Final answer:
The crosswise direction of a ship is described as 'athwartships' or 'beam,' relating to a direction perpendicular to the ship's length. This is related to the concept of relative velocity in navigation.
Step-by-step explanation:
The term used to describe the crosswise direction of a ship is athwartships or beam. This term refers to a direction at right angles to the length of the boat, from one side to the other. When discussing movement or forces such as relative velocity, this directional term is useful. For instance, when a ship moves through water and is affected by currents or wind, its actual path can differ from its intended course, a concept known as relative velocity. This is especially evident when a vessel, like a boat, attempts to travel straight across a river but is carried downstream, therefore, it moves diagonally across the river. Understanding the effect of relative velocity is critical for navigation to ensure the ship reaches the intended destination efficiently.