Topic 06 - Watch out for air bubbles
Air bubbles under water are the main nemesis to a fish finder. Ultrasound, when transmitted, travels straight down towards the seabed. However, when the ultrasound hits air bubbles on its way, the surface of the air bubbles reflect it back and it cannot travel any further. Ultrasound is very vulnerable to air bubbles. Even though ultrasound is transmitted with high transmission power, it may be reflected by the surface of the air bubbles or be attenuated when going through them. In such case, ultrasound may not reach fish schools in deep water or the seabed, only showing a strong echo return from the air bubbles.
Such interference from air bubbles can easily be observed in the following conditions:
- When your boat goes into another boat’s wake; or
- When your own boat goes astern.
In the latter case, a mass of air bubbles emanating from the propeller of the boat will completely surround the bottom of the hull, making the passage of ultrasound impossible.
Subsequently, a fish finder screen will only show the strong echo returns from the air bubbles. If the interference from the air bubbles is weaker, a fish finder screen can show a very fragmented view of the underwater condition.
When you find yourself in such a situation, the only way out is to escape from the area affected by the air bubbles. A couple of ways to avert the adverse effect of air bubbles include: taking into consideration the position where the transducer is mounted; and avoiding to manoeuver the boat in astern direction. If a fish school has been detected, it is advisable to let the boat continue on its course for a while and then turn around to go back to the point where the fish school was first detected. For those who are not used to using a fish finder on board their boat, the general advice is to always control the boat in a forward direction, when searching for a fish school.
