Depth Finder Showing a Depth of 15 Feet
A Flasher Depth Finder Showing a Depth of 15 Feet

When it comes to boating, let’s take it as read that knowing how deep the water is under your boat is pretty important. Any mistakes in this regard can ruin your whole day.

You already know how much water your boat draws, don’t you? Of course, you do. It’s right there in the owner’s manual, under the heading “Draft.”

So, how do you know out how deep the water is?

In lots of ways.

For example, if you can plot your position on a nautical chart, you can read the depth of the water where you are, right off the chart—maybe not for your exact position, but certainly for positions nearby.

You do know how to determine your position and plot it, don’t you?

Another way to find out how deep the water is, is to use a lead line. Lead lines come in a variety of shapes and sizes, from the traditional to the improvised. Essentially, they consist of a heavy weight and a long, stout line with marks on it. Traditionally, these marks are spaced at one-fathom intervals along the line and are marked with things like strips of red cloth, strips of leather, and so on.

The lead line I carry aboard my boat consists of a fairly heavy fishing weight tied to a braided cord that I’ve knotted every fathom—one knot for one fathom, two knots for two fathoms, and so on.

It is not a very long lead line, but then I don’t need very long lead line. I use mine to make sure that I have enough water under the stern of my boat when I’m stern tying, when I want to make sure my depth sounder is working properly, and so on.

A more convenient method is to use a depth sounder, also called a depth finder. Because sound bounces, or echoes, depth sounders can use sound waves to determine how deep the water is.

It works like this. The depth sounder emits a high-frequency pulse of sound that is shot into the water by a transducer. That same transducer then picks up the returning echo of that pulse and feeds it back to the sounder itself. The sounder then uses the length of the delay between the pulse and the echo to calculate the depth of the water.

That depth can be displayed in a variety of ways. Early on, depths were displayed as lines on a piece of paper or as flashes of light on a circular scale. These days, depth sounders that used the flashing-light method are called “flashers,” not to be confused with the other sort of flasher.

During the 1970s and 1980s, these flashing displays gave way to digital displays, which reported the depth as a number. My digital depth sounder displays three digits plus the decimal.

Digital depth sounders are still very popular and, for the most part, quite handy. They can be configured to display depths in feet, fathoms, or meters, and they can be configured to compensate for the depth of the transducer below the boat’s waterline.

I had one of these “depth instruments” on my previous boat, and I have one on my current boat.

However, depth sounders have come a long way, baby. Early on, like a tree branching, they evolved into fish finders.

Flashers were also sold as fish finders, and, indeed, there are currently several brands of flasher fish finders on the market, for use, primarily, in ice fishing.

Today’s common fish finder is an entirely different bread of catfish. Using multiple frequencies and television-like display screens, fish finders “paint” a picture of the water column beneath the boat, showing the bottom, rocks, logs, weeds, debris, and, oh, yes, any fish present.

Once introduced, it wasn’t long before screen-display fish finders were integrated into chartplotters, or, conversely, chartplotters were integrated into the fish finders.

These combination units can be optimized to show the contour of the bottom and the fish, or they can be optimized to show a stunningly detailed image of the bottom. So far, the units I’ve seen can’t really do both, owing, roughly speaking, to the way sound wave behave in the water, etc. The prices for one of these units can also be fairly stunning, too.

Hold onto that topic: the way sound waves behave in the water.

Anyway, this is all fine and dandy, but I just want to know how deep the water is under my boat. I’m not looking to kill fish, nor am I looking for the wreck of the schooner Hesperus.

I just want to know how far down the bottom is, and, if possible, to come by some idea about its composition: rock, mud, or sand.



And there by hangs a tale.

Which I will take up in my next post in this series.



Be the first to comment

Leave a Reply