Depthfinder? Fishfinder? Sonar?

by Blair Battye

Reprinted from "Just Fishing with 'Big' Jim McLaughlin" by permission of the editor.

Depthfinder, fishfinder, flasher, LCR, sonar, bottom-finder, sonar graph, .... plus at least a half-dozen other names I've heard them called. All these units have something in common - the ability to determine the a amount of water (depth) between you and the the lake bottom. Most have the added ability to distinguish underwater objects such as stumps, rocks and even fish! Sounds like just the tool every fisherman should own. As with any other tool, it is important to understand how a depthfinder works and how to use it properly in order to achieve the best results.

Over the years I have had the opportunity to fish with depthfinders from many different companies and have tried models of all shapes, sizes and design. In this article, my goal is to unravel the magic of sonar and to present the basic details without getting bogged down in the technical jargon.

How does it work?

Without going in to great detail the basic principle of "sonar" is as follows. A short burst of energy, called a pulse, is produced by the sonar unit and sent into the water by the transducer. The nature of this pulse is such that it bounces back from things it encounters underwater (rock, wood, fish etc.). The transducer picks up these tiny echoes and sends them to the sonar unit for display. The speed at which this pulse travels is known and therefore by measuring the time it takes for the echo to return, the sonar unit can determine the distance to underwater objects or the lake bottom.

The deeper the water you wish to measure, the more power or energy must be put into the pulse sent out the transducer. Remember that the more power you send down, the stronger those small return echoes will be and the easier the unit will by able to determine underwater information. Also keep in mind that soft muddy bottoms can absorb much of the energy and reflect very lityle back. The real magic takes place when these weak echo signals are received and converted into something you and I can see on the screen. The echo information can be processed by a digital computer within the unit or it may be simply displayed as "raw" echo information. It is the type of display and "signal processing" that distinguishes different types of sonar units such as the LCR, flasher or paper graph.

What's the difference?

Let's go back to the days prior to the arrival of the "digital" sonars, the time before words like liquid-crystal recorder (LCR) and liquid-crystal display (LCD) were buzzing about the tackle retailers. The two most common technologies in use were the paper graph (or chart recorder) and the flasher.

Flashers

Flasher type sonars use a tiny neon bulb mounted to a rapidly spinning disc. This assembly spins behind a clear faceplate that has depth scale markings typically from 0' to 30' . The 12 o'clock position would represent 0 feet and the dial is divided in equal increments around the circle clockwise back to the 12 o'clock position. The disc is driven by a small motor and each time the bulb passes the 0 foot point it lights up momentarily and at the same time a pulse of energy is sent out the transducer. In the time the pulse has travelled to the bottom and back, the disc has rotated a certain amount and the returning pulse energizes the neon bulb so for a brief moment. The depth is determined by reading the scale marking at the point where the bulb flashes. In shallow water the pulse returns sooner and the disc/with bulb will have rotated only a fraction of the circle and hence will indicate a shallow depth. In deeper water the Pulse will take longer to return and the disc will have had time to rotate further around the circular scale before the returning pulse lights up the bulb.

Paper Graphs

A paper graph has some similarities in that it sends pulses to transducer and then "listens" to the echoes, but the big difference is in how the echo information is displayed. The paper graph design has a wide spool of paper that un-rolls and passes behind a glass viewing screen and is then taken up on another spool. As the paper moves slowly along, an image representing the "echoes" from underwater structure and objects are drawn on the paper. A pen or stylus "writes" an image based upon even the slightest variation in the strength or delay of the echo. What results is a permanent record of the underwater situation presented in very high detail (resolution).

Paper graphs typically have a very high power capability compared to other units, and because of this, they are rarely usable in water less than 5 feet deep. This high power level is great for finding fish in deep water but in shallow water, it may overload the water with energy and cause multiple echoes that make it difficult for the unit to determine the real depth.

I often use the analogy of someone screaming in your ear and then immediately whispering something to you. It takes time for your ear to read just to be able to hear the faint whisper. It takes time for a sonar to be able to detect an echo after it has just delivered a blast out the transducer.

Digital Sonars, Computer Sonars, LCRs

Early digital sonars, or LCR's as they were called, were the hot ticket for every fisherman to have. Unfortunately, their performance level was nowhere near the level of demand for these novel new items. An all out war raged for several years as manufacturers competed against each other on issues such as power, number of pixels, screen size, depth range etc.... Many a fisherman was led astray as he desperately tried to catch one of those little black dots that would appear on screen. Modern era units (the last 2 or 3 years) have improved tremendously as manufacturers have developed smarter, faster computers for use in digital sonars.

