On the water
Downrigger true depth and blowback calculator
Quick answer
At 30 degrees of blowback with 100 feet of cable out, the ball is at 87 feet, not 100. True depth is cable out multiplied by the cosine of the blowback angle, which is published trigonometry, and the angle is read straight off the rigger with the boat under way.
A downrigger counter measures cable paid out, and cable does not hang straight down behind a moving boat. It trails aft at an angle called blowback, and the ball is shallower than the counter says by an amount that grows with speed and shrinks with weight.
Every input here has a definite source, which is unusual on this site. The trigonometry is published, the cable figure is on the counter, and the angle is read off the rigger itself with the boat under way at fishing speed. Nothing on this page is an estimate.
Run your own numbers
At 30 degrees with 100 feet out, the ball is at 86.6 feet. You are fishing 13.4 feet shallower than the counter says.
| Figure | Value |
|---|---|
| Counter reading | 100 ft |
| True depth of the ball | 86.6 ft |
| Depth lost to blowback | 13.4 ft |
| Percent lost | 13% |
| Cable needed for 80 ft | 92.4 ft |
| If blowback were 15 degrees | 96.6 ft |
Shown with the example figures above. Change any field to run your own numbers.
The working
Published figuretrue depth = cable out x cos( blowback angle ) 100 ft at 30 degrees = 86.6 ft to REACH a target depth: cable needed = target depth / cos( blowback angle )
Pure trigonometry. The blowback angle is read off your own rigger at fishing speed rather than estimated. A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
Riggers, weights and cable
A pancake weight tracks with far less blowback than a round ball at the same weight, which is free depth for no extra lead. Thinner cable helps on a long drop, where the cable contributes more blowback than the ball.
Cannon Optimum electric downrigger
$1,167.87Networked, programmable and able to run a search pattern on its own, which is the ceiling of this category.
Check price on Amazon
Cannon Magnum 10 TS tournament style electric downrigger
$711.77Adds bottom tracking and positive ion control to the Magnum 10, which is the tournament spec.
Check price on Amazon
Cannon Magnum 10 electric downrigger
$536.93The Great Lakes standard: a longer boom, a faster retrieve and the capacity for the heavy balls deep water needs.
Check price on Amazon
Cannon Magnum 5 electric downrigger
$460.91The cheapest genuine electric downrigger here, and the point at which running two or four riggers stops being physical work.
Best for: The first electric rigger on a boat that trolls seriously
Check price on Amazon
Scotty Strongarm 30 inch manual downrigger
$311.06Thirty inches of boom on a heavier frame, for bigger weights and rougher water than the Depthmaster was built for.
Check price on Amazon
Cannon Uni-Troll 5 manual downrigger
$288.64A dual axis rod holder and a line counter on a manual rigger, which is most of an electric downrigger without the electrics.
Check price on AmazonEvery figure shown is the manufacturer own published specification. The rating on the unit you receive is what governs, and the capacity plate on your own boat governs everything above it.
What blowback costs you
At 30 degrees you lose 13 percent of your indicated depth, and at 45 degrees you lose nearly 30 percent.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Blowback angle | Depth factor | True depth with 100 ft out | Depth lost |
|---|---|---|---|
| 0 degrees | 1.0 | 100.0 ft | 0.0 ft |
| 10 degrees | 1.0 | 98.5 ft | 1.5 ft |
| 15 degrees | 1.0 | 96.6 ft | 3.4 ft |
| 20 degrees | 0.9 | 94.0 ft | 6.0 ft |
| 25 degrees | 0.9 | 90.6 ft | 9.4 ft |
| 30 degrees | 0.9 | 86.6 ft | 13.4 ft |
| 35 degrees | 0.8 | 81.9 ft | 18.1 ft |
| 40 degrees | 0.8 | 76.6 ft | 23.4 ft |
| 45 degrees | 0.7 | 70.7 ft | 29.3 ft |
| 50 degrees | 0.6 | 64.3 ft | 35.7 ft |
| 55 degrees | 0.6 | 57.4 ft | 42.6 ft |
| 60 degrees | 0.5 | 50.0 ft | 50.0 ft |
Pure trigonometry. The only input that is not published is your own blowback angle, which you read off the rigger at fishing speed rather than estimating.
