Unstable ship
The stability of a ship determines how quickly the ship returns to an even keel when heeled by waves or for any other reason behaves in waves and it keeps the ship straight up. A ship could become unstable for whatever reason. If a ship lists too far, it can even capsize and sink. In any case, the weight distribution in listing can even cause the ship to break up.
Water entering the ship via a breach may result in a less stable ship for different reasons:
- The extra weight may increase the draught of the ship.
- If the water has entered on only one side of the ship, the centre of gravity of the ship is shifted which might cause a list.
- Water in a large space such as an engine room has a lot of ‘free surface’ meaning that the sea motion can cause sloshing in the semi-filled space In a listing ship, the liquid will move to the lower side,which will become even lower and so on. because of the extra weight moved to the side that is already lower.
Also incorrect placement of the cargo can result in an unstable ship. If packaged goods are not thoroughly secured, they can shift during a voyage due to the movement of the ship.
When a shift of cargo occurs, successive changes in the angle of repose can lead to can lead to progressive weight transfer to the lowering side which can increasing the list to the point of capsize.
The strength of the ships structure is designed for stable conditions. In an unstable ship, the forces and bending moments become very different. An unstable ship is therefore at risk of sustaining (even more) damage.
The crew can try to regain (full) stability by using ballast water or even redistributing the cargo. If the crew manages to regain stability by discharging losing/taking in ballast water or shifting/jettisoning the cargo the problem is solved and the risks for the ship, crew, and the environment are eliminated.
Keeping the ship stable during loading and unloading is a true balancing act. If too much cargo is placed on one side of the ship, it can capsize in the harbour.
Regulations are in place stating how stable a ship should be in which conditions. The blueprints of a new ship should be provided to a classification society such as Bureau Veritas or Lloyd’s Register of Ships for independent review. Ships are required to be stable in conditions they were designed for, also in damaged states. Several codes and regulations mention the extent of damage a ship should be able to withstand without losing stability.
