Drift angle (beta)

Drift angle beta is the angle between the ship’s centerline and its velocity vector at a point, positive when the bow points outboard of the track. In a steady turn it is set by the balance of hull, rudder, and centrifugal forces, and it grows from zero at the pivot point to a maximum near the stern. Beta drives the lateral hydrodynamic force through the sway derivatives Y_v and the yaw moment through N_v, so it governs how much side force a hull develops. Typical merchant-ship drift angles in a hard turn run 10 to 20 degrees. It is measured at the midship point for standard maneuvering reporting.