Gone Fishing

Lead Core Basics

By

Gaston Pratte

2007

 Credits

I want to thank my fishing buddy Gerald Bernier, a retired Mechanical Engineer, who has performed Hydrodynamic Analyses during his working days, for his guidance.

Overview

This article deals with nuts and bolts of why trolling lead core line acts the way it does and to demystify some of the perceived behaviors, which has taken on a life of their own as urban legends. This article will not provide a prediction of sinking rate of lead core line, there is insufficient data to create a model.  

How did this all come about? I was reading a post on Great Lake Angler forum that was discussing lead core sinking rates and referenced the Precision Trolling Big Water Edition book.

It came to my attention that it maybe possible to approximate the effects of speed variation on lead core depths. Data was generated and a graph was plotted. The graft was compared to  the stated values on the Great Lake Angler forum and there was good correlation that supports the generated data. I then purchased the book “Precision Trolling Big Water Edition”. The book is based on compilation of empirical data on lead core lines and other fishing devices, lead weights and dipsy divers. 

I Performed an Internet search for lead core line manufactures. One of the leading manufacturers did not have a web site and others had little to no information about their lead core line physical characteristics and sinking rates performance. I E-mailed the manufacturers for info and 2 out of the 3 replied with some info. Cortland was the most forth coming, see Appendix.

Sinking Rate

Sinking fly lines have a specification for sinking rate in the range of 2 to 7 inches per second depending on the construction of the line. In the days when I was fly fishing, I would row out to the middle of the pond, about 50 feet deep and cast a sinking fly. Sitting down while the fly line was sinking and watching the second hand on the watch measure out 30 seconds and then striping in the fly line with a nice 16 inch brookie, is called dredging for bookies. The forces on the fly line are gravitational, (weight), line tension and drag, since there was no movement of the boat.

In the lead core trolling scenario, where lead core sinking rate is measured in feet per color payed out, 10 yards, of 5 to 7 feet at some speed, additional forces are introduced on the lead core line.

Forces on Lead Core moving through water

For simplicity, the discussion will start with the effects of forces on the lead core line only. Other elements and affects will be introduced later in the discussion, water temperature/density, speed changes and mono leader with lures.

As the lead core line is placed in the water, using one yard as the increment, the tip of the line will sink to some depth determined by the weight of the lead core, drag, lift and the velocity in the water. Letting out an additional yard, the weight, drag and lift will double, with the speed of the boat held constant. So, for each additional yard, the forces increase in a proportional fashion, resulting in the lead core line making a straight line from the water entry point to the final depth of the tip, except for a discontinuity between the air and the water due to the difference in densities of the air and water. There is also a discontinuity between the first portions of the lead core line and the water causing drag coefficients to be different from those of the remainder of lead core. These effects are mentioned for clarity and will be disregarded because the effects are overcome quickly as the line is submerged deeper and deeper into the water. The density of dry air at sea level is about 1/800th the density of water.

There are four forces that come into play with lead core moving through the water: lift, weight, tension and drag. From Newton's first law of motion, we know that an object at rest will stay at rest, and an object in motion (constant velocity) will stay in motion unless acted on by an external force. If there is no net external force, the object will maintain a constant velocity.

Figure 1 Forces on Lead Core

·       Drag. Drag is the hydrodynamic force that opposes the lead core motion through the water. Drag is a force and is therefore a vector quantity having both a magnitude and a direction. It acts in a direction that is opposite to the motion of the lead core. We can think of drag as hydrodynamic friction, and one of the sources of drag is the skin friction between the molecules of the water and the surface of the lead core. Because the skin friction is an interaction between a solid and a fluid, the magnitude of the skin friction depends on properties of both solid and fluid. For the solid, a smooth surface produces less skin friction than a roughened surface. For the fluid, the magnitude depends on the viscosity of the water and the relative magnitude of the viscous forces to the motion of the flow. Another source of drag is the form and size of the body that is in contact with the fluid. Form drag is caused by the separation of fluid that is flowing over the lead core line and increases with velocity.

·        Lift. Lift is the force that directly opposes the weight of lead core. Lift is a hydrodynamic force produced by the motion of the lead core through the water. Because lift is a force, it is a vector quantity, having both a magnitude and a direction associated with it. Lift acts perpendicular to the motion of the lead core in the water. All that is necessary to create lift is to turn a flow of water.

·        Weight. Weight is generated by the gravitational attraction of the earth on the lead core which is a force that is always directed toward the center of the earth. The magnitude of the weight depends on the mass of the lead core and since it is a force, it is a vector quantity having both a magnitude and a direction associated with it.  

