A ship or submarine in waves can move in six basic ways called the six degrees of freedom. Three are straight-line motions along the vessel's axes, and three are rotations around those axes. Understanding these motions helps marine scientists, naval engineers, and ship operators predict comfort, safety, and control at sea.
The same ideas apply to research vessels, cargo ships, submarines, and small boats.
Key Facts
- Surge is forward and backward translation along the ship's length.
- Sway is side-to-side translation across the ship's width.
- Heave is up-and-down translation caused by waves lifting and lowering the hull.
- Roll is rotation about the ship's long axis, like rocking left and right.
- Pitch is rotation about the ship's side-to-side axis, like the bow rising and falling.
- Yaw is rotation about the vertical axis, changing the ship's heading left or right.
Vocabulary
- Degree of freedom
- A degree of freedom is an independent way an object can move or rotate in space.
- Heave
- Heave is the vertical up-and-down motion of a vessel in the water.
- Roll
- Roll is the side-to-side rotation of a vessel around its front-to-back axis.
- Pitch
- Pitch is the bow-up and bow-down rotation of a vessel around its side-to-side axis.
- Yaw
- Yaw is the left-or-right turning rotation of a vessel around its vertical axis.
Common Mistakes to Avoid
- Confusing surge with heave: surge is forward and backward motion, while heave is vertical motion.
- Calling any side motion roll: roll is rotation, but sway is straight side-to-side translation.
- Mixing up pitch and yaw: pitch raises or lowers the bow, while yaw changes the ship's heading left or right.
- Forgetting that the motions can happen together: real ships in waves often surge, sway, heave, roll, pitch, and yaw at the same time.
Practice Questions
- 1 A research vessel heaves upward 1.2 m from its lowest point and then downward 1.2 m from its highest point. What is the total vertical distance from lowest to highest position?
- 2 A ship surges 8 m forward, then sways 3 m to starboard, and then heaves 2 m upward. What are the three translation components of its motion?
- 3 A vessel's bow rises while its stern drops as a wave passes underneath. Identify the main motion involved and explain how it differs from roll.