Downrigger angle and depth are both important to get those trophy salmon. Here are two tricks of the trade that can make you fish more effectively for salmon when you are out on the water.
Most people troll at 1.5 to 3.5 mph. How do you know if you are at the right speed? Sure, you can look at your GPS speed over ground readings, but you see your downrigger lines are almost straight at 2 mph. Here is a simple trick: Remember the days with steel downrigger lines, when you could hear the humming sound when you were trolling at the correct speed? That’s not the case anymore. With 15-lb cannon balls, the general rule is a 45-degree angle between the downrigger arm and line. With 18-lb cannon balls, it’s roughly 30 degrees. If you are looking for coho salmon, you want to troll faster, which means the downrigger angle will be greater. It can vary from 45 to 60 degrees on the downrigger for 15-lb cannon balls and 30 degrees to 60 degrees on 18 lb cannon balls.
All downrigger wire and braided cables give you blowback when trolling. You can use blowback to your advantage to find more fish. When you troll with the tide, you can control how fast the lure is travelling, judging by the blowback angle. When you troll against the tide, the tide current is already providing the lure action, but it causes more blowback. The more you try to push ahead (trolling faster), the more blowback angle you will have. Try playing around with downrigger angles with the tide; that might improve your success rate in finding salmon.
Now with these angles, how do you know your lure is fishing at the right depth? Do you think your lure is at 150 feet when your downrigger is at 150 feet with blowback angle? Most likely it’s not. You thought you could ignore math when you’re fishing, but trigonometry is a useful tool that we can apply to fishing. When you have a downrigger angle, your lure depth will be slightly above your downrigger counter depth. We will use the cosine math rule learned in high school to calculate our lure fishing depth. Take the cosine of your downrigger angle and multiply with the downrigger counter depth. That number is roughly your lure fishing depth. For example, you saw fish at 150 feet and you put your downrigger at 150 feet, 150 ft x cos(45) = 106 ft. If you’re fishing at 106 feet, you’re missing the fish. To be fishing at 150 feet, you need to put your downrigger at 212 feet and troll at a 45-degree angle. The reverse equation is fishing lure depth divided by cosine of downrigger angle…
150ft / cos(45) = 212ft
Blowback is an indication of how much drag force you have while your lure, cable, and cannon ball are moving in the water. With laws of physics in the studies of fluid dynamics, you will have fluid resistance:
Fd is Drag Force p is Density of liquid v is speed Cd is drag coefficient A is cross sectional area
We do not know how much drag force we have with all the stuff we are trolling in the water, but this equation does tell you that we will have blowback. The cannon ball and cable is almost like a pendulum, so we can look at the downrigger angle as speed:
ω = θ/ t ω = angular speed in radians/sec θ = angle in radians (2π radians = 360 degrees) t = time, sec
Speed is proportional to angle. That is why downrigger angle can be used to determine trolling speed.
Now how do you put these two tips to use? By using the downrigger angle to judge the speed of your lure trolling, you can calculate the depth your lure will be fishing. By adjusting your trolling speed, you will adjust your downrigger angle. Then use your sonar to locate the fish and use simple trigonometry calculations to set your downrigger depth. Or, if you want to skip the math, you can use the chart I have created. Try this, and you’ll find more fish.
Fishing Depths vs Downrigger Angles
From your Scaly Friend (Montagu Lee)