A few years back, I found myself approaching the entrance to New York Harbor in a thick fog. There was no way we were going to be able to spot the big ships going in and out, and it was doubtful that their radar was going to pick up our little 31-foot boat. The solution to the problem came from an unlikely place: our depth sounder.

MaptechBoating is a three-dimensional activity.By staying in 10 feet of water all the way around Sandy Hook and into the harbor, we were able to avoid trouble. It was plenty deep enough for us but way too shallow for the ships.

Boating is a three-dimensional activity. Knowing how deep the water is can keep you off the shoals, help you navigate, tell you how much scope the anchor requires and even where to drop it. That makes a depth sounder or fish finder an essential piece of safety gear on just about any boat.

While the term "sounding" is an ancient reference to determining depth, the modern electronic sounder literally uses sound to do the job. The display unit is attached to a transducer in the water, which can be thought of as a combination of speaker and microphone. It sends out a high-frequency pulse of sound, and then listens for the echo from the bottom. Knowing the speed that sound travels through water, it can calculate the distance to the bottom, and does so several times a minute to provide a continuous depth display.

CHOOSING A SOUNDER

As with all electronics, depth sounders – meaning both display and transducer – come in a wide array of price ranges, with varying capabilities and features. Some are designed to stand alone, some are combined with GPS and plotter, and some are part of large networked systems that include radar and other technologies. There are also portable units – even some hand-helds – that can be moved between the dinghy and the mother ship. On the very cutting edge – and priced accordingly – are units featuring "forward looking sonar" that collect soundings in front of the boat, though these have more to do with collision avoidance than collecting traditional depth information.

FurunoAbove, standard readouts offer numbers in black and white. Below, more complicated "fish finders" offer more information and a graphic look.
LowranceBasic displays give a simple black-and-white readout in feet, while more complex units – usually considered "fish finders" – offer programmable alarms and a full-color screen that indicates bottom contours and characteristics. Transducers are products unto themselves, allowing choices between metal or plastic; the more high level fish finders offer dual frequency or multiple frequency; and models that offer variations on the width and direction of the beam. These provide more sophisticated information.

SOUND(ER) INSTALLATION

Choosing a unit is often a matter of how you use your boat and how much you want to spend, but there are some basic rules for installation that can be considered universal.

Most people mount the display near the helm, for obvious reasons. Before you do, however, be sure to activate the unit and ensure that it does not affect the compass in that location. If you find interference, choose another spot. Also, never shorten the cable between the transducer and the display, because that cable is part of the unit's calibration. Simply coil up any excess and tuck or mount it out of the way.

The more difficult operation is installing the transducer below the waterline. The traditional installation involves drilling a large hole in the bottom of the boat while it is hauled out, then mounting and sealing the unit much like a thru-hull water intake. But there are alternatives. Smaller boats can use transom-mounted transducers, which are attached to the exterior of the hull. Boats of solid fiberglass can use "shoot through" or "in-hull" transducers, which are mounted inside the hull in a well filled with oil.

Whatever the style, transducers should not be mounted where they may be damaged by a grounding or where turbulent water flowing past can cause erratic readings. Check with the manufacturer about whether or not your transducer can be covered by bottom paint. Most units must be pointing straight down to give an accurate reading, which sometimes requires a mount that accommodates a vessel's sloping hull. If you store your boat wet, in a slip or on a mooring, some transducers can be mounted in a sleeve with a matching plug, so that the unit can be removed when not in use. With practice you can pull the transducer and pop in the plug while only taking on a few sponges of water.

As purchased, the depth sounder will display the depth from the transducer to the bottom. That is not the same as displaying the depth from the bottom of your boat or from the surface of the water to the bottom. Many units allow you to enter an offset so that the reading indicates depth from one of those places. If you configure the offset to show depth from the bottom of the boat, you know how close you are to going aground. If you set it to show depth from the surface, you can compare it to the chart, making the instrument more useful for navigation.

