How do line drawings describe the shape of a ship’s hull?

In naval architecture, line drawings are essential for defining the external geometry of a ship’s hull, both above and below the waterline. These drawings describe the hull's complex three-dimensional shape by using the intersections of the hull surface with three series of planes oriented at right angles to one another.1

This process, often referred to as creating a lines plan, involves "fairing"—a technique used to mathematically delineate the surface to ensure it is smooth and free from unwanted bumps or discontinuities. Historically, this was done manually on a large scale, but modern design relies on computers to perform these calculations numerically. By defining the hull through these intersecting planes, naval architects can accurately calculate critical hydrostatic, stability, and capacity data, ensuring the vessel meets its performance requirements.21