Every fisherman who spends much time trolling wants to improve their understanding of trolling depth. However, if you try to learn more about trolling line curves and lure depth, you may run across a lot of confusing and incorrect information. This is not surprising, since very few people have observed an actual trolling line underwater or studied the relevant physics and math.
Trolling Line Shape
Apparently, even the basic shape of a trolling line curve is amusingly counterintuitive.
Many first impressions, diagrams or artist renditions of a side profile view of a trolling line show a straight line. Or commonly, the curve is shown in the wrong direction, as if the line is dragging over the ground. This is bogus.
In the water, the weight or diving lure pulls the end mostly down, while the water’s drag on the line pushes the rest of the line mostly backwards.
Rules of Thumb
Rules of thumb are bogus.
Some rules of thumb are better than nothing, but most are very imprecise and misleading. For example, a manufacturer may say that a lure dives 7-10 feet, without specifying line length or speed. Or, a manufacturer may only say that its trolled lure dives about 1 foot for every 3 feet of line.
A rule said to be used for saltwater big game is the “rule of fives” When trolling at 5 mph, with 50 pound test line, a 5 ounce weight, and 50 feet of line, the lure’s depth is 5 feet. If accurate, this is applicable only for this limited condition, and cannot easily be applied with different numbers.
Equations of the Line Curve
Nearly any line curve equation is bogus.
High school geometry can be used mentally by keeping a line angle of 30 degrees, and then the vertical distance from rod tip to lure depth is ½ of the total deployed line length. This is only useful for 30 degree angles, and also assumes the submerged line is straight, which is not the case.
If assuming a line is straight, one can use high school trigonometry for any line angle and line length. But the line is not straight.
When the ChatGPT AI was asked what the shape of a trolled line is, it answered that it’s a circular curve segment. That’s far too simple and physically incorrect. Perhaps ChatGPT will learn better as time goes on. Some internet posts have suggested using a circle segment as an approximation too, and may even be the source of the AI’s hallucination.
An engineer may say a depth curve can be approximated by a polynomial or spline. Well, that’s true, since any curve can be approximated that way, in a specific range of values. But without defining the coefficients, which must differ for every lure, line, weight, and speed, this is not very illuminating.
Some have claimed or assumed that the shape of a trolling line is a catenary, which is the shape of a hanging line such as a power line (though inverted). Now we are getting close. Both a catenary and a trolled line take their shape because line forces at each point are added to the sum of forces at each previous point. However, the catenary equation (of a hanging line) assumes that the force is proportional to a line segment’s weight, thus length, while the fishing line’s drag and lift forces are proportional to its frontal area and horizontal surface area and to drag/lift coefficients that change nontrivially as angle changes. (I apologize for the handwave calculus)
A single analytical equation is too simple to account for all the forces acting on different parts of the line, even if you know all the input variables.
Downrigger Blowback Charts
There are many charts and tables that relate cable length and boat speed to actual cannonball depth. Many are admittedly inaccurate, very approximate, and/or demonstrably bogus.
Some are apparently constructed by estimates and guesses, and few are constructed by direct testing or observation of the cannonball. Some are apparently constructed by observing actual cable line angle measurements, but use simple straight-line trigonometry, which cannot match the curved line.
Some may be constructed by observing readings from depth probe instruments attached near the cannonball. These may be more valid, except for potential probe inaccuracy and the effect of the probe’s drag, and they generally do not account for all the differences in drag caused by using different speed, cable size, cannonball weight and shape, lures, dodgers, and flashers.
Cannonball Depth from Depth Finders
Checking your cannonball depth with your 2-D sonar is bogus.
Except at speed=0, the depth of a downrigger weight as displayed on a 2-D sonar depth finder is always incorrect and far greater than the actual cannonball’s depth, because the depth finder is showing only the direct distance between the transducer and the target cannonball, which is blown back significantly at an angle behind the boat.
The depth finder shows the hypotenuse of the transducer-weight-depth triangle, not the depth, even though its display misleads you into believing it does show the depth Don’t even bother to angle the transducer or increase the sensitivity to show your cannonball. Its reading is no better than the cable length gauge. Use your fish finder as it was designed, to show the fish and the bottom, not your cannonball.
Advanced 3-D Sonar, down-scanning, omnidirectional, or “back-scanning” units might eventually have the transducer directionality and processing to do a better job, but cannonballs are usually not in range of their transducers, and displays are not designed to show this type of depth well.
Maximum Trolling Depth
It is generally obvious that deploying more line results in a greater depth, for both weighted and diving rigs. But not everyone understands the maximum depth limit of a rig.
Every rig, when trolled at a constant speed, has a maximum trolling depth, regardless of how much line is deployed. This is the depth that is reached when the drag and lift of the deployed submerged line fully counteracts the weight or dive force of the lure. If your rod tip is at the water surface, any additional line will simply stay on the surface. If your rod tip is elevated, increased line length will result in a smaller incidence angle with additional unsubmerged and submerged line, and the lure will never quite reach its maximum depth, but it will get vanishingly close.
As an aside, there are internet posts that claim that letting out more line can result in a shallower total depth because of the additional line lift. This is a bogus fallacy.
Depth vs Speed
Weighted Rigs
The depth of a weighted lure is very much dependent on speed. Obviously, a weighted lure has no maximum depth at all if the speed is 0 and will approach the surface if the speed is very high.
Diving Lures
The depth of diving lures is not theoretically dependent on trolling speed. This is because as speed increases, both the forces of drag and lift increase at the same rate: velocity squared, leaving the line angles unchanged.
However, many diving lures do run a little shallower as speed increases. A diving lure will blow out or move erratically and reduce its diving force if trolled fast enough. A lure that is designed to run very erratically may run at different depths depending upon speed. This effect is unpredictable but can be measured.
At any rate, the effect of speed on a diving lure is much smaller than the effect of speed on a weighted rig.
What Works?
It is not too much to say that there are only a few accurate, reasonably convenient, and versatile ways to determine the actual depth of a trolled rig: a probe, an app, or a rig-specific data chart
Probes
There are probes, or devices, that attach to a downrigger cannonball or end of the line, that measure water pressure which is converted to depth. They may also usefully measure speed with respect to the water and water temperature. Some transmit data in real-time to display on the boat. Others save or log their depth and are read after they are retrieved.
A probe system which does not transmit its real-time data is very inconvenient to use while fishing. Probe systems that do transmit data are very expensive.
The main disadvantage, apart from expense, is that a probe creates its own drag which contributes to the depth curve. This means that its depth information is applicable only to the rig that includes the probe. With a small probe and a large downrigger cannonball, this difference may or may not be negligible enough to ignore. For long-line trolling, a probe is not practical.
Other Apps
Apps or computer applications use various methods and data and may also be acceptably accurate. The best apps depend upon data that has been measured in actual tests of specific lures, lines, weights, configurations, speed, and conditions, and are not applicable with other conditions. Most apps other than Trolling Angles supply this data from the app vendor’s measurements.
Trolling Angles App
The Trolling Angles technique is described by its patent and on the trollingangles.com website. It provides an accurate trolling depth curve approximation based on your own measurements of the actual lure or rig in use.
There are currently no published charts or tables that have been constructed using the Trolling Angles app or its patented method. To publish one would be a violation of the Trolling Angles patent and/or the app’s Terms of Use.
But, why use a static chart anyway when you can use the interactive Trolling Angles app?
The Trolling Angles app is inexpensive, very versatile, theoretically sound, and accurate. Read https://trollingangles.com for much more information. Download Trolling Angles from the Apple App Store or Google Play.
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