The 6 movements of a ship are a fundamental element of navigation and affect its safety and behavior at sea.
Most people when on a ship may understand the basic types of a ship’s motion, for example if it rocks right and left or up and down, but they do not perceive what more often than not, neither the reason why a ship rocks nor the remaining movements besides the basic ones.
Sailors, on the other hand, perceive the type of a ship’s motion and the factor causing it better.
Most of the dynamic movements are not measured on the ship but are felt, and because they create serious problems, an effort is made to limit them.
Overall, in the consideration of dynamic motions, one translational motion and one rotational motion are encountered on each axis, all of which have a direct relation to the subject of accelerations and are:
1) HEAVE
The vertical oscillation along the “Z” axis (emergence or immersion, and sinking or submergence, parallel to the waterline, which is also called immersion or elevation oscillation), which becomes perceptible in rough seas.
It is more significant than the others because it is more connected to vertical acceleration. It increases compression and tension forces, creating damage (e.g., on rotating keys, etc.). An effort is made to limit it with appropriate handling. (Some call Heave Sinkage, which is wrong).
2) YAW
The rotational oscillation along the “Z” axis, (yawing of the bow, yaw, when the ship travels in waves or current, something like yawing, sheering, swing or sheer when the ship is anchored) which is perceived by observing the bow.
The yaw oscillation is not customarily measured on the ship, but if needed, it can be measured in degrees. This can be done to limit it in bad weather by changing course or to adjust the autopilot. With average helm and leeway, a drift of the ship to the right often appears due to the force exerted on the rudder.
3) SWAY (transverse oscillation)
The transverse oscillation along the “Y” axis. This movement-displacement of the ship in the transverse direction, as the ship falls off the other side of the wave coming from the beam, is not measured but it is known that it also contributes to accelerations as well as to the ship’s being thrown right or left from the intended (DR Track) course, since it is included in what is called the navigational sense or otherwise “Current” which becomes apparent when measuring two positions.
All the factors included in “Current” are ocean currents /or tidal currents /or river currents, the unknown compass error, the error in engine setting, friction on shoals, unusual behavior, a problem with the propeller, inaccurate steering, wind, etc.
4) PITCH (pitching)
The rotational oscillation along the “Y” axis, (pitching, plunging), which is immediately perceived and an effort is made to avoid it because it directly contributes to longitudinal and vertical acceleration, to the acceleration forces that create damage, as does leeway but in the longitudinal direction and with less intensity.
It affects the opening and closing of hatch covers etc.
Measured with a sensitive clinometer in degrees placed longitudinally and if necessary its period is measured with a chronometer.
5) SURGE (Longitudinal ταλάντωση)
The longitudinal (Longitudinal) oscillation along the X axis. This slowing down and speeding up of the ship forward and aft as the ship pitches passing over the large waves coming from astern, is not measured but it is known that it also contributes to the accelerations and to the apparent slip (Sapp) between two instants (e.g., noon reports along with the other factors mentioned that are included in “Current”).
(1. Sapp = [(E-Dapp):E] x 100%. E = engine miles calculated with the Revolutions and the Pitch of the propeller, Dapp = Distance in miles measured by the bridge in the same time interval).
6) ROLL (rolling)
The rotational oscillation along the “X” axis (rolling, roll, rolling motion, swaying), which is immediately perceived more sensibly than all other oscillations. It is the most important and harmful because the ship’s draft, the loads, the openings and closings of the holds, contributes to the accelerations mainly in the transverse acceleration, creates the racking force that causes damage to the vessel itself, also affects the compression force (Lifting force) and tension force, causes damage to rotating keys etc.
Its period (one complete oscillation) is measured in seconds and in degrees with the clinometer (Heel). An effort is always made to limit the rolling and to avoid synchronism (Synchronism), where the period of a simple roll is half the period of the wave, and to avoid the sea being on the side of the ship (Beam seas).
Appearance of all movements simultaneously
In a state of sea and because the ship usually has some small list and trim, rarely or never does one of these movements appear alone. Thus, underway in rough seas with heavy swell, all six degrees of freedom of oscillation occur simultaneously to a lesser or greater extent.
Also, it is impossible for the ship not to have the wave motions simultaneously. When the above movements are restricted, the composition of all forces ends up being the cause of the bad weather condition, which is characterized by the expression “the ship labours” or as we say “is labouring” (Labouring).
Stability: Each one of the six movements is controlled by the corresponding stability.
1. Roll: Transverse stability.
2. Pitch: Longitudinal stability.
3. Yaw: Directional stability.
4. Heave: Positional motion stability.
5. Surge: Stability in motion ahead and astern.
6. Sway: Lateral motion stability.
Conclusion
The ship in a seaway makes various dynamic movements, but a ship also makes some of these movements even when docked (static movements), e.g., from the weight of the cargo or from some slight list.
The dynamic movements of the ship that occur in a seaway are more significant and present interest because they create various conditions.
However, the study of these motions constitutes an extremely difficult problem, so in order to study the motions, some auxiliary simplifications and assumptions must be made and the problem must be approached progressively and methodically.
We see, therefore, that many of these terms of motions may seem unfamiliar even to sailors, but it is certain that all who travel encounter and face them daily on the ship.
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