buttock

In naval architecture, a buttock is one of a series of vertical planes that run from bow to stern, used to define the shape of a hull. Unlike frames (which are perpendicular to the centerline) or waterlines (which are horizontal), buttocks cut through the ship along planes parallel to the centerline and keel. When a buttock plane intersects the hull surface, it produces a curve that shows the longitudinal profile of the vessel at that location.

Buttock lines are fundamental to ship design and lofting. A typical vessel might be described by 10 to 20 buttock lines, each offset by equal distances from the centerline. On a ship's lines plan (the technical drawing that defines the hull), buttock curves appear alongside frame sections and waterlines, creating a complete three-dimensional picture of the hull form. These curves are essential for calculating displacement, determining stability characteristics, and guiding construction.

Practical use and naming

The term buttock comes from the bulging convexity seen in the stern sections of traditional wooden ships. The aft body of a hull often widens and rounds outward, which naval carpenters likened to the human form. Despite the colloquial etymology, buttock lines are used throughout the hull, not only aft, though they are most visually pronounced in the broad stern regions typical of merchant and naval vessels.

Shipyard workers and naval architects rely on buttock lines when fairing the hull, setting out frames, and checking that the construction stays true to the design. Modern computer-aided design has automated much of this process, but the buttock remains a conceptual tool for understanding and communicating hull geometry. Deviation from the design buttock curves indicates either a construction error or a deliberate modification, and either way demands investigation before the vessel proceeds to launch.