Deep Subjects

May 12, 2015
By Bill Jennings
originally posted July 1, 2012
The bottom line on depth sounders

The word ‘sounding’ describes the act of determining water depth. A Canadian engineer, R. A. Fessenden, is credited with developed the first electronic version in 1914. Such equipment became known as ‘echo sounders’ or ‘depth finders’. Fishermen refer to them as ‘fish finders’. The more sophisticated name ‘sonar’ is derived from “Sound Navigation and Ranging”, because it uses reflected sound waves to detail and paint underwater objects in much the same way a dolphin bounces sound waves off of underwater objects to help it swim. What we need to know is that all these names refer to the same technology.
Electronic sounder technology is used in many ways, including mapping the floor of a body of water, boat navigation, studying aquatic life and catching fish. Take note that meters have become the international standard when marking water depth on charts, however the NOAA in the US continues to uses feet as it’s standard unit of measurement.
Electronic sounders work by measuring the time it takes for sound to travel from the bottom of the boat to the seabed and the ‘echo’ return to the boat. Since the speed of sound through water is constant, the time measurement can be converted to distance. To perform this task and transmit the information to the boat operator, you need two electronic devices – a ‘transducer’ and a ‘display unit’.
A ‘transducer’ is simply a device that converts one form of energy to another. In this case, the motion of conductor coil waves to a magnetic field signal. A piezo-ceramic element, made from positively and negatively charged crystals, vibrates through the ‘acoustic window’ in the transducer, to cycle the vibrations to the bottom and back. In so doing it changes electrical energy to acoustical energy and back again to an electrical format that can be read on a digital or pixel screen.
Transponder frequency is the number of sound waves that the unit sends in one second and is measured in ‘Hertz’.  Most transponders offer dual frequencies, because a short wavelength, (i.e. high frequency), produces sharper images, while a longer wavelength, (i.e. low frequency), will allow you to see deeper into the water. A custom combination of the two wavelengths will provide a good balance of information for the viewer. Transducers for pleasure craft usually work with a signal frequency of 50 or 200 kHz. Both of these, incidentally, are too high to be heard by the human ear.
The ‘display unit’ for sonar can be anything from a simple LED digital read out of numbers, to a master monitor screen that can paint bottom information as well as a host of navigation and operation information. Specifically, your bottom digital data can travel from your transducer to your large screen display along the same NMEA 2000 connection as your GPS, weather, autopilot, radars and engine status details.
As with most electronics, when you buy a sounder, “you get what you pay for”. Basic units can provide positive depth information for under $200. As you add features you will move closer to $600. A high quality transducer can cost upward of $1,000 for a thru hull tilted to offset your boats ‘deadrise’ and more for a digital external mount. The sky is the limit for specialized commercial applications. Here are some important things you should consider when buying a depth/fish finder, to be sure you get what you want.

  • Decide if you need a colour monitor, or if grayscale will suffice.
  • Order a unit that can be set to read depth in feet, meters or fathoms.
  • A backlight screen is important.
  • In addition to the standard ‘auto’ function, look for a manual ‘gain’.
  • Fish finders should have a feature that displays a fish symbol for echos not connected to the bottom.
  • If it is not labeled as a daylight viewable screen – it probably isn’t.
  • Check that the ‘resolution’ rating will give you the detail that you want
  • An alarm is a handy option that can be set to ring when your water depth falls below a number that you set on your sounder display.
  • Sophisticated echo sounders offer add-on instrument pods, which will detect salinity, temperature, and the flow of current, all of which can interfere with the pulse of sound and cause a variation in the depth reading.
  • With playback feature you can rewind a portion of your sonar history to review what has passed across your screen.
  • Be sure your unit is designed to read the depths that you will normally need and has a beam width degree (cone angle) that displays the area size you want. The diameter of coverage area on the seafloor is dependent on water depth – – typically two to four times the water depth.

Boat speed as well as depth will decrease the performance of your depth sounder. The primary cause of this loss is that with speed, the water flowing past the transducer becomes aerated. Experts say that above 17 mph you can improve clarity by ordering a high performance fairing for the transducer.
A simple electronic depth sounder is a great aid for all mariners, but in our next issue we’ll report on the expanded technology of both broadband and forward looking sonar.