Despite all the bells and whistles such as speed/distance/temperature capability, the basic principle of operation remains the same -- send out a pulse and wait for the echo to return. The big difference between available models is the sophistication of their "digital signal processing" also known as DSP. This signal processing (not used in most flashers or paper graphs) filters out noise, interference and false echoes to create a display that more closely represents the underwater situation.

Which one should I use?

You probably have noticed that the more you fish, the more you crave new fishing equipment. A new rod that is lighter or more sensitive, a reel with more ball bearings, a bigger boat, more tackle boxes and so on........

If you are the occasional fisherman who fishes for a variety of species using both trolling and casting techniques, then a unit such as the Lowrance X-25 has the kind of features that will serve you well. Speed and temperature capability complement the other excellent features in this well performing, easy-to-use digital sonar. If you are a little more dedicated to the sport and fish once a week or more often, you may want to consider a more refined unit such as the Lowrance X-SSA for your boat.

If you fish competitively, or specialize in a certain species such as walleye or bass or lake trout then the answer to the "Which unit?" question becomes more complex. Big water, open water fishermen going after lake trout, salmon or walleye using downriggers and other trolling techniques use more paper graph units then any other type. Incredible resolution, depth probing power and ease of use more than make up for the inconvenience of having to change the paper roll. This combination of features has made paper graphs a favorite choice among charter captains and salvage crews everywhere. Unfortunately, Lowrance no longer produces these units, so if you want one, your only hope is to find a used one in the classified ads (Check out the TFN Buy, Sell & Trade conference in the TFN Forums! - ed.) or at a garage sale. The Lowrance X-70A or LMS350 represent the true state of the art in digital sonars and are your best choice for a paper graph substitute for this type of fishing.

If your fishing style sometimes finds you vertical jigging for walleye in 30-40 feet or going after some other species where pinpoint accuracy in presentation and boat control is critical, then you would probably enjoy an additional sonar unit for the bow of your boat. The type of sonar one should use in this situation has been a hot topic for debate around many tackle stores. Flashers, which manufacturers keep threatening to discontinue, are my number one choice when fishing jigs or plastic worms from the bow of my boat.

Flashers give you instantaneous feedback about what is going on underwater directly beneath the bow of your boat (assuming a trolling motor mounted transducer). With a conventional liquid-crystal display unit you need to wait as the unit creates a picture and then your eyes and your mind must interpret the display. A flasher presents you with a continuous stream of "raw" data and your mind does the interpretion. With a few hours of expexperience, you an distinguish between rock and muddy bottoms, between thick and sparce weeds, and between large and small fish. Digital sonars, of course, can interpret this for you, but nowhere near as fast or as accurate as the human mind.

Consider this: You are slowly moving your boat along, hoping to position yourself right above a small rock pile in 35 feet of water, the digital LCR unit's display starts to change, after several seconds you see it is starting to draw a hump which you interpret to be the rock pile and you take your foot off the trolling motor. Too late! you may now be 10-20 feet past the rock pile and must turn the boat around and try again. Add a 25 km/h wind and some waves and your task just became a lot harder. In this situation I prefer to use my flasher as I can stop the trolling motor the moment I see a depth change on the flasher and the instant the wind starts to blow me off the rock pile I can activate the trolling motor to hold my position. I apply the same technique when following a meandering deep weedline on a windy day. The flasher gives instant feedback so I can concentrate on my lure presentation instead of struggling with boat positioning and the reading of the sonar display.

Summary

As a consumer, the best advice I could give to you is to match the features and capabilities of a unit with your particular style of fishing. This may ultimately lead to you having more than one model of sonar for your boat should you decide to truly fine tune your techniques.

If your budget only allows for one sonar unit then it will probably be a digital LCR type. A unit with speed and temperature capability is nice to have but you are hopefully going to spend more time fishing so don't let the speed/temperature issue become the deciding factor. Try the unit out on a simulator - is it easy to use for you? Have a look at Zercom and their product line. These units offer the best of both worlds in my opinion, the ruggedness and reliability of a digital LCR with the instant feedback of a flasher type display.

Please send your comments on this or any other article in the TFN Web-zine to the Editor at editor@the-fishing-network.com