Flare is the angle at which a ship's hull plate or planking departs from the vertical in an outward direction with increasing height. A flared hull typically has a deck area larger than its cross-sectional area at the waterline. Most vessels have some degree of flare above the waterline, which is especially true for sea-going ships. Advantages of hull flare can include improvements in stability, splash and wash suppression, and dockside utility.^[1] Flare can also induce instability when it raises the center of gravity and lateral torque moment of a vessel too much (by negatively impacting its righting moment and metacentric height).

Tumblehome is the inverse of flare, where the hull becomes narrower with increasing height.