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| Searchlight sonar will provide a glass-bottom bucket's look - and then some - at what's beneath and around your boat. |
Tired of looking at only that narrow swath of bottom that your regular sounder shows? You might want to investigate several types of sonar available to recreational boaters above and beyond the familiar — forward-looking sonar (FLS), searchlight and side scan. FLS continues to evolve as a means of seeing what's up ahead. Searchlight sonar provides a picture of what's all around the boat — as does radar. Side-scan sonar lets you see to both sides.
Recreationally, Interphase and EchoPilot are the primary manufacturers of FLS units, with commercial-electronics company FarSounder recently attempting the crossover to the recreational market. At the last minute in 2005, Furuno opted against introducing a new FLS. However, Furuno, Simrad and Si-tex also make searchlight sonar, which displays an image around your boat as if your vessel were floating in the middle of a large bowl. Only Furuno and Si-tex have searchlight units aimed at the recreational market though.
If you have the opportunity to use or see any of these technologies, you will notice one thing they all have in common: They require users to practice often and learn the systems in order to interpret what they see on the screen, because the views can be like nothing you've seen before with traditional down-looking fish finders.
Sonar Basics
Sonar — regardless of type — works by shooting a sound signal via a transducer. Since the speed of sound in water is a known constant — 4,856 feet per second or about 3,000 mph — sonar measures the time it takes the signal to make the round trip down to a target (transmission) and back (reflection), divides the time by two and multiplies that by the speed of sound to arrive at the depth. It then presents that information in graphic form. All sonar/fish finders/depth finders work this way. The element that performs calculations and displays results is where much of the technology of sonar becomes apparent to the user, thanks to miniaturization, digital signal processing and screen-viewability advances.
You must understand two other issues concerning sonar: First, generated signal pulses are cone shaped and thus become wider the farther they extend from the transducer (form a tepee shape with your hands). Second, power, salinity, depth, water temperature and boat speed (and its concomitant bubbles) significantly affect transducer transmission and reception.
Location, Location, Location
Lastly, we consider the oft-overlooked part of the sonar package, the transducer. The transducer (or transducers) must properly match the power output of the sending unit and be properly mounted for maximum efficiency. You must place the transducer where it can't be affected by running gear or anything else on your hull that causes interference in water flow. Sound travels faster in water than air, so again, bubbles will detrimentally affect the transducer's transmission.
Remember: Lower frequencies give deeper range but lower resolution (less-clear target discrimination); higher frequencies offer better resolution but shallower range. Most recreational transducers fall in the 50 kHz to 500 kHz range, with 50 kHz and 200 kHz range transducers most common for down-looking sonar.
Forward-Looking Sonar
FLS has seen its most successful use as a collision-avoidance system, especially when used in vertical orientation. A vertical forward scan runs directly forward of your boat from just below the water's surface to straight down beneath your vessel, providing you with a look at the entire water column in front of the boat. The width (side to side) of the signal varies, but usually spans from 10 to 15 degrees.
In horizontal scanning mode, the signal sweeps from 90 degrees to port to 90 degrees to starboard. The center of this beam also angles slightly downward to avoid surface noise and to allow the beam to scan the bottom. This mode is best for fishing, especially when it shows you terrain features out to the sides (from 45 to 90 degrees on each side) that you may have otherwise missed. Interphase FLS units range from around $1,900 (list) to around $2,800 (list), as do EchoPilot's.
