Depth Finders
by Chuck Husick

Few of today's navigators have ever used the traditional lead line with its tallow-filled bottom sampling cavity. Few have ever heard the leadsman cry out "by the mark twain" or some other indication of water depth, or lack thereof. Perhaps not hearing that cry from the man on the bow is a reasonable price to pay for the convenience of not having to cast and retrieve the lead. Besides, good tallow is hard to find and who wants to stand on the bow when it is more pleasant to sit in the shade, managing the electronics at the helm station, with a cool drink close to hand. There are some traditions we willingly give up in the name of progress.

Sonar (Sound NAvigation and Ranging) technology, once limited to relatively simple depth sounders has developed into today's wide range of acoustic products. These include analog and digital readout depth sounders, fish finders, forward and side looking sonar systems and searchlight sonar equipment capable of providing a radar-like display of the water column surrounding and below the boat. As the variety of devices has grown, performance has improved and cost has diminished, in some cases rather remarkably. You can spend a fortune for a searchlight sonar with a large color CRT display. The challenge is to select the device that will be most appropriate for your intended use.

Depth sounders, fish finders and sophisticated sonar systems share a number of common elements. An electronic signal generator creates a specially shaped waveform that is amplified and applied to the system's underwater transducer. The transducer acts as both a loudspeaker, projecting a beam of acoustic energy into the water and as a microphone for detection of the returning sound echoes. The transducer delivers the detected information to the system's receiver circuits. The signals are processed to enhance the information content and to reject in so far as possible extraneous noise. The processed signals are then delivered to the display portion of the system for presentation to the user. Remarkably, all of these functions, when used for depth sounding in waters up to 240 feet deep, can be housed in a package no larger than a flashlight. The most complex searchlight sonar units, similar to those used on large commercial fishing vessels can weigh several thousand pounds and require a sonar head three feet in diameter that is lowered through the bottom of the vessel when in use.

The transducers used with most small craft depth sounders and fish finders operate at frequencies between about 25 and 400 KHz. As with radar, it is easier to achieve sharp focus of the transmitted energy at higher frequencies. Similarly, low frequencies can penetrate further, allowing measurements to be made at greater depths than is possible with high frequency equipment. The two most common operating frequencies are 50 and 200 KHz. The design of a transducer is always a compromise between the desire for a sharply defined energy beam and the need to cover a reasonable area of the water column beneath the boat. Typical beam angles range from 45-50 degrees to as little as 10-15 degrees. Single lobe transducers usually present a fairly wide acoustic beam to ensure that when used for depth sounding the closest object within a rather wide area will register as a possible obstruction whereas a narrow beam might overlook an obstacle just outside the beam. Although not too common in depth sounders, switchable transducer frequencies are often used in fish finders, with a wide, low frequency beam angle used to search wide areas and the more sharply focused high frequency used once the target has been spotted. The transducers used for forward and side looking sonar, including fish finders and depth sounders are typically made with a number of separate acoustic elements in one housing. This is in contrast to previous techniques in which a mechanism was used to rotate a single sensor element in azimuth and elevation. Some transducers use phased arrays in which the elevation scanning is effectively continuous, producing an effect similar to that of a radar beam's sweep across a target area.

Transducers are available in styles suitable for mounting on the boat's transom or for thru-hull installation. Transducers may incorporate temperature sensors and or speed sensors. Transom mount units are usually installed on I/O and outboard powered boats where the hull ahead of the transducer is free of protrusions that might create turbulence in the water flow, compromising the function of the transducer. Some owners of outboard or stern drive powered boats may choose to install a depth sounder transducer through the hull, somewhat ahead of the transom, while mounting a more elaborate fish finder transducer at the transom where it can easily be reached for cleaning.

Through-hull transducers are available with either plastic or bronze housings. Depending on the construction of the hull, it may be practical to install the entire transducer inside the hull, either fixed to the interior hull surface with a hard, dense adhesive or installed in an oil filled enclosure whose lower surface is the interior of the hull. In such installations the ultrasonic energy is projected through the hull during transmission and the reflected energy must pass through the hull to reach the transducer on its return. Some loss in signal strength accompanies this type of installation, making its use best suited to systems intended for modest depth capability. In-hull installations may not work in cored hull boats. A preliminary check can be made by placing the transducer on top of a water filled plastic bag laid against the inside of the hull bottom. Alternatively, a piece of "silly putty" can serve as an acoustic coupling medium for this test. Oil filled housings can be filled with mineral oil from the drug store.

