Trolling Depth Guide & Snap Weights Chart for Sinkers, Rigs, Baits
Thanks to technological advances, we can locate fish and know how deep we should be fishing. State-of-the-art fish finders can now read fish depth directly, or at least infer their location by zeroing in on schools of baitfish or analyzing bottom Structure.
We know what water temperature each species will seek and measure this at various trolling depths to pinpoint the zones where we will probably find fish.
Trolling Depth Guide for Lures and Rigs: How to Get to the Right Level
- 1 Trolling Depth Guide for Lures and Rigs: How to Get to the Right Level
- 1.1 Depth Calculations for Sinkers, Rigs, and Baits
- 1.2 Trolling Depth Variables with Speed
- 1.3 Velocity and Weight are Most Important
- 1.4 Velocity and Weight in Different Trolling Depth
- 1.5 Trolling Depth Guide on Velocity in Constant Speed and Different Weight
- 1.6 Use Low-drag Lures and Rigs for Trolling
- 1.7 Get in the Zone
So, we know what depth to fish. But can we take advantage of this information? How do we get our lure or bait to the right level? Using the spot lock machine will make reaching the right level more effortless, but it is always best to understand how it works.
Typically, we attach a weight that seems appropriate, estimate how many lines we should spool out, and hope for the best. The line usually loops back far behind the boat and the depth trolled is no better than a wild guess.
Sometimes anglers-particularly salmon trollers-get their bait or lure to the proper depth with heavyweights. The line goes almost straight down, eliminating most of the guesswork concerning trolled depth.
But heavyweights and light tackle don’t mix, and this sure takes a lot of the sport out of sport fishing for trout, catfish, salmon using the right rod and reel. Downriggers get the lure or bait to a known depth at typical slow trolling speeds but are pretty expensive.
Also, they aren’t practical on rental boats and are hazardous if they get caught on the bottom or in the prop. Many anglers avoid the complications of downriggers by not using them.
After knowing to troll at the proper depth, you can use the preferred ultralight tackle and fish.
Depth Calculations for Sinkers, Rigs, and Baits
There is a way to know how deep you are trolling; I’ve written and solved the equations that determine trolled depth and have applied the results to most trolling situations. Graphs in this and the following two chapters of this article show your line profile underwater and how deep you are trolling with a lure or bait.
Check out the trolling sinkers depth chart here at the right
Forces that lift an airplane or a kite are the same forces that act in the water. The hydrodynamics of trolling is the same as aerodynamics, except in air, while the other is in water. The density of water is about 1000 times greater than air, so the forces at a given velocity are about 1000 times greater in water.
By analyzing the force on the lure, weight, and line from the movement through the water, plus the force of gravity (the only two forces acting), I found the contour of the line in the water and exactly how deep the lure or bait is trolled. The concept is simple, but the equations are involved, and you need to know something about hydrodynamics to develop the equations. (A description of the analysis is in the appendix.)
Trolling Depth Variables with Speed
The underwater profile of the line as it is trolled through the water is shown for a typical freshwater trolling condition in the below picture (black graph). Using an anchor lock trolling motor will have different results, but if you are using a normal one, the reading is essential.
The trolling velocity is 2 mph using a one-ounce weight on 100 feet of a four-pound-test line.
Note how the line comes up steeply from the weight and flatten out near the surface. An important variable is speed, and the picture (Red Graph) shows the line contour for the same parameters, except the speed has been increased to 4 mph, twice the speed of the profile of Graph 9.1.
Note how the weight and lure are much shallower, only 9.7 feet deep for 4 mph but 28 5 feet deep for 2 mph.
Four factors influence snap weights trolling depth chart:
(1) line diameter-which can also be thought of as line strength:
(2) weight of the sinker or lure:
(3) trolling speed; and
(4) length of the line below the water surface.
Four factors are too many to graph easily. We can plot the depth for one factor, and we can understand two factors by putting several curves on each graph. How do we evaluate so many variables?
Using different graphs for different line strengths and making separate graphs for two-line lengths, we can make a family of graphs that cover any fishing situation.
Velocity and Weight are Most Important
Two of the factors have a relatively small effect. The least important is line strength-at least within the line strengths used for a typical trolling situation. What counts is the line diameter, and typical line diameters (shown in the picture below) were found by measuring the diameter of lines, etc.
The lower picture shows the difference in trolled depth for two-to 20-pound-test line trolled with a two-ounce weight at one and 4 mph. It shows (the right-side graph) similar curves for the 20-to 80-pound-test line.
Velocity and Weight in Different Trolling Depth
Note that these curves are very shallow, meaning depth changes are minor for significant changes in line strength. If, for example, we look at a narrow range of four and six-pound-test line-typical for freshwater, light tackle trolling-the trolled depth is more profound for the lighter line, but not by much.
Trolling with a two-ounce weight at 4 mph, 50 feet of six-pound-test line is trolled at a depth of 11.3 feet Four-pound-test is trolled 12.8 feet deep. That is a difference of 1.5 feet or a little more than 10 percent for a line that is 50 percent stronger.
The point is that whether you are trolling slow or fast, deep or shallow, the differences in trolling depth are negligible over a range of line strengths.
