The Troll Master Depth Calculator App

Not long ago I saw this interesting app on social media, recently developed by Trevor Lewis. I’ve never met the guy, I was intrigued with his app. I asked him to send me some info and thought I would be cool pass it along to you all. This looks like it will help take a lot of the guess work out of trolling.
But remember , boat fish don’t count anyway. Also thanks to Trevor for saving me some time by writing up how to use his app.

Troll Master – Trolling Lure Depth Calculation App for Android

Trevor Lewis

Taking the guesswork out of trolling

To say it was slow was an understatement.  The spring of 2018 had been excessively rainy in the Chesapeake area and the cold temperatures had put a real damper on fish activity.  The water temperatures were about 42°F and the fish were concentrated within 5 ft of the bottom in 35 ft of water right off Thomas Point.   At least we were marking fish!  Some folks had traveled much farther and found much less. 

Our trolling rods were set for the top half of the water column, not the bottom half.  I do a lot of shallow water trolling in the fall and these deeper water fish were outside of my comfort zone.  How much deeper would my lures run if I let out another 25 ft of line?  How about 50 ft?  How much more weight should I add?  Thanks to my fishfinder, I could clearly see fish on the screen, but how could I put those bucktails into the strike zone? As an engineer, this really bothered me.  I make my living by using my knowledge of math and science to solve problems. 

This was definitely a solvable problem!  This was one of those challenges that just sat in the back of my mind and gnawed away at me.  After doing some research, I was able to develop a mathematical approach to this problem.  To put it in basic terms, the depths that trolling lures run at is determined by two sets of forces: drag forces and sinking forces.  The sinking forces are pretty straightforward to calculate since we know the weight of our lures and trolling weights.  The drag forces on a lure are pretty easily determined as well.  The drag forces on the line are a different story!  The drag forces on the line are a function of the length of the line, the diameter of the line, and the shape that the line forms in the water.  This in turn affects the depth at which the lures will run.  I’m happy to say that this research came to a happy conclusion and resulted in a solution to this problem. 

The solution that I created uses the following inputs from the user:

• Speed of the lure through the water
• Line type (monofilament or braid)
• Line weight (8 to 80 lbs)
• Lure weight
• Trolling weight
• Leader length
• Lure profile

There are a couple of things worth noting here.  If your GPS says that you are moving at 2 mph and the current is running at 1 mph in the opposite direction, then the speed of your lure through the water is 3 mph.  If you are running at 3 mph in the same direction as a 1 mph current, your speed through the water is 2 mph.  Second, the leader length input is used to position the trolling weight on the line.  One important thing to note here is that the total line out is equal to the line length + the leader length.  The lure profile input allows us to compensate for changes in the overall size of the lure, which in turn affects the drag force on the lure.  Anyone who has attempted to muscle in a cownose ray understands this effect!  It’s like reeling in a barn door.

The screenshots below show how this information is displayed in the app.  Once you have entered all the information, you click the button labeled “Calculate Depth”.  The calculated depth is displayed below the button.  The last step is to subtract out the height of the rod tip. 

This is all fine and good, but how do I know the predicted depth is accurate?  One of the things that I did during the development of the app was to compare it against some of the trolling charts that are out there in circulation.  A good example of one of these comparisons is shown below. 

Trolling Setup
Line Diameter: .033 inches
Speed: 3 mph
Weight: 10 oz.

Table 1: Line Lengths Vs. Predicted Depths

Line Length (ft)	Depth predicted by the chart (ft)
50	15
100	20
200	25

I chose this set of data because it highlights some of the nuance that is not captured by these types of trolling charts.  They are a great reference but the Troll Master app is a more refined tool because of the ability to use the leader length, trolling sinker weight, and lure weight to predict depth.  A good case is If we were to use a very heavy trolling weight with a very light lure and a long leader.  The running depth of a lure will be greater if there is more weight further down the line.  For the sake of comparison, I used a leader length of 15 ft and varied the way the weight was split between the lure and the trolling weight.  The results from the Troll Master app are shown in the plot below in blue.  The values predicted by the trolling chart above are shown in orange.  

As you can see, the predictions from the trolling depth chart are right in the middle of the range of values predicted by the app.  One important observation from this chart is that, depending on how the weight is distributed between the lure and the trolling weight, there could be about a 9 ft variation between the values predicted by Troll Master and those in the chart.  This could mean the difference between being inside or outside the strike zone.  The chart below shows the calculated shape of the line in the water when a heavy trolling weight is used with a lighter sinker.

So if I was able to turn the clock back to that cold spring morning in 2018, how could I alter my rig to set some lines to run at 30 ft?  I was running 4 oz lures and I had the option of varying the line length, the trolling lure weight, or both.  A quick check on the Troll Master app shows that I could run a 4 oz lure at 150 ft back with a 10 oz trolling weight on a 15 ft leader to get right around the 30 ft mark.  The only thing left to do at this point would be to point the bow in the right direction and make sure the net is ready!

The Troll Master Android App is available on Google Play in the Play Store.  If you search for “Troll Master Depth Calculator” the app should be right near the top of the results.  The paid app will calculate a full range of depth values.  There is also a Free version of the app that I have deployed as a functional demonstration.  The Free version has limited functionality, but it will let you get a feel for the utility of the app without paying for the full version.  All upgrades are free once you purchase the full app.  For additional information, please don’t hesitate to contact me about this app at trollingcalculator@gmail.com.

Tight Lines,

Trevor Lewis