A ship has six degrees of freedom, three translational and three rotational, about the three mutually perpendicular axes through the centre of gravity (longitudinal x, transverse y, vertical z):

1. Surge - linear motion along the longitudinal (fore-and-aft) axis, caused by the variation of thrust and resistance, and by following/head seas pushing the hull. It affects speed and fuel consumption and can cause slamming of the bow in head seas.
2. Sway - linear motion along the transverse axis, caused by beam seas, wind and rudder action; it produces lateral acceleration, cargo shift risk and can combine with roll.
3. Heave - vertical linear motion along the vertical axis, caused by the passage of waves under the hull changing buoyancy; it produces vertical accelerations, slamming, and affects the effective GM and the comfort of crew and cargo.
4. Roll - rotation about the longitudinal axis, caused by beam seas, wind, and the heeling moment of rudder or cargo; it is the most dangerous motion for stability because it can lead to large angles, cargo shift, and in resonance to parametric roll or capsize.
5. Pitch - rotation about the transverse axis, caused by head and following seas; it produces bow/stern slamming, deck wetness, propeller emergence and affects speed and comfort.
6. Yaw - rotation about the vertical axis, caused by asymmetric wave forces, wind and rudder; it affects course-keeping, increases resistance and can lead to broaching in following seas.

Causes and impact: all motions are excited by wave forces, wind, and the ship's own propulsion; their amplitude depends on the encounter frequency relative to the ship's natural frequencies (resonance). They reduce stability margin (especially roll), increase structural loading (pitch/heave slamming), cause cargo damage and crew discomfort, and increase resistance and fuel consumption.

• Increase GM (stiffen the ship) by lowering KG (ballast low, remove top weight) to reduce roll amplitude, but avoid excessive stiffness which gives short, uncomfortable periods.
• Fit bilge keels, anti-roll tanks (passive/active U-tube tanks), fin stabilisers and gyro stabilisers to damp roll.
• Use active/passive anti-pitching and anti-heave devices, and design the hull with fine ends and adequate freeboard to reduce slamming and deck wetness.
• Operate at a speed and heading that avoids resonance (change course/speed to move the encounter frequency away from natural frequencies), and avoid beam seas for roll.
• Use ballast and trim to improve stability and reduce motions; press up tanks to remove free surface.
• Design with adequate GM, metacentric height and righting-lever (GZ) reserve, and use stabilisers, bilge keels and anti-rolling tanks; for comfort, reduce vertical accelerations by speed reduction and course changes.
• Structural measures: adequate freeboard, flare, and bow form to reduce slamming; and operational measures such as weather routing.