The Problem
While trolling, how do you really know how deep your lure is running?
It’s ironic that with modern electronics, we know our exact boat location and path on the 2-D surface, the exact depth and contour of the bottom, and the exact 3-D location of the fish, but we don’t know the exact depth of a trolled lure or bait.
How many times have you spotted schools or big suspended fish on the fishfinder, all near the same depth, but you could only guess how deep your lure is running?
How often have you known how deep the thermocline is, but you are not sure how to exactly reach it?
How often have you seen a reef or brush pile on your electronics, and held your breath, not knowing whether your trolled line would snag or run just above it?
The reality is that most anglers know for sure how deep their trolled rig is running only if they are watching the depth finder when the lure snags on the bottom.
Some Solutions
You know that to adjust depth, you can add or remove sinkers, use a diving crankbait with a different lip, let out more or less line, or change your boat speed. But how much?
There are rules of thumb, colors of lead core lines, and experience. There are depth charts for particular lures, apps of diving depths for specific lures, and apps that use physics to adjust depth for specific lures after you enter weight, line diameter, speed, and other fudge factors.
These work, and have their place, but also have limitations. The most critical limitation of methods that use such physical values is that the values for drag and dive force of a particular lure are unknown unless someone tests for and provides that information.
Angles and Line Length
Some people assume that they can derive depth (at least close enough) with simple high-school trigonometry, using the total line length and angle of incidence with the surface:
sin(angle) = depth/length, or
depth = sin(angle) * depth.
This is a good start, but assumes that the trolled line is straight. A trolled line is straight in only a couple of special cases:
- A lead-core line trolling a small light lipless lure or fly actually is pretty straight.
- An idealized line with zero diameter and no resistance in the water, would be straight when trolled.
Curved Line
Even a thin fishing line has a significant surface area when a lot of line is let out. Consider the surface area you can observe when the line is spooled onto the reel. The frontal surface area of a long line is typically greater than the frontal area of the lure. So, a trolled monofilament or braided line with a sinker or a diving lure has a very significant belly caused by hydrodynamic drag and lift on the line itself. The shape of this belly is heavily dependent on the diameter of the line.
It’s Complicated
How can we calculate the curve of that belly? This is not trivial math, and worse, we need to know a lot of other variables very accurately. Some of these variables like frontal area, drag coefficients, and the effect of a diving lip are highly dependent on the lure’s unique shape and are practically impossible to determine except by careful testing.
The belly of a trolled line has a smooth but complicated curve whose shape and dimensions are determined by all of the different factors which contribute to drag, lift, and down force at each point of the line, lure, and other tackle. Some of these factors are:
- Length of the line
- Trolling speed, with respect to the water accounting for current.
- Diameter of the line
- Frontal area, shape, weight/buoyancy of the lure.
- Frontal area, shape, and weight of any sinkers or other tackle.
- Position of tackle on the line, as in the case of snap weights.
- Buoyancy/weight and surface roughness of the line.
- Density of the water, especially the difference between salt and fresh water.
We would not have to actually calculate the curve, if we could simply observe and measure it. But the line and lure is in motion, underwater, and unobservable from the boat, so it is impractical to measure the line while it it is fully deployed.
Measuring Multiple Angles is the Solution
The angle of a trolled fishing line at any point is entirely determined by the force and direction needed by the line to oppose the combined forces of drag, lift, down-force, and weight of the line, lure, and other terminal tackle. It’s surprising until you really think about it, but these forces are not actually significantly dependent on depth. This implies that the angle of the tangent to the line at a given point of the line, and thus the curve profile of the whole portion of a trolling line below that given point on the line, does not significantly change as the line is lowered deeper into the water.
By measuring the line angle when a given point on the line is at the surface, we are also effectively measuring the tangent line angle at that point of the fishing line when that point is later submerged to any depth.
When we measure the angle of the line for many different total line lengths, these angles and corresponding line lengths are enough information to compute a close approximation to the full profile of the trolled fishing line.
The Trolling Angles App
The Trolling Angles app provides an economical, accurate, rapid, convenient, and versatile process and means with which a trolling angler can know how to present any trolled lure or bait at the desired depth where fish are most likely to perceive and strike the lure or bait.
In a calibration phase, the angler uses the app and the smartphone angle sensor to measure and record the angle between the water surface and the actual rigged trolling line, at many different line lengths.
In an analysis phase, the app calculates a curve data structure which approximates the actual curved trolling line profile.
In a trolling phase, after calibration and analysis, the angler specifies the desired depth, and the app uses the curve data structure to look up and display the line length which is required to troll at that depth.
This method used by the Trolling Angles app does not rely on knowing the values of variable physical factors such as weight, line diameter, or lure shape, or the mathematics of the physics which depends on those factors. It does not rely on external data for specific lure types. It does not rely on observing and measuring the submerged lure.
Trolling Angles relies only on the line lengths and angles of the line as it enters the water surface, observed with the tackle and conditions in actual use.
More Information
The article Line Angles Are Not Dependent on Depth presents an analysis showing that this approach is correct and feasible.