Before one can define the standards for desirable stability, it is necessary to consider the normal operations of the ship and what accidents might befall it.
In some cases this is a matter of safety, in others it is comfort. For instance, a ship with high initial stability will tend to roll back quickly when disturbed. Such a ship is said to be stiff and most people find this motion less pleasant than a slower roll, when the ship is said to be tender. In normal operations the level of stability provided must take account of:
• The range of operating conditions from light to fully loaded.
• Conditions during loading and unloading.
• The possibility of asymmetric loads or, in cruise ships, the fact that passengers may congregate to one side of the ship.
• The wind and wave conditions the ship may encounter on her trading routes. Wind could be particularly important for ships with high freeboard or large superstructures.
• The heel resulting from the use of rudder during manoeuvring.
• The possibility of ice forming on the upperworks and rigging.
The standards for passenger ships in the UK are laid out in the Merchant Shipping Regulations. The latest version of these regulations should be consulted but typically they require the GZ curves to possess certain attributes:
• The area under the curve should not be less than 0.055 m rad up to 30 degrees; not less than 0.09 m rad up to 40 degrees or up to the angle at which there are openings permitting flooding; not less than 0.03 m rad between these two angles.
• The GZ must be greater than 0.20 m at 30 degrees.
• The maximum GZ must occur at an angle greater than 30 degrees.
• The metacentric height, GM, must be at least 0.15 m. A reduction to 0.05 m may be permitted for ships with timber deck cargo.
When designing a new ship these standards can be met reasonably easily. They are minima and it is wise to exceed them subject to not creating too stiff a ship. This will allow for changes to the rules or to the ship during its life. Higher intact stability is required in some ships, for instance those carrying grain. This reflects the fact that grain can move, presenting a situation similar to a free surface.