How to read the angle
- Get the boat to fishing speed. Blowback depends entirely on speed through the water. An angle read at the dock or at idle is useless, and the angle at 2.5 mph is very different from the angle at 1.5.
- Let the ball settle. Pay out the cable and wait for the angle to stabilise. It swings while the ball is still dropping and settles once it is at depth and tracking.
- Read it against the vertical. Most riggers have a printed protractor on the boom for exactly this. If yours does not, sight the cable against the transom, which is vertical enough for this purpose.
- Reduce it with weight, not with speed. A heavier ball trails at a shallower angle at the same speed. A pancake or fin shaped weight tracks better than a round ball of the same weight, which is free depth for no extra lead.
- Re-read it when anything changes. Speed, current, ball weight and how much cable is out all move the angle. A figure taken once at the start of the day is wrong by the afternoon if the wind has come up.
Getting the ball deeper without more lead
Three things reduce blowback at the same speed, and only one of them is buying more weight.
- Weight shape. A pancake or fin shaped weight tracks with far less blowback than a round ball of the same weight, because it presents less frontal area and stabilises rather than tumbling.
- Cable diameter. Thinner cable has less drag along its whole length, and on a long drop the cable itself contributes most of the blowback rather than the ball.
- Speed. The unwelcome answer, since the speed is usually chosen by the fish rather than by you.
Frequently asked questions
How deep is my downrigger ball really?
Cable out multiplied by the cosine of the blowback angle. At 30 degrees with 100 feet of cable out the ball is at 87 feet, not 100. The counter measures cable paid out, and cable does not hang straight down behind a moving boat. The angle is read off the rigger at fishing speed.
What is blowback?
The angle the downrigger cable trails behind vertical as the boat moves. It is caused by drag on the ball and, on a long drop, by drag on the cable itself. It grows with speed and with cable out, and it shrinks with ball weight, which is why deep trolling uses heavy balls.
How do I measure the blowback angle?
At fishing speed, with the ball settled. Most riggers have a printed protractor on the boom for exactly this. If yours does not, sight the cable against the transom, which is vertical enough. An angle read at the dock or at idle is useless, because blowback depends entirely on speed through the water.
How do I get the ball deeper without more weight?
Change the shape of the weight and the diameter of the cable. A pancake or fin shaped weight tracks with far less blowback than a round ball of the same weight, and thinner cable has less drag along its whole length. On a long drop the cable contributes more blowback than the ball does.
How much weight do I need?
Ten pounds is the standard inland ball and the weight most blowback discussion is worked around. Deep water and faster trolling speeds push people to 12 or 15 pounds. The honest test is the angle: if you are consistently over about 35 degrees at your normal speed, more weight or a better shaped weight will buy you real depth.
Does the fish depth on my sonar account for this?
Your sonar reads depth under the transducer correctly, so the fish depth on screen is real. What is not real is the assumption that the ball is at the counter reading. That mismatch is why people mark fish at 60 feet, set the counter to 60, and catch nothing until they work out the ball is at 52.
Get the complete rigging shopping list
Every item in the beginner, intermediate and expert rigs for both a boat and a kayak, with running totals, in one printable list. No charge.
One list, no sequence of daily emails, and you can leave at any time.
Researched, not professional advice. This page is compiled from published engineering and regulatory figures, manufacturer specifications and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are boating convention rather than published standards, and they are labelled that way wherever they appear. Marine electrical work is not house wiring. Use tinned, finely stranded marine cable, size it for voltage drop and for the ABYC E-11 ampacity table rather than one or the other, and put overcurrent protection within seven inches of the battery positive terminal, because everything between the post and the fuse is unprotected. A lithium battery needs a charger with a lithium profile. Never load a boat past its capacity plate, and remember that canoes and kayaks carry no federal capacity plate at all, so their stated capacity is the manufacturer own figure. Wear the life jacket, and treat early season water as the hazard it is: cold water immersion takes your breath and then your hands long before it takes your core.