By Frank Patton
Lowrance Pro Staff

Introduction
How much and what does my fish finder show beneath the boat? What is the diameter of the coverage area of its cone angle at various depths? Am I looking through a soda straw or a megaphone for fish? Are the sonar pings or beams going out like a rifle shoot or a shotgun blast? These are a few questions frequently asked at boat shows and sonar seminars.

The standard answer is often something like: A) one-third the depth of the water for a twenty degree cone angle transducer, which is the most common transducer angle used in freshwater. That is, in thirty feet of water the diameter of the coverage cone would be about ten feet. B) Similarly, for an eight or nine degree transducer the rule of thumb might be one-seventh the depth of water. I too have wondered about the effective coverage area that I am seeing on my sonar unit while searching for fish. I think knowing the coverage area is helpful when searching for both game fish and schools of bait fish. There is more to the story of the sonar’s cone coverage but let us keep it simple for now in this first installment of two articles. I also recommend that the reader view the sonar tutorial at the www.lowrance.com web site as well as the GPS tutorial.

Background
Recently, Bill Vines, a friend and neighbor of mine who is a freshwater striper guide bought a new high powered Lowrance fish finder/GPS unit complete with a removable memory card and twenty degree (rated at the one-half power point) transducer for his work at the office. His office is behind the console of his boat. I showed him how to record with the new sonar log chart feature with which I was starting to use. This sonar log feature allows the sonar unit to capture data or recordings of the ftrimages seen on the unit’s screen and replayed later. Thus, this reviewing feature recaptures one of the features of the old paper graphs (i.e., a history record). Recordings might be replayed on the sonar unit at home or by using the Sonar Viewer software on a personal computer from the removable MMC (MultiMedia Card) or SD (Secure Data) flash memory card. The Sonar Viewer is a free computer program downloaded from the Lowrance web site. Another way is to download one of the sonar unit emulators and replay the logs.

A few weeks later Bill called me over to his house after a guide trip to show me his recordings of a school of stripers (Morone Saxatilis) on Lewis Smith Lake. Lewis Smith Lake is located in north central Alabama and is a deep clear lake stocked with Gulf strain striped bass. The Gulf strained stripers, as opposed to the Atlantic strain, are believed to be a bit more tolerant of the higher temperatures found in the Alabama’s southern waters. Because stripers typically prefer open water and move around a lot, they usually do not stay in one area long enough for sonar studies. Studies by State of Alabama researchers such as Jerry Moss show that stripers might move around a quarter of a mile or more a day and then suddenly leave for waters unknown.

When we played back his sonar log I was amazed at the number of fish recorded in a school. On this trip he caught stripers from ten to seventeen pounds. I copied his log onto one of my MMC memory cards for review on my personal computer. When I downloaded the log onto my computer and subsequently replayed the log, I realized here was a way to investigate the actual cone angle coverage for actual fish.

Using the pop-up feature of the Sonar Viewer program I could easily establish the distance or range from the top of the arch (i.e., the depth of the fish) to the beginning and ending of the characteristic arch signal (when the fish first entered and left the coverage cone). Thus, by using a little trigonometry and choosing a representative sample, I had the ability to approximate the actual degree of cone coverage of a known size and species of fish.

Discussion of Results
An examination of my six fish sample produced an average cone coverage angle of 58.6 degrees for these water conditions. Also, the angle is generally greater the shallower the fish are. This agrees with the fact that the signal strength has dissipated for these deeper fish. There was one exception in my six fish sample. Perhaps, that fish was larger and produced a stronger return signal relative to its schoolmates. Over three days Bill's clients caught fish that ranged from ten to twenty-one pounds. I even managed to catch a twelve and eighteen myself. Thus, I believe the school consisted of fish that averaged from twelve to fifteen pounds. Then, like silver ghosts, the school of stripers was gone. We will discuss sample selection and methodology used in the second part of the article.

The Importance of Using this Knowledge
These findings are important for both helping to visualize what the sonar unit displays below the water’s surface and to minimize search time. For example, when searching for fish I can also use the unit’s GPS feature to draw a trail as an aid in methodically running a search pattern such as S’ing, zigzagging, or spiraling and thus not overlapping or missing sonar coverage. Also, the 58.6 degrees average agrees closely with Lowrance’s literature that states up to 60 degrees of coverage for the twenty degree, rated at the one-half power, point skimmer transducer. This was certainly true for the large stripers of Smith Lake. Remember that when all things are equal, large fish make large arches when centered in your unit’s cone of coverage with high powered sonar. Smaller fish, such as black bass, will not give a strong a return; thus, the actual cone coverage angle will be less.

By using our sixty degree “megaphone” in searching for fish we can imagine that the diameter of the coverage cone will be about the same as the depth of water. That is, if we are searching in 40 feet of water we can expect to pick up large fish, with a correspondingly large swim bladder, that give a strong sonar return signal approximately 20 feet on either side of the boat (i.e. 40 feet across). On a typical fishing boat, we can expect a powerful sonar unit to display large fish about one boat length on either side of the boat as at a depth of forty feet. That is, a circle with a diameter of 40 feet. Likewise, at 20 feet the coverage diameter will be 20 feet or so. Therefore, this knowledge should help in running a search pattern methodically when look for fish.

Closing
In the second part of this article next time we will go into some of the trigonometry of the calculations. Also, we will explain some of the settings used on the sonar unit for its best utilization. These features include the use of Ping Speed, Colorline, Gray Scale, Sensitivity, Upper and Lower limit, and other.

Finally, thanks to Captain Bill Vines (www.stripefishingheadquarters.com) who recently joined Larry Salter and me on the local Lowrance Pro Staff team. After all, Bill did the hard part – he found the fish. Thanks also to researcher Jerry Moss for his kind patience while I quizzed him for hours on the movements these silver torpedoes.

Additionally, I recommend you drop by and talk to Michael Belcher the marine electronics specialist at Mark’s Outdoors concerning all your sonar and GPS needs.