SHIP STABILITY

There are certain principles of physics that you need to know in order to have an adequate understanding of stability.

Generally speaking, the weight of a ship in the water is “pushing” straight down, and the seawater that it displaces is “pushing” straight back up. You should be familiar with terms such as volume, density, weight, center of gravity, force, and moments

SHIP STABILITY Ship stability is an area of naval architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves. Stability calculations focus on the center of gravity and center of buoyancy of vessels and on how these interact. Most ships are now fitted with stability computers

The ship’s center of gravity, is the point at which all weights of the ship may be considered to be concentrated. The force of gravity is considered as acting straight downward

SHIP STABILITY A floating object is stable if it tends to restore itself to an equilibrium position after a small displacement. For example, floating objects will generally have vertical stability, as if the object is pushed down slightly, this will create a greater buoyancy force, which, unbalanced by the weight force, will push the object back up.

‘’Buoyancy” may be defined as the ability of an object to float.

Center of Buoyancy When a ship is floating at rest in calm water, it is acted upon by two sets of forces: (1) the downward force of gravity and (2) the upward force of buoyancy.

If an object of a given volume is placed under water and the weight of this object is GREATER than the weight of an equal volume of water, the object will sink. It sinks because the FORCE that buoys it up is less than the weight of the object.

However, if the weight of the object is LESS than the weight of an equal volume of water, the object will rise. The object rises because the FORCE that buoys it up is greater than the weight of the object; it will continue to rise until it is partly above the surface of the water.

The volume of the submerged part of a floating ship provides the buoyancy to keep the ship afloat. If the ship is at rest, the buoyancy (which is the weight of the displaced water) must be equal to the weight of the ship.

For this reason, the weight of a ship is generally referred to as DISPLACEMENT, meaning the weight of the volume of water displaced by the hull.

In science, buoyancy is an upward force exerted by a fluid that opposes the weight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid.

The magnitude of that force is proportional to the difference in the pressure between the top and the bottom of the column, and (as explained by Archimedes' principle) is also equivalent to the weight of the fluid that would otherwise occupy the column, i.e. the displaced fluid. 

Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object. ----Archimedes' principle Buoyancy = weight of displaced fluid.

The volume of water that is moved by the hull of a ship is known as “displacement.” Buoyancy = weight of displaced fluid.

Regulatory Authority/Operator PKA : Northport, Westport JPA : Johore Port Berhad, PTP PPC : Penang Port SB KPA : Kuantan Port Consortium, KmPA: Konsortium Pelabuhan Kemaman BPA : Bintulu Port SB

Watertight Integrity

Watertight Integrity means the water does not enter the hull and cause progressive flooding the structure or fitting will prevent the passage of water through the structure or fitting in any ordinary sea conditions.

Watertight integrity can be breached through any activity or happening that allows the ingress of water in unwanted areas or compartments of the vessel. Examples include Lack of maintenance to seals, screw threads and other locking devices. Cracks along welds in metal vessels

The Importance of Maintaining Vessel Watertight Integrity Ensure all watertight decks and bulkheads are inspected periodically to verify that there are no unprotected openings or improper penetrations that will allow progressive flooding and that closure devices (e.g. watertight doors, closures, etc.) are in place and in working order.

Ensure all crew members are familiar with the locations of the watertight doors (WTDs) and weather tight closures throughout their vessels. Knowing the locations of such WTDs and weather tight closures should be part of the crewmember vessel familiarization process.

Ensure WTDs and hatches are closed while at sea and as otherwise specified in the stability guidance provided to the master or individual in charge. The importance of keeping WTDs and hatches closed should be emphasized on a regular basis (e.g. at safety meetings).

Ensure electrical cables and conduits, piping runs, remote valve actuators, and other components that penetrate watertight bulkheads, and compartments are inspected frequently and properly maintained. Ensure water accumulation is minimized and all spaces are kept dry unless permitted by the stability instructions provided to the master or individual in charge.