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Determining Downrigger depth
when Trolling

     Friday, July 21, 2017
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There are several methods that have been used for determining the actual depth of the downrigger ball when trolling. The most important thing to remember is that the counters on the downriggers are measuring the amount of wire cable out, not the actual depth. The actual depth is affected by the speed the boat is traveling, the amount of underwater current and its direction, the size of the downrigger weights, the shape of the downrigger weights, and finally by the drag produced by the lure, downrigger ball, and cable.


Method 1

This method for estimating the depth of the downrigger ball is based on measuring the length of downrigger cableDownrigger cable angle measurement and the cable angle measured in degrees from vertical. Estimates can be calculated for the depth of the downrigger ball and the distance back from a vertical position that the ball would be located. The estimates for several angles and desired depths are provided in the chart below:

Cable Length and distance back from
desired Depth and estimated angle

 

20 degrees

30 degrees

40 degrees

45 degrees

Desired
Depth
in feet

Cable
Length

Back

Cable
Length

Back

Cable
Length

Back

Cable
Length

Back

10 11 4 12 6 13 8 14 10
20 21 7 23 12 26 17 28 20
30 32 11 35 17 39 25 42 30
40 43 15 46 23 52 34 57 40
50 53 16 58 29 65 42 71 50
60 64 22 69 35 78 50 85 60
70 74 25 81 40 91 59 99 70
80 85 29 92 46 104 67 113 80
90 96 33 104 52 117 76 127 90
100 106 36 115 58 131 84 141 100
125 133 45 144 72 163 105 177 125
150 160 55 173 87 198 128 212 150
175 186 64 202 101 228 147 247 175
200 216 73 231 115 261 168 283 200
225 239 82 260 130 294 189 318 225
250 266 91 289 144 326 210 354 250

Method 2

This method determines the proper downrigger weight size based on trolling speed and the depth required. The amount of weight required is a function of the speed you are trolling and the depth you are fishing. The deeper and faster you go, the more weight you need to keep the downrigger wire at a near vertical angle. Charts for different speeds and size weights are located at the Scotty Downrigger web site.


Method 3

This method determines the depth of the downrigger ball Troller's Handbook by Ray Rychnovskyand the distance from vertical that the downriggger ball is located. This method is described in a book "The Troller's Handbook" by Ray Rychnovsky, published by Frank Amato Publications. Calculations using differential equations and difference equations are used to calculate for hydrodynamic forces that act on the downrigger ball, downrigger cable, lure,Troller's Handbook by Ray Rychnovsky and fishing line. A series of charts are provided in the book that provide depth estimates for different size weights at different trolling speeds. Other sections of the book estimate the trolling depths at different speeds for different diameter fishing lines, leadcore line, and wire line.


Method 4

This method uses actual measurements taken with your Fish Finder or GTM 40 own boat and downriggers. The Fish Finder or GTM 40 is a device that measures water temperature and depth. When attached to the downrigger ball the unit will record water temperature at 5 foot depth intervals. The unit stops recording depth and temperature after staying at the same depth for 15 seconds. When the unit is retrieved you press a switch on the unit that will cause the unit to playback the depth and temperature in 5 foot increments. All you need to do is reach your favorite trolling speed, attach the unit to a downrigger ball, lower 50 feet of cable, then wait 15 seconds. Raise the downrigger and display the actual depth of the downrigger ball.

The GMT 40 has been discontinued by the manufacture and is no longer available




This web page was last modified on Wednesday, July 09, 2008

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