The sounder has several useful functions that are hard to do without:
- Choosing the exact spot to anchor in
- Guiding oneself through the riverbed where sand and mud banks move.
- Navigate at dead reckoning by referring to the sounding lines on the map
This instrument is extremely accurate: modern depth finders have a margin of error of 3%, i.e. 3 centimetres per metre of depth. However, in order to use it properly, it is necessary to know the reference level from which it calculates the water level. Then it is possible to calibrate it, i.e. to choose this reference level.
So here's how to calibrate a sounder:
- 1. Check the displayed measurement with a handheld probe
Measure the height from the bottom to the surface of the water beneath your boat by standing as close as possible to the transducer. For that the bottom must be hard enough. Avoid bottoms of liquid or too soft mud in which your probe lead may sink. - 2. Compare the actual height and the displayed height to find out the correction to be applied.
- 3. enter the correction into the unit so that it displays the actual height from the surface.
Now your reference level is the water surface. Some sailors stick to this setting. However, it is recommended to add a pilot foot to this measurement.
The pilot's foot refers to the height of water that is taken off the depth sounder to give a margin of safety. For my part, I recommend that you remove 50 cm at the actual water height. So when the depth sounder indicates 2.50 mc (4), there is actually 3.00 m below the surface (3).
- 1 - Depth under the keel
- 2 - Measured depth
- 3 - Actual depth
- 4 - Depth minus pilot's foot
Why the pilot's foot?
The pilot's foot is useful because when you see the probe going up, you will have more time to slow down and thus avoid hitting the ground suddenly. Even if you calculate water heights very accurately, atmospheric pressure and storms offshore can change the tidal range by a few tens of centimetres. Sometimes uncharted obstacles litter the bottom, these 50 cm may preserve the keel of your boat from these bad encounters.
Placing the reference level under the keel
This solution allows you to know immediately how much water is available under the keel.
If you want to calibrate the sonar in this way, simply enter a pilot foot equal to your draft. For a keelboat that would set 1.80 m, the sonar will display 1.20 m (1) when the actual depth is 3.00 m (3).
Each yachtsman will choose the solution with which he feels safest: 50 cm pilot's foot or depth under the keel. The solution chosen depends very much on the psychology of the skipper. But don't forget to inform the crew members in charge of navigation...
