What is the main purpose of a compass on a vessel?

The main purpose of a compass on a vessel is to establish the vessel's heading. A compass provides navigators with the direction in which the vessel is pointed relative to magnetic north. This information is crucial for determining the correct course to follow in order to reach a destination safely and efficiently. Understanding the heading allows the crew to make necessary navigation adjustments based on tidal currents, wind, and other environmental factors.

While the other options refer to important aspects of navigation or vessel operation, they do not directly relate to the primary function of a compass. For instance, determining speed requires a different type of instrument known as a speedometer or log. Measuring distance typically involves navigational charts or electronic systems, and avoiding obstacles relies more on situational awareness, radar, or visual observation rather than the compass which solely indicates direction.