Ever wondered how a massive structure like a cruise ship doesn’t tip over even though having a huge volume outside of the water compared to what’s below the water?
So, the answer to this question is balancing the forces. Let’s say we have two main forces acting on the hull, one is the weight of the vessel that pushes it down and the second is the buoyancy force that acts on the bottom of the hull that pushes it up.
How Does A Cruise Ship Float?
When both forces balance with each other the vessel is stable and remains afloat. Now, these forces act on certain points in the ship called the center of gravity, where the weight acts, and the center of buoyancy, where the buoyancy or upward forces act on.
Now since all the heavy machinery on board like engines, generators, and so on are placed on the lower levels in the vessel compared to lighter materials which take up spaces like cabins, furniture, entertainment spaces, and so on, are mostly found on the upper levels or decks in a vessel, the center of gravity will be located towards the bottom half of the vessel.
Whereas the center of buoyancy is found to be acting below the water surface on the submerged section of the hull and is defined as the center of the waterplane area which is almost half of the submerged section.
Let’s suppose an external force like wind acts on the side of the vessel, the vessel tends to heel towards one side and while doing so the amount of body submerged inside the water also changes, bringing forward a shift in the center of buoyancy of the vessel whereas the center of gravity remains unchanged.
In stable conditions the center of gravity is aligned with the center of buoyancy in a single vertical plane, whereas in unstable conditions like the one described above, the center of gravity and center of buoyancy will be not aligned on the same plane which thereby causing a twisting moment which tends to bring back the vessel to its stable upright condition, once the wind is over.
Now if there is a shift of weight within the vessel, the center of gravity changes and again a twisting moment is produced if the center of gravity aligns with the center of buoyancy again the vessel remains stable but with an angle of heel, and this condition can be correct by balancing the weights on-board.
Ship stability is divided into, Intact stability and damaged stability. Intact stability determines the vessel’s ability to stay afloat in all operational and weather conditions. Whereas damage stability determines the vessel’s ability to stay afloat such that safe evacuation procedures can be carried out, during collision or grounding of the vessel.
Let’s get into the details to find out more.
How Are Cruise Ships Designed In Terms Of Stability?
The stability of a cruise ship is not only depends on its geometry but also on how the structures are arranged inside the vessel to help it stay afloat during undesired events such as flooding due to collision or grounding of the vessel.
In cases of flooding, water-tight compartments and water-tight bulkheads help in retaining water in specific spaces inside the vessel, thereby increasing the reserve buoyancy of the vessel and not causing it to spread throughout the vessel which can ultimately cause sinkage.
Looking back into history, one of the main reasons why the world’s biggest passenger ship, The Titanic sank was the absence of watertight bulkheads.
Classification Societies
All the new build designs are governed by a regulatory body called IACS (International Association of Classification Society), which has put forward a set of safety standards that must be met by the designer/builder during the entire shipbuilding process.
All vessels are designed so they operate in certain sea states and the standards have a high safety margin in terms of adverse weather conditions the vessel may ply through during her operational life. Also starting from the design stage till the final launching and sea trials there are regular class survey’s done by surveyors under the classification society to ensure the standards are maintained.
They also verify the structural strength and integrity of components of the hull to ensure the safe operation of the vessel. Also, periodical surveys are done to check if a vessel is properly maintained in compliance with class requirements throughout its operational life to ensure the overall safety of life on board the vessel.
Inclining Test
Near to completion of a vessel, an inclining test is carried out to assess the stability of the vessel. The primary aim of the experiment is to find the center of gravity, and the metacentric height (used as an indicator of stability), and further estimate the stability of the vessel.
This test involves shifting weights within the vessel so that the vessel’s heel is to one side, the corresponding heel angle is marked with help of devices such as a pendulum or stylograph, and computations are made to get the above-mentioned values and this is repeated for different heel angles.
The inclining test is not only done for new builds, but also for vessels that have undergone a huge change in their overall weight for example refit of the superstructure and so on.
