3.6 Ship Motions and Movements at Sea
Understanding how a ship moves in relation to the sea is crucial for safe and efficient navigation.
Ships experience two main types of motions:
Movements of Translation: These describe the linear movement of the entire vessel along a specific axis.
Movements of Rotation: These describe the angular movements of the vessel around a specific axis.
Movements of Translation (Linear Movements)
There are three primary movements of translation a ship encounters:
Surge: This refers to the forward or backward movement of the ship along its longitudinal axis. In simpler terms, it's the direction the ship is headed. Engine power, currents, and waves can all influence surge.
Sway: This describes the lateral movement of the ship to either side, port or starboard, along its transverse axis. Strong winds and waves can cause a ship to sway.
Heave: This refers to the vertical movement of the ship, up and down, along its vertical axis. Heave is primarily caused by wave action.
Movements of Rotation (Angular Movements)
There are also three primary movements of rotation a ship experiences:
Roll: This describes the side-to-side tilting motion of the ship about its longitudinal axis. Imagine a seesaw; rolling is similar, with the ship tilting to port or starboard. Wave action is the main cause of rolling.
Pitch: This describes the seesaw-like motion of the ship about its transverse axis. The bow (front) of the ship rises and falls alternately, while the stern (back) does the same. Pitching is also primarily caused by wave action.
Yaw: This describes the turning motion of the ship about its vertical axis. Yawing essentially changes the heading of the ship, turning it port or starboard. Yawing is primarily caused by rudder control but can also be influenced by wind and currents.
What is the difference between Sway and Roll ?
Sway and roll are both motions a ship experiences at sea, but they differ in the axis around which the movement occurs:
Sway: This is a lateral movement of the ship from side to side, along its transverse axis. Imagine the ship moving to the left (port) and right (starboard) without tilting. Strong winds and waves pushing the ship sideways cause sway.
Roll: This describes the tilting motion of the ship from side to side, about its longitudinal axis. Think of a seesaw - the ship leans to port or starboard but doesn't move sideways significantly. Wave action is the main culprit behind rolling.
Here's an analogy:
Imagine a rectangular box (representing the ship) on a table.
Sway would be like pushing the box sideways across the table (left and right).
Roll would be like tilting the box on the table, leaning it on its long sides (port and starboard).
Additional Notions:
Ships can experience multiple movements at the same time. The combined effects of these motions can create a complex, rocking, and swaying movement for the ship, especially in rough seas.
Ship design and size can influence how a vessel reacts to these motions. Larger ships tend to be more stable and experience less dramatic movements compared to smaller vessels.
Ballast water, which is seawater carried onboard for stability, can be adjusted to improve a ship's response to various sea conditions.
Understanding ship motions is essential for crew comfort and safety, cargo protection, and efficient navigation.