·        Tension. Tension is a reaction force applied by a stretched lead core line. Because tension is a force, it is a vector quantity, having both a magnitude and a direction associated with it. The direction of the force of tension is parallel to the lead core line towards the fishing pole. Tension is the resultant vectorial addition of the forces of drag and weight minus lift. This is one of forces that can be measured at the boat by using a sensitive scale and measuring the pull of the lead core.

     ·        Velocity.  Velocity is speed with direction.

 The formula for Drag force is F_d = C_d * 0.5 * ρ * V² * A

Where:

• F_d is the force of drag
• C_d is the drag coefficient which varies with speed
• ρ is the density of water
• V is the velocity of the object relative to the fluid
• A is the reference area

 The formula for Lift force is F_L = C_L * .5 * ρ * V² * A

Where:

• F_L  is the force of lift
• C_L is the lift coefficient which varies with speed
• ρ is the density of water
• V is the velocity of the object relative to the fluid
• A is the surface area of the lifting surface

 Density of water.

The density of water changes with temperature. As we all know that cooler water sinks and warmer water is displaced to the surface, cooler water is denser than warmer water. In the preceding equations for Lift and Drag forces, they are dependent and proportional to the density of water. Water density is equal to 1, when the water temperature is equal to 39.2 degrees Fahrenheit.

 Figure 3, Water Density

To calculate the effects that water density has on the Drag and Lift forces, two temperatures are selected, which are encountered during the summer and fall trolling seasons, along with their associated water densities. At 50⁰ the density is 0.9997 and at 68⁰ density is 0.9987, for a difference of 0.001 which is 0.1% change in density. The 0.1% is a minuscule change in Drag and Lift forces since they are directly proportional to density changes.

 In summary, water temperature does not have a large effect on the forces of Drag and Lift and thereby does not affect depth, sinking rate of lead core perceivable. This is contrary to published literature that alluded to temperature effect has an observable influence on sinking rates.^{1, 5}

Velocity.

A small change in velocity will cause substantial changes on the Drag and Lift forces which in turn are translated to a proportional change in leadcore depth.

At 2 mph, a trolling speed that is commonly used for LL Salmon, on a steady course and making a positive speed change of 0.1 mph, will cause a Drag and Lift force change of ≈10.25%. This is caused by the parameter “velocity” which is squared in the previous equations for Drag and Lift forces. At a speed of 2 mph squared 2^2 = 4 and the increase speed of 2.1 mph squared yields 2.1^2 = 4.41; 4.41 – 4 = 0.41; (0.41/4) * 100 = 10.25%. Refer to Figure 8, Lead Core Depth Estimator.

In summary, minor speed changes in velocity has a substantial affect on the forces of Drag and Lift and ultimately on sinking rate of lead core. Again, this is contrary to publish literature that alluded to a stable sinking rate for speeds of .5 to 1.8 mph or for slow to medium speeds.^{1, 5}

Trolling lead core through underwater currents has the same effect and sensitivity on sinking rate as boat speed changes. There are currents in the lake that may be going in the same, opposite or broadside to the trolling direction that will modify the depth of the lead core drastically.

 Observations:

·        Performing slow “S” maneuvers while trolling will make the lead core line on the inside slow down; therefore, increasing depth and the line on the outside of the curve will speed up and rise. These falls and rises do not happen instantaneously. First the lead core has to sink which is a function of the deceleration change and then the combination leader and lure has to follow, all of this takes time. 

·        There is a performance difference in where you position the lead core line in the water in relation to the boat. If it is placed down the chute, (over the stern), in the wake of the prop wash, the lead core will feel the affects of the water speed changed. The prop wash is water that has a higher velocity than the water 8 feet on either side of the boat. The prop wash water velocity (energy) will dissipate at a rate of distance squared. Hypothetical example:  the boat is at a speed of 2 mph and the prop wash speed was measured at 10 mph at a distance of 3 feet back from the boat, then at 27 feet from the boat, the prop wash speed will have dropped to 0.15 mph.  For every doubling of distance from the reference point (3 feet), the force will be quartered. This holds true for the water of the prop wash that does not intersect the surface. The wash that intersects the surface will bounce off the surface at a reciprocal angle of engagement and adds to the wash that comes directly of the prop.

Area of the lead core.

 Table 1, % Increase of Area

A change in the area of the lead core line, i.e., a change from one test rating to next higher rating will cause the Drag and Lift force to increase proportionally to the area change. The percentage change is shown in the table for Cortland’s Kerplunk mean diameter and will affect the depth proportionally and ultimately the sinking rate.

 Effects of mono leader and lures.