NAVIGATING WITH DEPTH

Gene BjerkeBy taking a string of soundings, correcting them for tide and plotting them at the appropriate scale on tracing paper, you can match the paper to a chart along your course and determine your approximate location.How much you depend on your depth sounder will vary according to where you do your boating. If you spend a lot of time in the deep waters of the West Coast, the readings will be useful for anchoring and mostly academic the rest of the time. But if your local waters are shallow, like the Chesapeake Bay or in Florida, your depth sounder will be your best friend. In areas where the bottom shoals gradually, it will give you ample warning of approaching trouble, but it will be less helpful in areas where rocks and reefs rise abruptly.

Whatever the case, a good navigator uses all the information at his disposal, including depth readings. One simple technique is to follow a depth contour. Charts indicate depths in numbers, but they also show contour lines as a demarcation point between deep and shallow waters. Using your chart and sounder, you can follow these contours when it is advantageous to do so – like we did coming into New York Harbor.

Just remember that in tidal waters, you will need to take the stage of the tide into account. Chart datum is usually Mean Low Water or Mean Lower Low Water, meaning there will usually – but not always – be more water than shown. You will often need to subtract some amount from your sounder readings to get to the charted depth. For example, let's say your depth sounder reads 18 feet but the chart says 15 feet. You know it is halfway between high and low tide, and that the range that day is six feet. So you would subtract three feet from what your sounder shows to get 15 feet, the charted depth.

So how do you know how much to adjust your instrument's reading? As a rule of thumb you can figure that the tide will rise or fall one-twelfth in the first hour of the cycle, two-twelfths in the second hour, three-twelfths in the third and fourth hours, then two-twelfths in the fifth hour, and one-twelfth in the last hour.

Depth can also help determine where you are. If you are able to get a bearing on a fixed object, you can plot the bearing on the chart. Where the bearing line crosses the depth as shown on your finder, adjusted for tide, is most likely where you are. A more complicated technique that works where the depth varies frequently involves taking a string of soundings at fixed intervals. These are corrected for the tide and plotted at the appropriate scale on a piece of tracing paper. By sliding this paper around on the chart and matching the numbers lined up along your course, you should be able to get an approximate location.

Depth information can be critical when anchoring. The amount of anchor rode you release, called scope, is calibrated using a ratio of line to depth. For example, a 5:1 ratio means five feet of line for every one foot of depth. A 7:1 ratio means seven feet of line for every one foot of depth. The information on bottom characteristics some fish finders can reveal may also be helpful, even if it is just an indication of "harder" or "softer."

Some cruisers use an even more complicated system, taking a portable sounder in the dinghy and mapping a course into an anchorage. They then return to the big boat and mirror that course. If the portable is a combo unit with GPS and plotter, a tracking function can be used to get even more precise. Some units can even be plugged into the mother ship's systems. It's high-tech and high-work, but it is a very safe way to reach an unfamiliar anchorage.

Gene BjerkeDepth can tell you your location. Get a bearing on a fixed object, then plot the bearing on a chart. Where the bearing line crosses the depth on your finder (adjusted for tide), is most likely where you are.While sounders are available cheap enough to accommodate just about any budget, there are still low-tech ways to measure depth that are cheaper and more reliable still. The traditional instrument is called a lead line, consisting of a heavy weight – usually a tear-drop shaped piece of lead – attached to a length of light line that has markings to indicate various depths. True lead lines often had a hollow in the bottom of the sinker, which contained wax, tallow or another sticky substance used to bring up a sample of the bottom.

There is still a place on a modern boat for the old-fashioned lead line. Electronics sometimes fail, batteries go dead and even the best sounders are often rendered useless in a grounding, when transducers are too close to the bottom to get a meaningful reading. In this situation, the sounder is unable to show the way off the shoal. But a lead line always works if it is used properly. If you run aground, you can use the lead line all around the boat, or even take it out in the dinghy, to find the best way off the shoal – which is not always the way you went on.