Bob Maindelle Guide Lines March 3

This screenshot taken on a Lowrance Carbon 15-inch sonar unit shows side-by-side down-imaging (left) and colored sonar (right) views of an aggressively feeding school of fish found in the lower third of the water column recently on Belton Lake. Careful observation reveals schooled fish, a recently released fish swimming back toward bottom, air bubbles released by that fish, the angler’s lure returning to bottom and some mild interference from another sonar unit’s transducer pinging nearby.  

In his book “When Pride Still Mattered: A Life of Vince Lombardi,” author David Marannis wrote: “He took nothing for granted. He began a tradition of starting from scratch, assuming that the players were blank slates who carried over no knowledge from the year before. … He began with the most elemental statement of all. “Gentlemen,” he said, holding a pigskin in his right hand, ‘this is a football.’”

In this day and age of scanning sonar, side-imaging sonar, down-imaging sonar, high CHIRP sonar and so on, it is tempting to ignore the fundamentals of sonar, switch on a $4,000 to $5,000 sonar unit, put all of the features in auto-mode, and expect to find fish.

More often than not, that will lead to much disappointment. The fundamentals of sonar must still be understood.

I have provided on-the-water sonar training to several hundred anglers over the past several years and have witnessed a general lack of understanding about the basics which allow sonar to assist anglers, when tuned and used properly.

Although there is quite a list of fundamentals, these four I have focused on are the most essential to grasp.

NO. 1 — Understanding

the shape of sound

Despite the misleading name of “2-D sonar,” traditional colored sonar is actually the closest we have to three-dimensional sonar of all the sonar technologies out there. The roughly 22-degree cone angle of the sound emanating from a colored sonar transducer’s crystal produces a beam of sound which has a height (surface to bottom) a width (left to right) and a depth (front to back).

The reason colored sonar has so many adjustments and is the most difficult to interpret is because these three dimensions are being forced onto a two-dimensional screen (which has a height and a width, but no depth). It is like taking a globe representing the world and attempting to flatten it onto a poster board and making it perfectly fit — that just cannot be done. And so it is with colored sonar — it brings with it much distortion (a negative), but it covers a tremendous amount of water volume (a positive).

Compare this to down-imaging. Down-imaging sound is shaped in a thin, pie-wedge shaped sheet of sound sent down in just two dimensions with a height (top to bottom) and a width (left to right), but essentially zero depth (front to back). The downward angle of down-imaging crystals is roughly 20 degrees, depending on manufacturer. This thin-beam technology allows the targets it senses to be miniaturized onto a sonar screen with little distortion, thus making interpretation more intuitive. When a two-dimensional segment of water is miniaturized onto a two-dimensional screen, little is lost in the translation. Hence, a fish looks like a fish, a sunken car looks like a car, and hydrilla looks like strands of vegetation stretching upward from the bottom toward the surface.

Finally, side-imaging is accomplished by taking the same thin-beam technology used to generate down-imaging, and canting it about 90 degrees left and right so as to look out to the sides of the boat. In actuality, side-imaging beam width is about 78 degrees from directly downward and out to the right, and from directly downward and out to the left, thus creating about 156 degrees of coverage, not the 180 degrees of coverage many imagine they are viewing. There is actually about 12 degrees of cant downward from the surface toward the bottom.

Remember, side- and down-imaging is intended to be used when the boat is moving slowly forward and in a straight line. Using side-and down-imaging from a stopped boat will cause the bottom and all other targets to blur greatly. Using side-imaging while turning sharply will curve and stretch objects being detected such that they are no longer easily recognizable.

I have seen many fairly advanced anglers strap side-imaging transducers to their bow-mounted trolling motors only to be disappointed that the view they get is nothing like that which they get from the console. Had they understood the fundamentals of sonar, this disappointment (and expense and hassle) could have been avoided. Again, the thin beam technology works best when moving straight and slow (under 4 mph).

No. 2 — Understanding

sonar’s limits

At the risk of getting a bit too “mathy,” follow me on this example. With a bit of geometry, we know that a 22-degree cone angle on a colored sonar transducer will see a 1-foot diameter circle of bottom in 3 feet of water. Hence, since the transducer’s angle is fixed at 22 degrees, in 30 feet, that unit will “see” a 10-foot diameter patch of bottom; and, in 60 feet of water, that unit will “see” a 20-foot diameter patch of bottom.

So, if I am holding perfectly still and want to see my slab or drop-shot rig on my bow-mounted color sonar unit 100 percent of the time while in 30 feet of water, I need to keep my bait inside that 10-foot diameter circle.

Failing to do so, regardless of sonar brand, price, amount of fine-tuning, setting selections, etc., will guarantee the bait will not show up. This is why understanding how the sound is shaped is so important. It equips the angler to use the sonar within its physical limitations.

Similarly, due to the thin-beam technology of down-imaging, when the down-imaging and colored sonar are played side-by-side, down-imaging will typically show fewer targets with better resolution, whereas colored sonar will show more targets but with poorer resolution. This is because that thin-beam technology is covering only a fraction of the area covered by the colored sonar.

Likewise, due to the 12-degree angle of depression manufactured into side-imaging transducer crystals, there is a limit as to how far out away from the boat side-imaging can reach. It does not matter how far out an angler sets the range on side imaging if the sound intersects with bottom, for example, 140 feet to the left and right of the boat. All else beyond that distance will appear dark and uninterpretable.

Understanding sonar’s limitations, and then realistically working within those limitations, will make you a better, less frustrated, angler.

NO. 3 — Understanding


Sound is a vibration. When a vibration occurs repeatedly over a span of time, that is defined as frequency. Transducers emit sound at a given frequency. Typically, colored sonar emits sound at around 200 kilohertz (kHz), and down- and side-imaging transducers emit sound at around 455 kHz, 800 kHz or 1,200 kHz (also known as 1.2 megahertz, hence the nickname “Mega” used by Humminbird).

A transducer will detect the sound frequency it is designed to detect, no matter what is producing that sound, including another transducer.

Physics aside, if you have multiple transducers sending out sound using the same frequency, “cross-talk” occurs, thus producing interference which manifests itself as undesirable vertical and/or diagonal lines on your sonar screen.

When an angler moves from console to bow or from bow to console, simply placing the unit which will no longer be observed into “standby” mode will eliminate this problem simply and easily. Standby mode simply halts the emission of vibrations from the transducer until the sonar unit is taken back out of standby mode.

No. 4 — Transducer alignment

When anglers hire me to come aboard their boats to train them on their sonar units, tweak the variables to give the best readings, etc. the first thing we do is to inspect and level their transducers. I “preach” about this at every seminar I give, as well.

If a transducer is not parallel to the bottom of the lake as the boat moves slowly forward (in gear), the picture generated on the screen will not be as good as it can be. Particularly impacted will be the down-imaging view.

Although the procedure I used is a bit more complicated, a “quick check” on this simply involves comparing the angle of your transducer to the scum line that forms on the side of your boat. If these two are at the same angle, you are close, if not right on, with your transducer’s degree of levelness. If your transducer is angled forward or rearward as compared to the scum line, you are in need of an adjustment.

The list of fundamentals goes on, and the list of advanced features and settings is even greater. Suffice it to say that a basic unit, if well-adjusted and understood, will outperform a unit costing 10 times more if that expensive unit is not set up properly and understood by its user.

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