Fish finders(sometimes also referred to as sounders) are great ways to help you find the right spot when out on the water in your fishing boat. They consist of two parts: a main unit(with a display) that you keep with you on the deck of the boat, and a transducer, which is connected to the main unit and is mounted on the hull.
Fish finders use sonar technology which was developed during the second world war to show what is directly below your boat. They also borrow some of their technology from fathometers, which are devices used by marine vessels to figure out how deep the water is and what potential hazards(like large rocks and corals) are below. This helps them navigate safely.
When you turn your fishfinder on, the transducer that you have mounted on the hull of your boat sends out sonar waves. These waves, like sonar waves in a radar, bump into objects in the water, as well as the waterbed, and bounce back up towards the transducer, which receives them again. The transducer then sends the received waves up to a component in the main unit called transmitters, which measure the new sonar waves, and sends those measurements up through a wire into the main unit. The main unit then displays the readings on a screen.
Depending on how advanced your fish finder unit is, these readings are displayed either as spikes on a graph, where flat areas represent the sea bed and spikes represent objects. These objects could be fish, rocks, or conceivably anything solid somewhere in the water.
More advanced units have more sensitive transducers that are able to pick up signals and display little fish icons on the screen. They differentiate between fish and other objects by measuring how the sonar waves bounce off of the contours of the fish’s body.
Using a fishfinder can really help take a lot of the guesswork out of where to take your boat. Whether you are a professional fisherman or fish for sport and hobby, using a fish finder can really help turn a calm day out on the water into an exciting day that ends with a few trophies and a huge barbeque!