When you look at the ocean on a still day it may appear to be flat. However, it is not. When you take a closer look, the movement of the ocean causes a ship to have six different degrees of motion.
These are: heave, surge, sway, roll, yaw, and pitch. Plus, weather generates a quickly changing, dynamic plan that each cruise ship moves across. Here, we’ll examine the various types of ship rocking.
Cruise ships move forward/reverse, up/down, side-to-side, and, during a large wave, they can heave into the air. All six types of motion can happen in combination. Understanding this factor enables engineers, scientists, and bridge operators to assess what is going on so they can make informed decisions regarding the ship’s operation.
Cruise ships are designed to withstand stresses, whether it is structural, wave loads, turning, or wind forces. To counter the motion, people must be able to understand a vessel’s response to each.
Ship Motion: Across the Six Planes
As mentioned, there are six ways a ship moves. Let’s explore each of these terms in greater detail.
- Surge: This happens when large swells arise. A surge pushes the ship forward, impacting the vessel with front-to-back motions. This occurs when a ship rapidly decelerates and accelerates. The cause is from waves hitting the ship from stern or fore.
- Sway: The sliding motion that happens when the ship’s hull is pushed by currents or wind is called a sway. The motion is transverse, happening when waves strike the ship perpendicularly. This causes a rocking across the starboard and port sides. The force distribution in a sway is uniform, and there is an absence of uneven loads.
- Heave: This is the up-and-down sensation as a large swell heaves a cruise ship vertically on the troughs and crests of waves. Heaving involves a difference in weight and buoyancy forces. This imbalance of force generates the heaving motion.
- Yaw: This will spin a ship on an “invisible middle line.” It is like swiveling around on a chair. The cause of this may be waves moving perpendicular to the ship’s motion, changing the direction or where it is heading. Proper rudder corrections reduce the yaw’s effects.
- Roll: This is the tilting motion of a ship as it rocks side-to-side. Waves and wind push against a ship, causing the vessel to rock. This is caused by waves moving in the same direction as the vessel.
- Pitch: This is the movement of a ship going up and down. This happens due to the falling and rising of the ship’s stern and bow like a teeter-totter. A pitch tends to be experienced over open waters and in bad weather conditions.
Wait … Is There More?
Hogging and sagging don’t necessarily rock the ship, but they are reactions to the motion. Simply put, this is when a ship rises in the middle or sags. It is simply some ship’s ability to flex in response to crests.
Bow, Stern, and Bottom Slamming
Slamming is the quick deceleration of a cruise ship as it strikes the surface of water. It is caused by large shiploads exerting a force around the three main ship parts. The main parts include the stern, bow flare, and the bottom.
The bow is the very front part of a ship, while the flare identifies how a ship’s beam will increase from a waterline till a forecastle deck. Vessels containing large flares typically encounter a larger loading condition attributed to hydrodynamic behavior and overall design. To counter the effect, draft corrections are made so flares can be utilized without damaging the hull.
Bow flare slamming happens when keel portions of a vessel unexpectedly slam into the water. This happens when the ship’s weight found at the fore is not supporting buoyancy forces. Therefore, it suddenly drops.
Bow slamming is very common on open waters. It exerts a tremendous force on the cruise ship and, if not accounted for, it can cause structural failure. To reduce it, sections are reinforced. They provide the necessary rigidity and distribute the load evenly. By increasing the draft drastically, it is possible to reduce bow slamming.
This type of slamming is common when the ship is further away from the mainland. This is where large waves can exert extreme loads against the hull’s structure.
Stern slamming is like bow slamming. The only difference is it happens only at the stern when its weight becomes imbalanced by a lack of buoyancy forces. Since ships have heavy engines, rudders, and propellers, this slamming can damage the aft.
The good news is that it is a less common type of rocking due to hydrodynamic characteristics and the shape of the ship.
The last type of motion is bottom slamming. While less common, it occurs at the keel area close to the midship section when it slams into water.
For most passengers, they probably won’t notice the different types of motions aboard a ship. You may notice the boat swaying during turbulent waters or as it moves, but usually not much else. The ship’s captain and crew tend to be good at navigating through anything the ocean throws at them. They know the ship and how it is constructed.
Therefore, they consider these different types of rocking to provide safe passage to various destinations and to reduce any ill effects against the ship.
Are Ships Safe?
Absolutely! These types of things are taken into consideration when the cruise ship is being designed. Most ships have been strengthened sufficiently at various components that receive most of the stress. They also have additional support.
The main point that is kept at the forefront when ships are designed is that they can withstand any loads acting as a force on the vessel. These forces must be evenly distributed – which they are.
While there are different types of ship rocking, as mentioned, guests rarely notice them. While some may experience motion sickness, many just notice the ship rocking back and forth. This is because crew members in charge of navigation can account for various types of motion, plus ships tend to travel at a speed at which passengers enjoy a smoother ride.