Six types of ship sway you should know

When you look at the ocean on a calm day, it can look flat. However, this is not the case. When you look closer, the motion of the ocean causes a ship to have six different degrees of motion.

They are: heave, thrust, swing, roll, yaw and pitch. Additionally, the weather generates a dynamic and rapidly changing plan that every cruise ship passes through. Here we are going to look at the different types of ship sway.

Cruise ships move forward/backward, up/down, side to side, and in a big wave they can rise into the air. All six types of movement can occur in combination. Understanding this factor allows engineers, scientists, and deckhands to assess what is happening so they can make informed decisions about the operation of the vessel.

Cruise ships are designed to withstand stresses, whether structural loads, waves, rotation or wind. To counter the move, people must be able to figure out a ship’s response to each.

Movement of the ship: through the six planes

As mentioned, there is six ways a ship moves. Let’s explore each of these terms in more detail.

  • Surge: This happens when large swells occur. A thrust pushes the ship forward, impacting the ship with back and forth motions. This occurs when a ship rapidly decelerates and accelerates. The cause is waves hitting the vessel from the stern or bow.
  • To balance: The sliding motion that occurs when the ship’s hull is pushed by currents or wind is called rocking. The motion is transverse, occurring when the waves strike the vessel perpendicularly. This causes rocking on the starboard and port sides. The distribution of forces in a swing is uniform and there are no uneven loads.
  • Swell: It’s the up and down feeling when a big swell lifts a cruise ship vertically over the troughs and crests of the waves. Lifting involves a difference in weight and buoyant forces. This force imbalance generates the lifting movement.
  • Yaw: This will rotate a ship on an “invisible center line”. It’s like swiveling in a chair. This may be due to waves moving perpendicular to the movement of the vessel, changing direction or course. Appropriate rudder corrections reduce the effects of yaw.
  • Roll: It is the tilting motion of a ship as it rocks from side to side. Waves and wind push against a ship, causing it to tip over. This is caused by waves moving in the same direction as the ship.
  • Ground: It is the movement of a boat going up and down. This happens due to the sinking and rising of the stern and bow of the ship like a seesaw. Land tends to be experienced over open water and in bad weather conditions.
Cruise ship bow (Photo credit: LarsZ / Shutterstock)

Wait… is there more?

Squatting and sagging do not necessarily rock the ship, but are reactions to motion. Simply put, it’s when a ship rises in the middle or sags. It is simply the ability of a vessel to sag in response to crests.

Slamming bow, stern and bottom

Slamming is the rapid deceleration of a cruise ship as it hits the surface of the water. It is caused by large ship cargoes exerting force around the three main parts of the ship. The main parts include the stern, the bow flare and the bottom.

The bow is the most forward part of a ship, while the flare identifies how a ship’s beam will rise from a waterline to a forecastle deck. Vessels containing large flares typically encounter a greater loading condition attributed to hydrodynamic behavior and overall design. To counter the effect, draft corrections are made so that flares can be used without damaging the hull.

cruise ship bow
Photo credit: Paul Vinten / Shutterstock)

Bow flare slapping occurs when parts of a ship’s keel unexpectedly slam into the water. This occurs when the weight of the vessel found ahead does not support buoyancy forces. Therefore, it suddenly drops.

Bow slamming is very common in open water. It exerts tremendous force on the cruiser and if not taken into account can cause structural failure. To reduce it, the sections are reinforced. They provide the necessary stiffness and distribute the load evenly. By considerably increasing the draft, it is possible to reduce the bow slam.

This type of slamming is common when the ship is farther from the mainland. This is where large waves can exert extreme loads against the hull structure.

The stern slam is like the bow slam. The only difference is that it only happens at the stern when its weight is thrown out of balance by a lack of buoyant forces. Since ships have heavy engines, rudders, and propellers, this slamming can damage the stern.

The good news is that this is a less common type of rocking due to the hydrodynamic characteristics and shape of the vessel.

Cruise ship waves
Photo credit: Steve Heap / Shutterstock

The last type of movement is the bottom slam. Although less common, it occurs in the keel area near the midsection when slamming into the water.

For most passengers, they probably won’t notice the different types of movement on board a ship. You may notice the boat rocking in rough water or when moving, but usually not much else. The ship’s captain and crew tend to be good at navigating through whatever the ocean throws at them. They know the ship and its construction.

Therefore, they consider these different types of tipping to ensure safe passage to various destinations and to reduce any adverse effects against the vessel.

Are the ships safe?

Absolutely! These types of things are taken into consideration when designing the cruise ship. Most ships have been sufficiently reinforced in various components that experience the most stress. They also get extra support.

Also Read: Is It Safe To Go On A Cruise – Everything You Need To Know

The main point that is kept in the fore while designing ships is that they can withstand all loads acting as a force on the ship. These forces must be evenly distributed – which they are.

Although there are different types of ship sway, as mentioned, customers rarely notice them. While some may experience motion sickness, many simply notice the ship rocking back and forth. This is because crew members tasked with navigation can account for different types of motion, and ships tend to move at a speed at which passengers enjoy a smoother ride.

ship scale

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