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May 12, 2011

Interpreting Your Sonar

Learning to properly use and understand sonar takes time on the water.

wide and narrow beam

Upping the Ante
Technology does exist now that can improve an angler's fish-finding ability, but much of it costs tens of thousands of dollars ($12,000 to more than $65,000) - a cost barrier only serious tournament anglers and commercial fishermen cross. For instance, Furuno builds searchlight and omni sonar systems that use a deployable transducer that extends and retracts from the hull of the boat.

"It drops out from the bottom of the boat and spins. You can see 360 degrees or as little as 6 degrees," says Steve Bradburn, assistant product manager for Furuno USA. "It's like an underwater radar."

Furuno also builds "slewable" sounders that anglers can tune, dialing in the frequency they want to use. Bradburn says the company expects to release a CHIRP-type sounder soon.

Raymarine also has a CHIRP machine on its drawing board. Currently, its high-end DSM 400 black-box unit ($2,795) supports two transducers across four frequencies - 28/38 kHz and 50/200 kHz - all capable of operating simultaneously. "It marks depths in excess of 10,000 feet; it's pretty close to CHIRP," McGowan says.

Sonar Symphony
The current dazzle, however, comes from Simrad's BSM-2 black-box sounder ($2,495) and Garmin's GSD26 module ($1,999.99), expected in August. Each features CHIRP technology, though Garmin uses the phrase "Spread Spectrum." CHIRP units send pulses containing a whole band of frequencies instead of just one frequency. They literally "chirp" instead of "ping," and consequently a lot of information comes back.

As a complement to those units, Airmar is introducing a full line of broadband transducers with CHIRP abilities; one includes a medium-frequency ceramic - 85 kHz to 135 kHz - commonly used by commercial fishermen, who claim that band captures some species other frequencies don't. The two transducers available at press time included the R209LH/R209LM ($2,995) and B265LH/B265LM ($1,795) through-hull units. Airmar will launch other transducers in different mounting styles this spring and summer.

"The big thing I think this [technology] will do is greatly improve the clarity between the bottom and structure, and the separation of that structure and fish around it," says Garmin's DeVries. "Spread Spectrum allows the transducer to spread frequencies over a single chirp. Spread Spectrum will go a long way for sport fishermen toward getting deeper water penetration with less power, better target separation and superb bottom detail."

The GSD26 sends a pulse over any frequency range available from the transducer. Both the GSD26 and the BSM-2 feature two separate transceivers; one can work with a fixed frequency while the other uses CHIRP/Spread Spectrum.

Simrad's BSM-2 uses low (25 kHz to 45 kHz), medium (40 kHz to 60 kHz) and high (130 kHz to 200 kHz) bands of frequencies, based on the transducer setup. "Traditional sonar sends out a ping that's fairly long," says Lucas Steward, product manager for Navico (the parent company of Simrad). "If there are multiple fish together, all you see is one return. Multiple frequencies separate that out."

This frequency sweep should also show species that might not appear or might appear faintly in a 50 kHz or 200 kHz beam. And while the technology won't resolve the issue of where in the beam the fish resides, it does add detail and provide bottom clarity that anglers can greatly use.

Slice of Life
One tool that might help better pinpoint fish is side-imaging sonar, a technology now used by Johnson Outdoors (for Humminbird and Geonav) and Navico (for Lowrance and Simrad). In shallow water, in particular, even a wide sonar beam has a fairly small bottom-coverage footprint. Comparing what's out to each side of the boat - as much as 240 feet left and 240 feet right - with what traditional 2-D sonar shows helps anglers locate structure and fish.

One caveat about using side-imaging sonar: Its fore- and aft-beam width is very narrow, about 1.5 degrees. Picture a set of fans emanating from the transducer, aimed toward either side of the boat. One side of each fan touches the bottom and the other might be just below the waves.

If your boat is under way, a fish that swims through the beam fairly quickly might not even register a blip. However, structure such as rock piles or wrecks show up in splendid, photolike detail. Slow down and watch for streaks on the screen to better identify fish.

Testing, Testing
Regardless of what sounder technology you embrace, learning to properly use it and understand what you're seeing takes time on the water. One particular tip that can help: Motor repeatedly over a piece of structure such as a wreck; if it's a wreck you might have dived or snorkeled, so much the better. Approach it from a variety of angles and watch how the sonar returns paint the picture.

Pay attention to the fish marks you see compared with the fish you catch. Patterns should emerge.

Know the beam width of your transducer's pulses so you can determine how much of the water column and the bottom you might be seeing. (See table) Having all the puzzle pieces together can make every day on the water more productive and rewarding. Plus, imagine how you'll amaze your friends.


Manufacturer Links:

Sound Producers
Airmar (transducers)
Milford, New Hampshire

Camas, Washington

Olathe, Kansas

Geonav (Johnson Outdoors)
Racine, Wisconsin

Humminbird (Johnson Outdoors)
Eufala, Alabama

Lowrance (Navico)
Tulsa, Oklahoma

Merrimack, New Hampshire

Simrad (Navico)
Nashua, New Hampshire