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Lead Line Trolling Depth

October 7, 2010

One of the biggest problems with fishing for cold water species is depth control. Of course you can solve this problem with expensive down riggers, but I am a cheapskate with a mathematical bent and a little background in classical physics. A search of the web turned up little more than rules of thumb. There was one interesting scientific paper which took an engineering approach to the problem: “Sink depth of trolled fishing lines” G Spolek – Sports Engineering.

What Spolek did was to make some simplifying assumptions about the behavior of weighted line and developed a simple model for calculating the depth of a trolled lead core line based on its weight, its diameter and trolling speed. In any case where drag is involved the drag increases with the square of the speed unlike that of other factors. He then validated his model by depth and speed measurements.

Professor Spolek’s paper has already been referenced in Fly Fisherman Magazine in reference to sink depths of wet flies, if you are interested in that kind of thing.

What I did was to measure diameters of lead core line with calipers and a feeler gauge (+/- .05 mm) and measured one color of the line to the nearest gram – if no published data were available. I then calculated the specific gravity theoretically and ran the numbers through Professor Spolek’s model. This gives a speed dependent view of depth to line out. At 2.5 MPH I get roughly 5 ft down per color, which agrees with observations seen around the net, and on lead core line web sites. As you can see from the table below, however speed, diameter, and the line’s grain weight vary this number considerably.

Sink Depth in Feet per Color

Speed – MPH Kerplunk #18 Kerplunk #27 Cabella #12 Cortland LC 13
.5 26 22 21 20
1 16 13 12 11
1.5 11 9 8 7
2 9 7 6 6
2.5 7 5 5 4
3 6 4 4 4
3.5 5 4 3 3
4 4 3 3 3


I had assumed while doing the above calculations that the lead core line would take on a catenary shape as more line was paid out, based on my interpretation of this Wikipedia article which touches on, but does not completely explain the problem of calculating the depth of a towed cable.  Once I went to the trouble of creating a model which took catenary effects into account the effects cancel out. It turned out that there are no additive effects on a simple lead core line, that it lays straight in the water; with the small exception of density changes due to temperature changes.  So theoretically, one could use this table to calculate the depth out to ten colors or more. According to this model then, ten colors of Kerplunk #18 at 1.5 MPH could get you deeper than 100 ft, assuming no current, and that your lure was of neutral buoyancy, such as a light salmon spoon.

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