The monofilament leader and lure combination generates additional and separate Drag and Weight forces.

Figure 4, Mono Leader and Lure

Figure 5, Lead Core, Leader and Lure

When the combination of the leader and lure is attached to the tip of the lead core, an additional drag force will pull the lead core tip backwards. Pulling the lead core line back has the effect of raising the lower portion of the lead core thus, decreasing the sinking rate and creating a bow in the lead core line. The bow in the lead core line will progress up the line from the tip to the fishing rod. The bow diminishes as it gets closer to the rod tip and goes asymptotic to the path of lead core line without a leader and lure.

A hypothetical example, plotting the weight to drag ratios will be used to explain the difference between trolling lead core line only and with the addition of the monofilament leader with a lure or fly to the line. The hypothetical will use values based on a per color of lead core weight of 0.75 oz and a drag of 2.9 oz which are in the realm of being close to perceived actual values for a trolling speed of 2 mph.

Refer to Table 2, Ratio. For Lure and Fly, values of 1.5 and .75 oz for drag and 0.0625 and 0.0165 oz for Weight respectively.

Reviewing the table, Lead Core, Ratio 3 represents the lead core line without leader and lure. In row 0, Lead Core Color, refers to the absence of mono leader and lure data. The remaining colors are incremented with values of 2.9 oz for drag and 0.75 for weight per color dispensed into the water. Referring to Figure 6 Ratio Graph, it depicts a straight line at a weight to drag ratio of 0.2586.

Refer to Table 2, Lure. Where the drag and weight for lure are set to 1.5 oz. and .0625 oz. respectively. The quantity of 2.9 oz. for drag and 0.75 for weight are used for each additional color of lead core. Refer to Figure 6 Ratio Graph, the red curve Ratio 1, depicts the ratio of weight to drag ratio varying as lead core descends into the water. The bow that is generated is due to the leader and lure drag.

 Refer to Table 2, Fly. Where the drag and weight for fly is set to .75 oz and .0165 oz. respectively, the quantity of 2.9 oz oz. for drag and 0.75 for weight are used for each additional color of lead core. Refer to Figure 6 Ratio Graph, the black curve Ratio 2, depicts the ratio of weight to drag ratio varying as lead core descends into the water. The bow is less than that generated by the lure.  

Monofilament lines have a density slightly greater than water, so they tend to be buoyant neutral, as where the density of lead core line is 11.3 times greater than water. The lure has a weight and a drag associated with it. The weight and drag can vary greatly when using flies or spoons. 

In summary, sinking rate will be minimal with the least lead core in the water and will increase as more lead core line is released into the water.  

Table 2, Ratio

Figure 6, Ratio

 Mono leader and lure discussion.

Different fisherman have different prospectives on how long the leader should be on the end of lead core line. There is a Winnipesaukee Lake charter that says he uses 150 feet of mono for a leader others use 100 or 50 feet.

 Figure 7, Distance vs. Time  

My prospective is this: the objective of a long leader is to disassociate the boat from the lure for finicky fish. With a 150 foot leader and two (2) colors of lead core in the water, the distance between the boat and the lure is now 210 feet. Using 210 feet as a hypothetical gold standard and with the objective of keeping the mono as short as possible to minimize the drag and maximize the sinking rate, then when fishing deep it would be prudent to shorten the leader and still maintaining the 210 feet. Referring to Figure 7 Distance vs. Time depicts the elapsed time for various boat to lure distances.

Some fishermen will run a butt leader of higher lb test monofilament line for 50 feet or so and then tie a swivel and a lighter fluorocarbon leader of 10 feet or so. To minimize drag, it would be a good idea to run a smaller monofilament all the way from the lead core to the lure.

Doubling the test lbs. of the monofilament line will double the drag.

Lead Core Depth Estimator

Lead Core Depth Estimator graph was generated using the premise that velocity changes can be used to ball park depth changes. This graph is based on velocity only and does not include effects of other forces, underwater currents or any other effects. The graft is to be used by troller to adjust the dispense line when speed changes are implemented.

The only requirement to use this graph is that the speed for the reference point must be at 2 mph which equals a ratio of 1. The user must set his/her gold standard of colors of lead core dispensed into the water to achieved depth less any external currents.

• Determine the number of colors of lead core to be used: i.e.         5 colors
• Multiply by the depth constant per color:  i.e. 6 ft.:    6 X 5 colors = 30 ft.
• Determine the trolling speed to be used:                                         1.8 mph
• Find on the graph the corresponding ratio associated with 1.8:           1.24  
• Multiply the ratio time the depth:                              1.24 X 30 = 37.2 feet

 37.2 feet is the estimated depth of the lead core at 1.8 mph or for a simpler way is to estimate the amount of change and add i.e. 25%.

