Q. I am confused about which transducer frequency — 50 or 200 kHz — is best when using my sonar for fishing and family cruising.
A. The higher 200 kHz frequency provides greater detail and resolution to track the bottom, as well as to spot and identify fish in depths down to about 350 feet. It is more sensitive to smaller objects and bottom structure. It is especially useful in lakes, rivers and shallow waters off the coast.
The lower 50 kHz frequency packs a more powerful punch and penetrates deeper — depths to 1,000 feet or more — where 200 kHz cannot reach. This makes 50 kHz best for the ocean and deep lakes. While lacking the sharper resolution of 200 kHz, the 50 kHz frequency offers a wider view of the bottom while also allowing you to spot fish and bait schools that the narrow-beam 200 kHz frequency might not pick up. Think of 200 kHz as a moderate-distance spotlight and 50 KHz as a long-range floodlight. You can get transducer/sonar systems that offer both frequencies, and many displays allow you to view both at once in split-screen mode.
Cone Angle Counts
A transducer sends its signal downward in the shape of a cone that tends to widen the farther it travels from the transducer. This can be helpful for both fishing and navigating. A 200 kHz transducer typically has a beam width between 9 and 12 degrees. The beam of a 50 kHz transducer ranges from 28 to 40 degrees. Your manual, dealer or manufacturer’s website can tell you what beam angle your transducer produces.
When fishing, it is important to understand how much area your display is showing you at the depth you are fishing. Instead of attempting to do the math (beam angle versus depth geometry) to determine the area viewed, you can use a calculator, like the one on the Furuno website under its Learning Center (furunousa.com/LearningCenter/Transducer-Beam-Angle-Calculator.aspx). This will tell you exactly what to expect the coverage area to be at your fishing depth.
Added Fishing Information
Many fishermen will use a split-screen view of both frequencies side-by-side to see a wide area and high-resolution picture at the same time. The angler will also benefit if the transducer he uses has a built-in temperature sensor to track the water’s changing surface temperature, which aids in finding where fish species are likely to be.
A cruiser can monitor its depth not only to stay clear of shallow waters but also to follow how the bottom is trending. This can help preview and predict what the water depth will likely become in the direction you are headed. This can be very helpful when entering unknown waters or an unfamiliar harbor.
Power is a Factor
Higher power can add both detail and range to what you are seeing on the screen of your fish finder. Six-hundred-watt transducers are the most common. Some fish finders are capable of producing 1,000 watts or more. This can increase target resolution to improve bottom-structure detail and help distinguish the bottom from bottom-feeding fish at greater depths. Transducers of 1,000 watts or greater are physically larger, requiring bigger fairing blocks, and add to the cost or your total depth-sounder/fish-finder investment.
Mounting Style Helps
How your transducer is mounted can affect your depth sounder’s performance. Through-hull transducers with a high-speed fairing block perform best. An inside-the-hull transducer that shoots its signal through the fiberglass may experience signal loss and may have reduced resolution detail and depth. Transducers mounted on the transom are most subject to cavitation and interference on the screen, and work best at slower boat speeds. A through-hull, flush-mount transducer with a titled element is a good alternate to the transom transducer. It is mounted almost flush to the hull and aimed directly downward, even though the hull may be at an angle.