A ship’s motion at sea is fully described by six degrees of freedom — three rotational (roll, pitch, yaw) and three linear (heave, surge, sway). Heave, surge, and sway are the linear, non-rotational components.
Heave
Heave is vertical up-and-down movement of the entire vessel — the ship bodily rising and falling with wave action, distinct from pitch (which is a rotation causing bow and stern to move in opposite vertical directions). Heave affects the whole ship together.
Surge
Surge is forward-and-backward linear movement along the ship’s fore-and-aft axis — the vessel momentarily speeding up or slowing down due to wave action pushing against or with the hull, separate from the vessel’s actual propulsion-driven speed.
Sway
Sway is side-to-side linear movement perpendicular to the ship’s fore-and-aft axis — the vessel being pushed bodily sideways, distinct from roll (which is a rotation).
Why These Six Motions Matter Together
Real ship motion at sea is essentially a combination of all six — roll, pitch, yaw, heave, surge, and sway simultaneously, to varying degrees depending on sea state and the vessel’s heading relative to the waves. Understanding each individually helps in analyzing specific operational concerns (heave affects deck wetness and cargo securing loads; surge affects mooring line loads in port; sway affects positioning during offshore operations).
Practical Relevance
Beyond general seamanship, these motions matter concretely in specific contexts — offshore support vessel operations (where precise position-keeping against sway/surge matters for close work near platforms), mooring line load calculations (surge and sway forces), and cargo securing calculations (which account for combined motion accelerations, not just static weight).
Exam Relevance
The six degrees of freedom, including heave, surge, and sway, are covered within Ship Knowledge and seamanship topics in deck officer competency exams.