Most of the time fishfinders work great on auto mode. But not always.
Fishfinders don’t just find fish; they also find the places fish are likely to be, like reefs, wrecks, and drop-offs where baitfish and predators gather. But sometimes they need a little help to do their job.
Fishfinders produce short pulses of high-frequency sound that travel down through the water and reflect back from objects they strike, returning to the transducer as echoes. Knowing that sound travels through water at an almost constant speed, fishfinder software calculates the distance from the transducer to whatever produced each echo and translates that data into a graphic representation. But sometimes it can’t tell you exactly where the echo came from or what created it.
Some experts suggest that the first move in dealing with this problem is to switch off all automatic settings, which is a bit controversial because automatic modes usually do a better job of tuning than most humans. But while automatic settings may be fine for everyday use, when conditions are out of the ordinary, knowing how to adjust your fishfinder’s manual controls can make a big difference.
The most important control is the range selector, which you should set to keep the seabed visible near the bottom of the screen. If you select the 120-foot range in water that’s 50 feet deep, useful information will be crowded into the top half of the screen, compressing the picture so that echoes from two fish at about the same depth run into each other and look like one fish. Switching to the 60-foot setting will make those small fish stand out and separate groups into individuals. (The Zoom function achieves much the same effect, by enlarging a particular part of the picture.)
The other essential control is Gain or Sensitivity. If this is set too low, the fishfinder won’t display weak echoes, so you may miss small fish or subtle features. Too high and it will fill the screen with small echoes that hide important details. To start, set the range to at least twice the water depth and increase sensitivity/gain until you get a copy of the echo produced by the seabed but at twice the true depth. Then switch back to the range that is most suitable for the actual depth.
Scroll speed isn’t as important as range or sensitivity but it can make a big difference. A slow scroll speed on a fast boat can exaggerate seabed features, making a gently shelving bottom look like a cliff. A fast scroll speed on a slow boat flattens important features such as rocks or wrecks.
Most of the time your fishfinder will do just fine in automatic mode. But when it doesn’t perform exactly as you want it to, a little manual intervention can make the difference between a successful trip and an empty fishbox.
This article originally appeared in the January 2010 issue of Power & Motoryacht magazine.