September 23, 2021

BOB MAINDELLE: Simple sonar solutions | Outdoor Sports

As I surveyed the weather forecast for this past week, I could not get excited…

As I surveyed the weather forecast for this past week, I could not get excited about what I saw.

Monday’s winds were due to be from the north, thanks to the hurricane which hit Louisiana, then winds were due to be calm to very light on Tuesday through Thursday.

Seeing this developing, I scrambled to reschedule fishing trips and, instead, conduct sonar training sessions which are not so weather-dependent.

I wound up conducting on-the-water sonar training Monday through Thursday with anglers with an array of equipment and prior experience. Each student had different end goals in mind, from competitive bass tournament fishing to fishing at a leisurely pace for whatever bit.

I noted that despite their differences, there were some commonalities in the needs these sonar owners had when it came to their training. Although the list is lengthier than the handful of issues I share in this column, what I share is nearly universal amongst those I have trained in the hundreds of sessions I have conducted.

Leveling the transducer

Failing to level a sonar system’s transducer is like building a house on sand. It is a fundamental flaw which will negatively impact everything else the system is supposed to do.

By using a short carpenter’s level, a few coins as shims, and investing all of 25 minutes with your boat on the water, a transducer can easily be leveled.

I spelled out, step-by-step, just how to do this in my May 3, 2020, column. Go to kdhnews.com and use the search tool with the phrase “Maindelle transducer.” The article entitled “Leveling the transducer is a sonar fundamental” will pop up and provide all the details.

A leveled transducer directs sound directly downward despite the boat’s nose-up attitude as it is operated at a slow forward speed. Sound directed straight down is more likely to bounce (or echo) straight back to the transducer, thus necessitating the use of less sensitivity and reducing unwanted interference for a clearer overall image.

Understanding sonar beam shape and coverage

Most of my students significantly overestimate the coverage of the bottom provided by their sonar unit.

As a rule of thumb, your traditional sonar beam (also referred to as 2D sonar and/or colored sonar) operates by shooting a 22-degree, three-dimensional cone of sound downward.

Doing the math on a pair of 11-degree right triangles, one can calculate the diameter of this cone of sound.

The result is a fixed, 3-to-1 ratio of depth-to-cone diameter. Thus, in 30 feet of water, your traditional sonar beam will see a circular patch of bottom roughly 10 feet in diameter. In 60 feet of water, this coverage expands to a 20-foot diameter circle, and so on.

Your down-imaging beam is also roughly a 22-degree beam, however, it exists in only a single plane, like a wall of sound or thin sheet of sound, going directly downwards and 11 degrees to the port side and 11 degrees to the starboard side, parallel to your transom (for a transom-mounted transducer).

As with traditional sonar, this beam also has a 3:1 ratio, but the ratio is depth to beam width, not depth to cone diameter. So, in 30 feet of water, your down-imaging beam will “see” a rectangular patch of bottom which is 10-foot wide, with a thin front-to-back dimension of roughly three-fourths of the length of your transducer (about 5 inches or so).

Finally, your side-imaging consists of two beams; both beams are roughly single-plane, 78-degree beams with one beam oriented directly downward and then out to the left, and another beam oriented directly downward and then out to the right. As with down-imaging, these thin beams exist in a single plane and are oriented parallel to your transom (for a transom-mounted transducer).

If these beams were 90-degrees, they would shoot just under the waves on the surface, but this is not the case. These beams are 12 degrees shy of 90 degrees, and are actually canted 12 degrees downward from the surface.

This 12-degree angle of depression produces a sonar blind spot. This is why a sonar user can pass by an object, like a tree or bridge piling, and only see that portion of the object closest to the bottom, and not see that portion up near the water’s surface.

Although bottom slope impacts side-imaging greatly, a good rule of thumb to estimate the maximum reach of side-imaging is to simply multiply the depth by a factor of 3 or 4. Hence, in 30 feet of water, the maximum reach of your side imaging, due to this 12-degree angle of depression, will be about 90 to 120 feet to the right, and another 90 to 120 feet to the left.

Setting side-imaging sensitivity too high

The most sound-reflecting portion of a fish’s body is the gas-filled sack in the fish’s abdomen known as the swim bladder. This gas reflects sonar sound back to the transducer much better than the fish’s flesh and bone (which consist primarily of water).

With sensitivity properly set on side-imaging, fish will consistently appear as bright, rice-grain shaped objects on the left and right fields of the sonar screen.

Most students arrive for training with side-imaging sensitivity set way too high.

In my opinion, Humminbird has the best side-imaging adjustments built in, followed by Garmin, followed by Lowrance.

On Humminbird units, one of the two amber colored palettes tend to perform best, whereas the blue palettes tend to perform best on Garmin and Lowrance units.

On a Humminbird unit, for example, with the contrast set at a “middle of the road” setting of 10 on a scale of 1 to 20, the sensitivity is the only variable which then needs to be manipulated to get an ideal color tone for identifying fish.

Although a picture is worth a thousand words here, I coach my students to reduce the sensitivity until a “toasted marshmallow yellow” color is witnessed on the screen. This is brighter than the black/brown tone which results from a sensitivity setting which is too low; and, this is darker than a yellow/white/gold tone which results from a sensitivity setting which is too high.

Lowrance and Garmin units have similar “sweet spots” when it comes to screen tone.

These few things are but a sampling of the breadth of items covered during on-the-water sonar training, but are perhaps the most critical to address before a sonar user can progress any further in his or her understanding of the technology.

If you feel sonar training may benefit you, feel free to contact me by phone or text at 254-368-7411.