Searchlight Sonar
Picture pointing a searchlight up to the sky, then aiming it below the boat, and you have the path searchlight sonar follows — a giant circle. This very pricey sonar has been most popular with commercial fishermen, though recently Si-tex introduced a model — the ESR-140 — aimed somewhat at the recreational angler; list price comes in around the $8,000 mark. The transducer assembly that hangs down dramatically below the hull recedes into its tube when you reach a preset speed.

Furuno also makes several searchlight units including a dual beam. Expect to pay between $13,000 and $18,000 or so for these excellent systems. Certainly, whether that qualifies as "aimed at the recreational market" depends on the size of your recreational purse. The transducer assembly on these models also recedes into the hull, some automatically at a    preset speed. Additionally, Furuno makes all its searchlight models available as black-box units, which brings the price down somewhat.

Both Furuno and Si-tex have some form of auto-stabilization to maintain the transducer in the correct orientation relative to a boat's pitch and roll.

Why searchlight sonar (if you can afford it)? This technology comes as close to the glass-bottom bucket as a sonar possibly can. It shows you what's around, ahead of, to the sides of and straight down under your boat. If there are fish near your boat, searchlight sonar will pick them up — thus its great popularity among commercial fishermen.

Side-scan Sonar
And then there is side scan, which, as the name applies, scans out to the side of your boat. Until recently, side-scan sonar has been available only as a very expensive towed array — i.e., a torpedo-shaped object containing the transducers that tows behind a craft.

(An aside: There's a fairly active do-it-yourself community working on homemade side-scan units, though usually the towed kind; check www.thunting.com/geotech/forums/archive/index.php/f-10-p-3.html; www.portup.com/~dfount/sidescan.htm; and www.marine-group.com/sonarprimer/sidescansonar.htm# applications).

As the name implies, side scan sees out to the sides of the boat in a narrow beam. Side scan requires that you also install down-looking sonar for fishing purposes.

Humminbird produces the most economical and, for that matter, the only side-scan unit currently available for the recreational market. These combination units consist of side scan that sweeps from just below the water's surface on each side down to 84 degrees. Humminbird covers that 12-degree gap directly under the keel with an additional down-looking transducer beam.

"Back in early 2003, I was at a show where commercial side-scan sonar products were being displayed," says Humminbird's Mark Gibson. "I was completely shocked by the quality of the images they were showing. My thoughts were that if somehow we could figure out a way to shrink the     6-foot-long torpedo like device that gathered the images and attach it to a boat, we could rule the fishing market."