How Electronic Depth Sounders Work

Electronic depth sounders are a great aid to modern mariners, allowing them to know the precise depth of the water at all times and thus avoid underwater hazards and shoals.

Electronic depth sounders are a great aid to mariners of all types, whether they pilot an ocean liner or a small outboard powered runabout. Contrary to the perception of most land-lubbers, the greatest dangers a mariner faces are not found offshore, in the depths of the open ocean, but rather are located in near-shore waters, where there are many hazards. To sailors, land can be a safe haven, but it can also be grave danger and more ships have been lost near shore than in all the world's deep oceans.

For safe navigation in near-shore waters, no information can be more essential to the mariner than the depth of the water. In past times, ascertaining the water depth involved a difficult process called "sounding," which was done by throwing a weighted line over the side in an attempt to find the bottom. This line, called a "lead line" was knotted in increments that allowed the user to measure the water's depth in feet or more commonly, in "fathoms" a nautical unit of measure equal to 6 feet. Using a lead line from a moving vessel was of course problematic, and subject to inaccuracies. The user had to stand on the bow of the ship or boat and toss the line, wait until the lead weight hit the bottom, and then haul in the line and count the number of knots that were submerged. All the while the vessel was still moving and the bottom contour could, of course, have already changed by the time the sounder called out the depth to the captain.

Today's electronic depth sounders have changed all of this. Depth sounders provide instant and continuously updated readings of the water depth as a vessel speeds along. Depth sounders work by the principle of "sonar." A sound signal is emitted from the bottom of the hull and this signal travels through the water until it reaches the bottom and then bounces back, to be picked up by the depth sounder's receiver. Since sound waves travel at a known rate, the depth can be determined by calculating the amount of time it takes for the sound waves to hit the bottom and return to the vessel. This is all done automatically and instantaneously by the instrument.

Modern depth sounders consist of two parts: the instrument face that is mounted in a location where the helmsman can see it while navigating, and the transducer, which is a through-hull fitting mounted in the hull of the boat, usually near the deepest point, and connected to the instrument face by a wire. This transducer is the part that actually sends out the sound signal, and it must be mounted in a place where the signal can travel unobstructed to the bottom. Transducers are mounted with a waterproof seal so that no leaking can occur through the fitting.

The instrument face of a depth sounder provides the information on water depth in whatever units the navigator desires. Most can be set to read depths in feet, meters or in the old-time units of fathoms. This flexibility is desirable because navigation charts for different areas may be printed with charted water depths in any one of these units.

Paired with accurate charts that show such surveyed water depths, the modern navigator can use the depth sounder as another of his tools to determine his position off of a coastline. In many places the bottom gradually slopes to greater depths the farther offshore you go, and these incremental depths are shown on the charts as contour lines. Cruising along with continuous feedback from the depth sounder, the navigator can determine which contour line he is following or crossing and thus get a pretty good idea of his location to back up input from other navigational aids such as the compass and GPS receiver.

Electronic depth sounders are a great comfort to mariners operating in areas of shoals, underwater reefs or manmade obstructions on the bottom. This instrument should be installed on just about any boat larger than the smallest skiff, as it can help prevent boat owners from running aground or worse, wrecking their vessels in dangerously shallow water.

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