Sunday, December 08, 2019
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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 cable 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
and 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, 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
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
