What is the purpose of a depth sounder on a boat?

• A. To measure water salinity.
• B. To determine water depth and help avoid grounding.
• C. To provide weather forecasts.
• D. To identify fish depth.

Answer: (B)

A depth sounder is used to measure the distance from the boat to the bottom, which helps you avoid running aground and navigate safely in shallow water. It works by sending a short sonar pulse from a transducer into the water and timing how long the echo takes to return from the bottom. Using the speed of sound in water (which varies with temperature and salinity), the device converts that travel time into depth and displays it on the screen. This information is vital for judging whether you’re in safe depths, spotting shallow areas, and planning routes or anchoring. The other functions listed aren’t what a depth sounder provides—it's not for salinity, weather forecasts, or specifically identifying fish depth, though some displays can show fish targets as an extra feature.