Final answer:
The correct fog signal for a boat under sail is to sound one prolonged blast every two minutes. Sonar is utilized to measure ocean depths by reflecting sound waves and calculating distances based on the time interval and speed of sound.
Step-by-step explanation:
The signal that a boat under sail should use in the fog, according to the International Regulations for Preventing Collisions at Sea (COLREGs), is to sound one prolonged blast at intervals of not more than two minutes (Option 1). This sound signal notifies other vessels of the sailboat's presence in conditions of reduced visibility, such as fog.
In the context of navigation and safety at sea, sonar technology plays a significant role. For example, sonar can be used to determine the depth of the ocean by transmitting a sound wave that bounces off the seabed. To calculate the depth, you would multiply half the time interval it takes for the sound to return by the known sound speed in seawater. If the time interval is 2.5 seconds and the speed of sound in seawater is 1450 m/s, the calculated ocean depth would be (2.5 seconds / 2) * 1450 m/s, which is approximately 1812.5 meters.