To review . . .
When it comes to being out on the water, one of the better ways to ruin a perfectly good trip is to run out of water, that is, to run aground. Just ask Joseph Hazelwood, the captain of the ill-fated Exxon Valdes. Hell, just ask me. Ask just about anyone who’s been boating for any length of time.
Eventually, almost everyone runs aground.
Ah, yes, but how to avoid this potentially lethal calamity? (And running aground can be and often is lethal, even in a small boat.)
The simple answer to that one is that you have to make sure your boat has more water under her than she needs float. If your boat draws six feet of water, as mine does, then you have to make sure you keep her in water that is over six feet deep. Duh! (Frozen water doesn’t count.)
So, all in all, you have to know how deep the water is.
Which brings me back to the close of Part One.
When I’m out on my boat, I want to know how far down the bottom is, and, if possible, what sort of bottom is down there: rock, mud, sand, gravel, weed, or some combination.
Enter the problems with digital depth sounders.
By and large, these are fine instruments, and I am certainly not about to remove the one I have on my boat.
However, digitals do have a couple of critical weakness.
As far as I can tell, these spring from their nature as digital instruments, that is, they report depth as a number.
“What’s the big deal?” I hear you ask.
Hang tight, and I’ll tell you.
Depth sounders work by sending out sound pulses, waiting for the echoes, and then measuring the elapsed time between sending out any given pulse and receiving back the echo from that pulse.
The root of the problem is that sound will bounce off, echo off, a lot more than just mud, rock, sand, gravel, weed, etc. It will also bounce off fish, turbulence in the water, the layers between water masses of varying temperatures, baby jellyfish, adult jellyfish, plankton, whales, seals, submarines, garbage, and a lot of other stuff.
And we’re only getting started.
The pulses that depth sounders shoot into the water spread out, and as they do, they trace a cone-like shape. Therefore, by design, they strike a much wider area than would, say, a focused beam of light travelling through air and hitting a wall.
Moreover, sound waves do not bounce off only one thing. Indeed, they bounce off anything and everything they can. They’re very conscientious in that regard.
So, for example, a depth sounder sends out a pulse. A short time later, back comes the return. But it’s not just one echo. Rather, it’s a mass, a swarm of echoes, all originating from that one pulse.
Some of these echoes are strong, some are weak, some are slightly ahead of or behind the others, and so on. Those wildly differing echoes could have come from water turbulence, fish, weeds, debris, soft mud, hard mud, rocks, algae, eel grass, kelp, and the bottom—in multiple places and at multiple depths.
Now, in one way or another, to one extent or another, flasher depth sounders and fish finders report all, or most, of these echoes, ether as a series of flashes around a scale or as a television-like “picture” of what’s under the boat. (More about this in Part Four.) The point here is that the report is visual. The operator gets to interpret and adjust the display.
With digital depth sounders, the report is strictly digital: a number. What you see is what you get. Take it or leave it. Period. Believe it, or don’t.
That number results from feeding that buzzing swarm of echoes, their delays and probably their strengths into a highly complex and equally sophisticated algorithm. In go the delays and the strengths, the machine goes crunch, crunch, crunch, and out comes a number: 21.7 feet, 245.9 feet, 34.6 meters, 61.2 fathoms.
Pretty cool, huh?
Yep, pretty cool . . . right up until the time that something goes wrong and reliability flies out the old porthole.
And that, girls and boys, what can and does go wrong, will be the subject of Part Three in this series of posts.