diving lure is not dependent on trolling speed.
The Trolling Angles web pages and FAQ explain why this is so. One can examine the drag and lift equations and become theoretically convinced.
- For a sinking lure, downward force is directly proportional to its weight, while the drag is proportional to the square of velocity, so drag dominates as the velocity increases.
- For a diving lure, both drag and downward force (negative lift) are proportional to the square of velocity, so they cancel each other out.
A Sinking Rig
Here is the graph of the submerged line profile of a sinking rig. This is a 2-spinner, 2-hook nightcrawler harness, with a long plastic worm and a 3/4 ounce sinker, measured with Trolling Angles at speeds from 0.5mph to 2.5mph. Clearly, speed makes a big and consistent difference. At a slow speed, we can reach almost 50 feet with 100 feet of line at 0.5mph, but only about 9 feet at 2.5mph.
A Floating Diving Lure
This is the profile of the largest size of a popular and venerable diving lure, measured at 1.0-3.0 mph. This lure has a substantial buoyancy. In fact, it can be retrieved very slowly on the surface without diving at all, but at greater speed, the diving force overwhelms its buoyancy. It has a small diving lip, and is trusted to have a very consistent tight wobble across many speeds. Notice that there is much less variation than with the weighted Worm example above. In fact, all lines are probably the same within the limits of experimental error, except for the run with the highest speed. In general, we could say that the diving depth is not dependent on speed, in practice. Except above about 3.0 mph.
Another Diving Lure
Here is a comparison of another popular lure at various speed. This has nearly neutral buoyancy, a large diving lip, and a wide wobble. Contrary to theory, it seems to dive less deeply as the speed increases, and consistently so.
Note that the variation of depth with respect to speed is still far less than the example of the heavy weighted lure.
Explain That!
Why is there any variation at all, though? Lets look more closely at the lift and drag equation: Lift L is equal to the lift coefficient Cl, times the density of water R, times half of the square of the velocity V, times the area A.
L = Cl * R * .5 * V^2 * A
In practice (and in theory by the way), the Lift constant Cl is not necessarily constant. It is a mechanism to group together all the variables that are hard to determine theoretically, and is usually determined experimentally. It is highly dependent on the shape, texture, and orientation of the object.
A wobbling fishing lure is a highly dynamic thing. I think its reasonable to expect that as the SR9 lure moves faster, it wobbles faster and wider, and spills more water, losing some downforce compared to the theoretical V-squared expectation. Cl changes as speed increases. On the other hand, the F18 with its longer more stable body and smaller lip, keeps a more consistent downforce constant Cl until a higher speed is reached.
Conclusion
Every lure and rig is different. An approach that measures your lure and rig at only one or a few conditions of speed, weight, line, etc., and then theoretically calculates based on your own set of variables is not necessarily as applicable as the Trolling Angles approach, which lets you measure and calculate your own exact rig in practice.
