Digital depth sounders, sometimes called digital depth instruments, report depths as a series of single numbers. For example, 23.0, 23.5, 23.7, 22.9, and so on.
Herein lies the key to the problem with digitals. They can only report one depth at a time. By their very nature, digitals display those numbers one after another, like the times displayed on the face of a digital watch: 12:31, 12:32, 12.33, and so on. Whether that one depth is the depth you’re interested in or, effectively, noise is another matter.
So, where do those numbers come from and why all the guessing, or what amounts to guessing?
Digitals, like all depth sounders, send out pulses and measure the time between when the pulse was sent out and when the echo of that pulse returns. Let’s call that difference the delay.
Using the delay, the depth sounder can then calculate the depth of the water. The longer the delay, the deeper the water. The shorter the delay, the shallower the water.
Makes sense, right?
You see, there’s a fly buzzing around in the ointment. Well, several flies, actually. For every pulse sent out, a whole bunch of echoes come back: echoes from the bottom, from nearby rocks, from weeds, from fish, from kelp, from just about anything that can return an echo. That’s a whole lot of stuff; that’s a whole lot of echoes.
Once received back at the depth finder itself, that buzzing swarm or echoes is fed into a highly complex and equally sophisticated algorithm. In go the echoes, the machine goes crunch-a, crunch-a, crunch-a, and out comes a number: 21.7 feet, for example.
Pretty cool, huh?
Yep, pretty cool . . . until something goes wrong. When that happens—and in my experience it happens a lot—reliability flies out the old porthole. (The trick at that point is to make sure that the keel doesn’t slam into the old mud!)
Sound behaves differently in salt water, brackish water, and fresh water. It behaves differently in warm water, tepid water, and cold water. It behaves differently in water that swarming with algae and water that’s clear.
And that poor little algorithm, despite its sophistication and decades of development by the finest minds in the industry, often can’t cope. It has to make assumptions, it has to pick and choose, it bases current calculations, in part, on past calculations, and a lot of the time, it literally, for lack of a better term, gets lost.
I’ve found that when I’m in cold, clear water, my digital works just fine. I’ve also found that when I’m in deep water, rather than report “no bottom,” it tells me I’m in impossibly shallow depths. In the relatively warm, relatively cloudy waters of Puget Sound, it often delights in telling me that I’m in less water than my boat draws. Frequently, it can’t decide whether I’m in 158, 126, 75, 58, or 3.5 feet of water—all within the course of a few seconds. You can imagine how much fun this is when I’m trying to nose into bay to drop the hook.
Complicating matters still further, there’s no way for the user to adjust the gain or sensitivity of the units. That is, there’s no way to “tell” the digital that there’s a lot of “crud” in the water and to pay attention only to the strongest echoes. Think of this as being the rough equivalent of having a volume control.
Overall, digitals are fine units, but they do have their limitations, and often, those limitations make the difference between an enjoyable day on the water and a day spent in the enthralling anticipation of becoming suddenly and intimately familiar with the local underwater landscape, be it mud, rock, sand, kelp, or Junior’s discarded bicycle.