Here are some thoughts on the geometry of downrigger lines and 2-D sonar displays. This example is based on the data from the previous post “Trolling Angles with Downriggers“.
Transponders
The transponder is below the surface, in this example by 2 feet. The green area shows the effective span of the transponder’s signal cone. A typical sonar transponder can use either 83Hz, which has a 60 degree cone, or 200Hz which has a narrower 20 degree cone. These cones do not have discrete boundaries. Rather, the signal is 1/2 as strong at the edge of the cone than at the center of the cone, and the signal strength drops off rapidly outside the cone.
Fish Echoes
If one sees a fish echo on the display, it’s telling us that the fish is the indicated distance from the transponder, which is not necessarily the same as its actual depth. As you can see from the dotted red line on the diagram, with a 83Mhz transponder, with an echo shown at 60 feet, the fish may actually be anywhere from 53-62 feet deep.
Cannonball Distance vs Depth
Typically, when using downriggers, the cannonball weight is outside the cone and not displayed, due to “blowback” caused by the drag on the cannonball and cable. But, if we do see it on the display because the transponder is angled backwards, and/or sensitivity is turned very high and/or boat speed is slow enough, the cannonball is shown at a depth much greater than its actual depth. In this example, with 90 feet of cable, the sonar shows the cannonball’s echo at 83 feet, but its actual depth is only 62 feet.
Conclusions
When you use a 2-D sonar, keep in mind that fish may be quite a bit shallower than shown on your display, if they are to the side of the cone.
And if the fish or bottom is actually below your boat, and you are trying to set your downrigger to reach them, keep in mind that neither the sonar and nor the downrigger line length indicator show your actual cannonball depth. They both will read substantially deeper/longer than the actual depth.
And, although you can perhaps ballpark a very rough estimate, you will not be accurate unless you have an instrument or observer at the cannonball, or measure those line angles and do some high order math.
Get Trolling Angles
The Trolling Angles app can easily calculate the actual depth of your downrigger cannonball and lures. It’s very inexpensive for a subscription with a free trial period, and for the rest of the year, the Android version can still be upgraded in-app to a permanent license for only $20.
