Final answer:
The term for the vertical distance from the bottom of a ship's keel to the waterline is 'draft'. It's critical for safe navigation, and changes with the ship's load. Modern ships use sonar technology to measure ocean depth, which was historically done with a fathom rope.
Step-by-step explanation:
The term for the vertical distance from the bottom of a ship's keel to the waterline is known as the draft. This measurement is crucial for determining if a vessel can safely pass through shallow waters without running aground. The draft varies depending on the load and distribution of weight on the ship, which is why it is often closely monitored by sailors and shipmasters. It is an important safety measure, and understanding a ship's draft is essential for proper navigation and operational planning.
Ships also utilize technologies such as sonar for navigating and understanding the oceanic environment around them. Sonar, which stands for Sound Navigation And Ranging, is a technique that uses sound propagation to navigate, communicate with or detect objects on or under the surface of the water. The depth of the sea, which used to be measured by a fathom rope, is now accurately determined by sonar.