How does it work?

All sounders, graphic or not, function on the same principle. A transmitter/receiver (sensor,) located under the boat sends a sonar signal vertically toward the bottom. When this signal encounters an obstacle, itis returned to the sensor. The sensor’s clock calculates the time taken (transmission/reception) and calculates the depth. This is then displayed, digitally or graphically.

Selection criteria

-The number of vertical pixels
Whether monochrome or colour, the screen must be very fine to displaythe details. This is related to the number of vertical pixels taken into account. The horizontal pixels are used only to display the history of the probe as it travels from left to right.
-The sensor frequency
The sensors operate on two frequencies: 50 and 200kHz. A 200kHz probe does not work beyond 200m. Its sweeping cone is large. The 50kHz probe goes deeper but has a narrower cone. We choose the sensor according to the types of fish sought and the depth of the fishing zone. Note; certain sensors are dual frequency and have the ability to switch from one to another.
-The power of the sensor

The power of the device is especially important if one opts for the 50kHz sensor. Attention; depending on the manufacturer the power is given in watts CC or RMS. The difference between the two is in a ratio of 8 to 1. In fact a 1200watts CC sounder corresponds to a model of 150 watts RMS.

Installing a sounder

The sensor should not be disturbed by the wake. It is necessary that water flows over it without air bubbles. There exist, several types of sensor, depending on their location on the hull. There are transom probes, through-hull and those that are stuck to the bottom of the boat. Ensure that the probe is an extension of the boats shell. For in-board engines, the probe should be positioned in an area where the water streams are not disturbed. Seek to position it as horizontally as possible. On a sailing vessel, it is necessary a transverse probe or a through hull is bonded. Its location is in front of the keel. It must be perpendicular to the bottom. There are models that provide tri-data; depth, speed and water temperature.

Specialized sensors

Sounders are not limited to the two standard frequencies. There are multiple frequency sounders and angled to cover important surfaces (often to the detriment of range.) Associated with some particularly fine sounders are top of the range models like Lowrance or Humminbird. These give amazing 3-D readings.


Capable of seeing forward, sonar permit the detection of obstacles up to 100m in front of the boat. They are useful for sailing vessels, which travel in poorly defined mapping areas (presence of coral for example,) or forfishermen looking for a rise or fall. Selecting a mooring with such a strong vision of the bottom becomes child’s play.
Professional advice
“In indicating wrecks, or the nature of the bottom; rock or sand, the graphic sounder is the ally of the fisherman.”