A down imaging fish finder consists of two parts: main unit (with a screen) and transducer. 

The main unit should be located where it can be easily accessed in the cabin or wheelhouse, etc. The transducer should be mounted on the boat's bottom either through hull or in-hull installation. Please bear in mind that the performance of a fish finder depends largely on whether the transducer is properly mounted on the boat's bottom. An improperly mounted transducer may result in failure in capturing reflected waves or other serious defects. To avoid these troubles, you should consult with a technical dealer on how to properly install the transducer prior to actual installation.

Overview of operating mechanism of fish finder

A down imaging fish finder transmits ultrasonic waves from the transducer mounted on the bottom of the boat. The transmitted ultrasonic waves advance straight towards the seabed, but if there is an object, i.e. fish school or rocks, they will reflect. Part of them will reflect back to the ship's bottom, which will then be captured by the transducer. By measuring the time elapsed between transmission and reception of ultrasound, the depth of the fish school will be calculated.