Also, to abide by safety standards classification societies have brought forward a rule where all vessels must be weighed over a period of 5 years to ensure that the vessels are lightweight (that is the weight of the vessel without any cargo, fuel, passengers, and so on, onboard) haven’t changed.
If there’s a noticeable increase higher than 2% an inclining test must be done so as to assess the stability of the vessel because when there’s a change in lightweight the position of the center of gravity also changes which can affect the stability of the vessel.
Cruise Ship Stability Systems Onboard
Back in the day, vessels were stabilized by adding more weight/ballast on the lower sections so as an effort to bring down the values of the center of gravity. Not only does this add more weight overall but also required more power to push this weight, which thereby increased fuel consumption and reduces range.
Understanding this was not a feasible way forward, designers started thinking of other means by which they could achieve a more stable vessel and one of them was by improving the values of metacentric height.
The absence of a completely filled tank gave rise to the free surface effect which had a direct relation to metacentric height and methods were adapted to reduce this effect by compartmentalizing the tanks.
Again, the overall effects were observed to be very small and other systems were developed which proved to be more effective.
Degrees Of Freedom
A ship normally has a motion in 6 degrees of freedom out of which 3 are translational and 3 are rotational. The translational motions are, a surge which is the forward and aft motion of the ship, sway which is the port to starboard motion, and heave which is the up-down motion.
The rotational motions are roll, motion with respect to the longitudinal axis towards port and starboard, Pitch which is the motion about the transversal axis forward and aft, and yaw which is a swing motion about a vertical axis.
Stabilizer systems are used to dampen these motions which can lead to travel discomfort problems like seasickness for passengers and crew onboard the vessel.
Based on the source of energy utilized to stabilize a vessel, the systems can be divided into active and passive stabilizers. Passive stabilizers are those which may work utilizing energy from the ship’s motion and they can either be fixed or mobile whereas Active stabilizers are those which are linked to cruise ships’ internal means and require power to operate which can be either electrical or hydraulic.
Do Cruise Ships Have Stabilizers?
In order to reduce rolling at sea and improve the experience of passengers onboard, cruise ships are equipped with stabilizers. However, it is a more complex system and includes both passive and active stabilizers.
Passive Stabilizers:
- Bilge Keels
Bilge keels are normally plates welded to the hull and are located in such a way that they create an obstacle to the water flow generated during roll motion, thereby producing a damping motion that reduces roll.
They are the most common stabilization systems used on most vessels and are effective at all speeds. Also, they do not require much maintenance as they are fixed structures.
- Passive tanks
Passive tanks are tanks that are designed inside the vessel and they use ballast water to add and discharge weight as a means of counteracting the roll motion. If designed correctly, flow motions inside the tanks due to roll act as a damping force.
It is highly effective at slower speeds and since there is no additional structure protruding out of the vessel there are no additional drag forces created on the hull making it more efficient.
Active Stabilizers:
- Fin stabilizer
Stabilizer fins are the most common active stabilizers. They are movable lifting surfaces and are free to rotate about an axis. Its motion is governed by sensors to counteract external forces.
- Gyro stabilizer
A gyroscope is a device that has a wheel-shaped structure in a box that can spin at very high speeds about its own axis. They produce a moment that can oppose the external forces thereby stabilizing the entire vessels.
Some vessels use two or more stabilizers depending on the size of the vessel. They are effective at all speeds and also when the vessel is at rest.
- Autopilot
The autopilot system uses electronic signals to control the rudder of the vessel. They can help the vessel keep a steady course without any other external forces acting on the vessel which could lead to a change in course.
- T foils
They are used to stabilize the vertical motions in a vessel. They work in a similar manner to fin stabilizers and are usually retractable so as to reduce drag and also so that the vessel could easily maneuver through shallower water without the risk of grounding.
Is It Safe To Take My Family On A Cruise Ship?
Yes, it is totally safe to go out with your family to the sea and have a great time on a cruise ship, as modern vessels are designed and built in such a way, strictly adhering to all safety measures and protocols. They are also maintained and surveyed periodically to ensure the overall safety of life on board and also are equipped with the latest life-saving appliances such as boats and rafts for undesired events.
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