The installation of through-hull transducers is well within the reach of many boat owners, provided due care is taken to ensure that the location chosen will be in a area free of turbulence when underway and that the transducer will be submerged when the boat is on the plane. When choosing the transducer location be certain to check the interior structure of the boat for any interfering structure. Drilling a two inch hole for a transducer, only to find that the hole is directly beneath the fuel or holding tank will present you with the unwelcome opportunity of learning hull repair. An owner / installer must be prepared for the trauma of drilling a two inch diameter hole through the bottom of the boat. If the boat has a cored hull special procedures will have to be followed. The core in the area around the transducer hole will have to be removed and replaced with a suitable structural filler material.

The circuits that power the transducer to create the ultrasonic energy pulse can properly be called a transmitter. As with the more familiar radio transmitter, the power rating assigned by the manufacturer can be mislead the technically uninformed. Output power levels are often quoted as peak pulse power, peak-to-peak pulse power, rms power or average power. (Root-mean-square, RMS power is an engineering measurement means for equating the total energy in an alternating current waveform to the average or heating power in a direct current circuit.) Interestingly, some of the very smallest and least costly depth sounders and fish finders may carry power specifications of 2,000 watts while drawing only one quarter of an ampere from the boat's 12 volt power system. This seeming impossibility is explained by the fact that the high power quoted for the unit, 2,000 watts, exists only for a small fraction of the time the unit is operating. In this example, the average input power is only 3 watts. The rms power rating is 200 watts, indicating that pulses are being transmitted only about 1% of the time. The 2000 watt peak-to-peak power rating is accounted for by the extremely short duration of each pulse. The rms power of most recreational depth sounders and fish finders range between 100 and about 600 watts. As with most complex systems, raw power is not necessarily a good measure of overall results. The quality of the transducer, the sensitivity and selectivity of the receiver circuits and the type of data processing used in deriving information from the returning acoustic echo will determine the results achieved with a given sounder or fish finder.

For many boats a simple depth sounder will suffice. Unless you are fishing or want to view the changing contour of the seabed knowing the depth of the water beneath the transducer may be all you require. Always remember that these devices tell you what is more or less directly below, they do not look ahead or to either side and won't, by themselves keep you off a shoal. Most depth sounders have an adjustable minimum depth alarm. Some offer a second alarm that can be set to indicate when the depth exceeds a preset value. Used in conjunction with the shallow water alarm this capability can be useful when trying to follow an underwater contour. Many sounders provide a transducer depth offset adjustment so that the depth shown will be from the water's surface rather than from the face of the transducer. This feature enables the sounder to show actual water depth and is quite useful when navigating in areas having a significant tidal range. Some users may choose to set the offset adjustment so that the indicator shows the depth of water beneath the keel.

Virtually all of today's LCD display depth sounders and fish finders are spray proof, if not waterproof. Be sure to check the manufacturer's specifications carefully, some units are spray or waterproof only if gasket mounted on a surface so that rear connectors are totally protected from the environment. In many units the cable connecting the transducer to the display must be kept at its original length. Cutting the excess cable can detune the system, compromising performance. Where an installation results in an excess of cable it is usually best to coil the unnecessary length and secure it in an out of the way location. Do not leave the coil unsupported, the motion of the boat may eventually result in damage to the cable.

The most common depth sounder display is a monochrome LCD. The instrument may be a dedicated display or it may be shared with other functions such as boat speed, water temperature or other sailing data. Multi-function displays are a common feature of many vessel information systems. Displays combining depth, hull speed and distance run are common. Depth displays are usually equipped with night lighting. If your boating involves much night operation be sure to check the brightness range of the night lighting before purchasing a unit. The minimum intensity of night lighting in some instruments can be too high. The check should be made only after your eyes have dark adapted, become more sensitive to light by excluding all normal light for a period of time. Red LED internal lighting is preferred from the standpoint of preservation of night vision. The monochrome LCD displays used in most depth sounders are usually highly visible, however if you normally wear polarized sunglasses be sure to check the visibility of the LCD display through the glasses. A poorly aligned polarizer in a LCD can make the numerals difficult to see or even invisible when viewed through polarized glasses.

Some instruments provide NMEA 0183 data output allowing transfer of information to other on-board systems. Although not available as a standard, stand alone product, depth data can be integrated with GPS derived position plotting information and recorded in an on-board computer, producing useful sets of sounding information in areas where up-to-date depth information may be lacking. If this type of system is attempted be sure to include tidal information so that the results can be referenced to the local datum.