 Figure 8, Lead Core Depth Estimator

SWR (secret weapon rig)

The SWR was developed several years ago, early 2000, out on the Great Lakes (GL). Many GL anglers have adopted this technique with good success. The purpose of this technique is to present the lure in a stealth environment for when the fish are spooky or finicky. The idea is to present the lure/bait some distance below and back from the ball to disassociate the lure from the ball and boat, stealth mode of fishing. Some anglers will do a three color rig, but two colors is the most widely accepted rig for what its worth. With the two colors of lead core, the lure will drop and rise when the boat changes directions, providing additional action. Keeping in mind, the lead core will rise and fall with speed changes of the boat and subsurface currents.

SWR setup can be used to target lake trout that are hovering at or near the bottom. Instead of dropping the downrigger ball to the bottom, the ball can now be raised several feet above the bottom and letting the lead core drop to the bottom with the lure. This will save downrigger weights and in some cases downriggers that have the potential of being ripped off the boat.

It also can be used in the spring time for surface fishing, (flat line), when the fish are hitting up high.

Monofilament and Lead Weight Trolling.

Lead weight trolling does not act in the same manner as lead core line. In the case of lead core trolling, the weight and drag forces are distributed over the length of the lead core line. Where lead weight trolling, the weight and a large portion of the drag force is concentrated at the weight, due to size of the weight and the remainder of the drag forces are distributed over the length of the line. Again, the diameter of the line is critical to minimize drag forces.

Adding Weight to Lead Cored Line

There are many different devices that can be used to attaching lead weights to lead core line increase sinking rate. Some devices have a large cross sectional area and as such increase the drag substantially. The following table delineates different devices, their drag performance and where they maybe purchased.

Table 3 Weight Clips

Stainless Steel and Copper Line Trolling

Stainless steel and copper are of a lower density when compared to lead. The advantage of using copper or stainless steel is there is no sheathing as in lead core providing tensile strength to the lead core line, sufficient tensile strength is inherent in the material. For the same overall diameter as lead core line, copper or stainless has more mass, subsequently weighs more than the core of lead core line. The following table shows the diameters of the core for lead core line and a comparable SS wire and copper with the associated areas. 18 lb lead core line has a core diameter of 0.020 and an over all diameter with sheathing of 0.032. The table compares the differences in materials and the resulting weight difference over the lead core trolling line.

Table 4 Stainless Steel & Copper Comparison

 Applying the % Weigh Increase values from the previous table to one’s favorite sinking rate; 5, 6 or 7 feet per color @ 2 mph, the following table shows the approximate sinking rate improvement for the various trolling lines. An additional benefit, assuming that the same leader and lure combination is use in all cases would a smaller amount of a bow on the sinking line.

 Table 5 Sinking Rate per Color

Several sources for copper and SS lines; Opti Tackle and Fishing Line.

The detractors in using copper and SS trolling lines are kinking, curling and using special rods with rollers or with Twili tips. The pluses are more weight for similar diameter lines, increasing the sinking rate and less pull back by the leader lure combination.

Lead trolling lines provides stealth. Due to the low sinking rate, more line has to be dispensed to achieve the desired depth, thus separating the boat from the bait. The SS and copper lines will sink much faster and thus reducing the separating between the boat and bait. The SS and copper lines are a great choice for Great Lake fishing when wanting depths of greater than 60ft, also when the trolling speed is increase to 2.5 to 3 mph.

GPS Speed

GPS speed accuracy stated by Garmin is 0.1 knot, 0.115 mph, RMS steady state. There is also a displayed resolution accuracy of 0.1 mph. Combining the two potential errors yields a total error of 0.215 mph. The inability to determining speed precisely puts a depth ambiguity of up to 20% from your desired depth. Satellites change position all the time with respect to one’s position. This causes the errors to change thus causing the speed accuracy to change through the day.

Appendix

Cortland's lead core manufacturing specifications for Cortland's Kerplunk

http://www.cortlandline.com/catalog/2007_Sportfishing.pdf

References

1)     Per Gudebrod lead core packaging;

 “Approximate sink rate – 2 yards out = 1 foot depth. After 25 feet of depth thermocline or extreme water temperature change effects lead core sink rate.”

2) Cortland Line

3) Western Filament Inc

4) Gudebrod

5) 4Fishin

6) Precision Trolling Big Water Edition

7) NASA

 © 2007, All Rights Reserved
Gaston Pratte

landlock@pattiandken.com