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Practical Aspects of Stability

Practical Aspects of Stability

Water on Deck

If water is shipped on board, then the effect is three fold. Firstly a weight is added high up in the vessel, thus reducing stability. Secondly, that water has a free surface effect, which will further reduce stability. Thirdly, the added weight causes the vessel to sink further in the water, thereby reducing freeboard, and reducing seaworthiness. Freeing ports are provided on deck, so that the water shipped on board can be cleared rapidly. These freeing ports should never be blocked.


You may recall from Section 2 that reserve buoyancy is the volume of watertight hull areas above the waterline. As weight is added to a vessel and it sinks in the water, the volume of space above the waterline decreases. When this space (reserve buoyancy), is gone the vessel will sink.

If part of the engine room or the vessel’s hold is above the waterline, then providing that they are enclosed they will contribute to the vessel’s reserve buoyancy. Hence, the reason that all watertight doors are to be kept closed (except for access), at all times.

Figure 4.9

It is necessary to have a certain reserve buoyancy as, when in a seaway with the ends or middle unsupported, the vessel will sink down to displace the same volume as it does in smooth water. This could result in the vessel foundering.

If a vessel is damaged, and water can enter a compartment which was previously watertight, the compartment is said to have been bilged. When a compartment is bilged the buoyancy provided by the underwater volume of that compartment is lost, as is the reserve buoyancy of the enclosed volume above it. Before bilging, the reserve buoyancy was the entire enclosed volume above the original waterline. After bilging it is the enclosed volume above the new intact water plane area.

If this compartment is to one side of the centre line then the vessel will take up an angle of list. Depending upon the location of the compartment, the vessel may also trim by the bow or stern. In any case, draught will increase, freeboard and therefore reserve buoyancy will decrease and the effect is always to reduce stability.

In case of flooding, the biggest danger is the loss of watertight integrity and the subsequent loss of internal buoyancy from the damaged areas. Your immediate action in this case should always be to close all watertight doors through the vessel to prevent further loss of buoyancy. It may be possible in some cases to bring the damaged area out of water deballasting the vessel or providing a list on the opposite side to the damage.

Structural Changes

If a vessel is changed structurally, for example if a new wheelhouse is added or if an extra mast or winch is installed, the effect on stability is exactly the same as though these items were added weights. Because structural changes are usually complex and old material is often taken off the vessel as well as adding new material it is a survey requirement that all of the vessel’s stability is reworked after structural changes have taken place.

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