Monday, July 13, 2009

Stability of Cruise Ship

In initial stability assessment, naval architects will always have to make sure that the cruise ship they are designing has positive GM. It is the distance between the center of gravity of the ship and the metacenter, an imaginary point which is created when a line from center of buoyancy is vertically drawn and intersect with the vertical axis of the ship that passes through or divides the ship into two equal halves. Watch the following video where naval architects conduct model test to assess or to find out how stable the  Oasis of the Sea , a new cruise ship will be when it has been constructed into full scale. Cruise or passenger ships are sophisticated floating structures that are constructed to transport people from one place to another. Today's modern passenger ships are equipped with advanced navigational equipments that will help the ships' crews handle the ship safely.
In addition, cruise ships or passenger ships now have more decks which make them taller. An example of such ship is Carnival Destiny. This beautiful ship is built by Fincantieri Cantieri Navali in Italy and classified to Lloyd's Register, one of the best and the most experienced classification societies in the world. Besides having 16 decks, the luxurious cruise ship has a swimming pool at the top or sun deck. From the stability perspective, the mass of water in the swimming pool raises the center of gravity of the ship.
In addition, the taller the superstructures, the higher will be the center of gravity. Ships that have higher vertical center of gravity (VCG) will have smaller righting lever, GZ. Such condition is dangerous for traveling around the ocean. To have lower VCG or higher metacentric height (GM), ship designer will balance the passenger ship with ballast tanks that are filled with sea water.
In addition, ship designer will use lighter materials such as composite materials, aluminum, plastics and wood, for higher decks so that increase of height of CG will not jeopardize the ship's righting lever (GZ). Ship with larger GZ will be more stable compared to ship with smaller GZ. The ship will be unstable if the metacentric height is zero or negative.
Source: K.J. Rawson and E.C. Tupper, Basic Ship Theory 1, page 105
Cruise ship with positive GM will be more stable when sailing at sea. The position of VCG (Vertical Center of Buoyancy) depends on the loading condition. Sometimes when there are too many passengers on the uppur decks, ballast tanks which are at the bottom of the ship will be filled with sea water to balance the ship and to lower the position of VCG and balance the LCG.With this arrangement, the ship will be stable and safe to travel across the ocean around the world.
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Anonymous said...

Stabilizing fins have nothing to do with a ship's initial stability and the two subjects should not be confused. They involve completely separate requirements. The author of this column needs to rewrite it so it is more clear to the reader.

Charles Roring said...

Thanks for your suggestion, I will write another article about stabilizing fins in relation to the rolling of ships

Anonymous said...

AFAIK modern cruise ships have negligible ballast water capacity, which is mainly used to offset consumables such as fuel along the voyage. If a fully-laden cruise ship requires fixed or water ballast to fulfill the stability requirements, the engineers haven't done their jobs properly...