Sway

Sway is the lateral translational motion of a ship along its transverse (y) axis. It has no hydrostatic restoring (c_22 = 0), so a free-floating hull does not oscillate about a sway equilibrium; its linear equation reduces to (m + a_22) y-double-dot + b_22 y-dot = F_2(t). Sway couples strongly with roll and yaw in oblique and beam seas through the off-diagonal added-mass and damping terms, which is why beam-sea roll prediction must solve the sway-roll-yaw set together. In maneuvering, sway velocity v combines with surge to set the drift angle and the hydrodynamic side force and yaw moment on the hull.