Length of Line
Out The second consideration, the length of line spooled out when the rig is trolled far behind the boat, and the line is lying reasonably flat on the surface, has a negligible effect. For example, with a standard four-pound-test line and a two-ounce weight, 50 feet of line trolled at 3 mph will troll at a depth of 11.3 feet.
Increasing the line out to 100 feet increases the depth to 14.1 feet. Even spooling out 200 feet of the line only increases the depth to 17 feet. Sure, these changes are significant, but if you are in the fish catching zone with 100 feet of line out, you are probably still in the zone with less or more line out.
Of course, if you are trolling with a heavyweight and the line goes almost Straight down, a change in the length of line out makes a significant change in trolling depth. If the line goes into the water at a steep angle, the depth is about the same as the length of the line below the surface.
Little Changes in the Chart: Velocity in Constant Speed and Different Weight
Trolling velocity and the weight of the sinker or lure are the two most important variables in determining trolled depth. If you use the right downrigger reel for salmon and trout for trolling, you can use heavier rods. Better reels will allow you to try different weights.
The picture below relates depth fished to speed through the water trolling with 50 feet of four-pound-test line and a one-ounce weight.
This graph shows that for a 4 mph trolling speed, the trolled depth is 7.8 feet. If the trolling speed is cut in half to 2 mph, the rig’s depth is more than doubled to 19.5 feet.
Small changes in trolling speed make significant changes in trolled depth. The graph is the same as the chart except for curves for two additional weights added. For a one-quarter ounce of weight trolled at 2 mph, the trolled depth is 8.8 feet; for one ounce, it is 21.7 feet, and with four ounces, the depth is 39.9 feet.
As you see, increasing or reducing the weight makes a significant difference in trolled depth. Changing weights is an excellent way to change your trolling depth. Letting out more lines or bringing in line may be used to make fine adjustments.
Trolling Depth Guide on Velocity in Constant Speed and Different Weight
All of these calculations assume that we know our trolling speed. If a current is flowing in a river, we must make our measurements relative to the water.
Speed along a shoreline, even if we can measure it is only part of the answer. Trolling velocity is an essential factor independent of depth, so getting our bait to the proper depth may not necessarily result in fish on the line. However, it is the first step toward improving our chances.
How do you know how fast your boat is moving through the water?
If you have an excellent speedometer that will accurately measure your low trolling speed, you can read your velocity and choose how much weight you will use and how many lines to let out.
On a recent salmon fishing trip, I found that wind can affect trolled depth dramatically. We were trolling for salmon with the right tackle at the minimum throttle setting with about a 15-knot wind out of the south.
We caught chinook salmon on shallow lines with four-ounce weights, but we were only catching fish when heading south against the wind. As the wind pushed us on our northerly route, we didn’t catch any fish.
We knew we were moving through the water faster, and the trolled depth was shallower on the windward leg, but we didn’t know how much weight to add to get our bait to the right depth. Our speedometer said we were going about 2 mph against the wind and 3 mph going with the wind even at the lowest engine speed.
How much difference would that velocity change make to our trolled depth?
A four-ounce weight on 50 feet of 20-pound-test line trolled at 2 mph into the wind would take our rig down 29 feet on the productive leg at 2 mph but only 17 feet on the faster wind-aided return trip at 3 mph-probably enough difference to take us out of the fish zone.
With this graph, we know that we need 10 ounces of weight on the wind-aided leg to get to the 29-foot depth we were trolling using a four-ounce weight against the wind.
This analysis assumes the water velocity is the same from the surface to the depth of the weight or lure. This may not be true for trolling in rivers, and subsurface currents in the ocean or a large lake may also affect the depth. However, most of the time, the depth shown on the graphs will be very close.
Use Low-drag Lures and Rigs for Trolling
These charts apply directly to trolling with a weight, well behind the boat, using a lure or bait with low drag. Many lakes are crammed with small baitfish like minnows, threadfin shad, and silversides, and most game fish go for these at low-drag imitations of them.
Check out few low drag lures:
Small lures like Triple Teazers, Needlefish, Dick Nites, Mepps Spinners, or Panther Martins imitate these baitfish and are trolled at the depth shown in these graphs. Lures like Kastmasters, Rays, and Cripplures are heavy, and the weight of the lure is the weight to use when reading the graphs.
If the lure trolled behind a weight is heavy, it will run deeper than shown, but if it is light and has a lot of drag, it will run shallower. Use a slightly heavier weight or slower trolling speed to get high drag lures to the desired depth.
Flicker Shad Dive Chart
Get in the Zone
Fish are usually in a depth zone. One will be at 15 feet, the next one maybe 18 feet deep, and another one will be 14 feet deep. Not all fish are at the same depth, so your goal is usually to get your lure into the fish zone, not to a precise depth.
With these graphs, if you know your parameters, you can get accurately to your depth. If you don’t know your velocity or other parameters exactly or lure drag makes your lure shallower, you can usually estimate and still get to the fish catching zone.
Trolled depth is one of the most essential factors in catching fish, but most people don’t know how deep they are trolling.
Armed with these graphs, a suitable selection of weights, and some form of trolling speedometer, you can master any trolling situation. Use them to find your trolled depth and see if your catch isn’t vastly improved at the end of the day.