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Sportfishing

Send a Strong Signal

Want a razor-sharp image like this 17" Furuno’s? Then you’ll need a top-quality sounder module.

Everyone’s familiar with fishfinders; they’ve been around for years. But if your boat operates in deep water and you need outstanding resolution for, say, serious offshore fishing, your boat should have a modular electronics network composed of individual components instead of an all-in-one fishfinder. A key component of such a system is the sounder module, which powers and controls the transducer and feeds data from it to the network display. That data contains the bottom profile and images of fish and any other objects in the water column, images that are much more accurate than those from a fishfinder. A properly selected, installed, and working sounder module can not only help you catch more fish, it can also improve your navigation skills by giving you accurate depth and bottom-contour information that you can compare with paper charts. In that regard you could say that a sounder module is a piece of safety equipment.

The sounder module does all this by sending a high-power, high-frequency signal to the depth transducer, which may be mounted on the boat’s transom, pass through the hull, or be bonded to the inside of the hull. Once it receives this signal the transducer sends a “ping” that travels down through the water column until it hits something—the bottom, a piece of structure, or a fish—at which point it travels back up to the transducer. In the meantime, after having transmitted the ping, the transducer has turned into a receiver; it’s now listening for this tiny return signal. When it’s received it sends the signal back to the processing unit in the sounder module.

Since the processor knows when the ping was sent, when the return signal was received, and the speed of sound through water, it can accurately calculate the distance—or more accurately, the depth. By further analyzing the input, the module can also determine which echoes are fish and which are bottom, since the quality of an echo is partly determined by the target’s density. Higher-quality systems are able to further break down the return signal into shapes that are accurate enough to let you determine the number and even the species of fish.

Of course, these images eventually appear on some kind of network display, which is independent of, but usually must be compatible with, the sounder module. Although most high-speed marine networks are based on Ethernet, they are actually proprietary, which is why you generally cannot attach a sounder module from one manufacturer to the network or display of another.

The sounder module’s output power partially determines the maximum depth of the water the system can operate in. Because the transducer changes from transmitting a high-power pulse to receiving a small signal, the amount of interference or “noise” generated by the module is critical to how well the quality of this signal is preserved. A well-designed, low-power system with a low-noise sounder module can read deeper and detect more fish than a poorly designed, high-power sounder with high noise.

aymarine’s DSM400 is compatible with the company’s G and E Series displays.

Ping frequency is measured in kilohertz or kHz. Higher-frequency units are typically designed for use in shallower water, lower-frequency ones for deeper water. Specifically, sounder modules typically support 200 kHz for shallow water and 50 kHz, 38 kHz, or 28 kHz for deep water. Lower-frequency transducers usually cost a good deal more than higher-frequency ones.

This article originally appeared in the March 2009 issue of Power & Motoryacht magazine.

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