By
Markus Aarnio |
In theory, ship stability is straightforward. In reality, it means two things. Intact stability refers to how safely the ship floats in all weather and operational conditions it may encounter. Damage stability refers to the ship’s state following a damage to her hull – she should be able to float safely enough to provide a secure evacuation. In practice, adequate stability is maintained by the ship’s officers ensuring that the metacentric height (GM) is always above the intact stability and damage stability limit curves, which is calculated by a loading computer.
When a ship is delivered from a shipyard, she should have at least some stability margin. However, the situation changes as the ship ages and gains weight. Maritime regulations demand that a ship is weighed every five years in a lightweight survey, which measures the ship’s weight when empty of tank contents, passengers, crew and provisions. If the results show that the lightweight has increased by more than 2 per cent in that time, then the centre of gravity must also be measured in an inclining test.
The first lightweight survey is commonly based on tests carried out before the ship is complete. These missing weights are therefore estimated but regularly fall on the low side, resulting in a ‘weight gain’ later. The weight of ships increases for many other reasons, including modifications, technical upgrades, painting, dust and dirt accumulation, and even humidity retained in insulation materials. In addition, lightweight surveys and inclining tests are subject to inaccuracy and are often done in less than optimal conditions. A one-centimetre error in a draft reading – which is easily done when read in choppy water – can mean a 100-metric-tonne inaccuracy in lightweight.
As the lightweight rises, the centre of gravity increases, which makes it difficult to maintain adequate stability. Sailing below stability limits is not an option and reducing weight is usually impossible. So how can this be addressed?
The traditional solution is to use a permanent ballast; putting something heavy in the bottom of the ship, or maintaining high deadweight continuously (for example, keeping high levels of fuel in tanks). However, a fixed ballast is not a good solution as it can increase the draft by too much, add to fuel consumption and carbon emissions, and reduce operating range.
Fortunately, there are alternatives. One option is to lower the stability limit curves. Damage stability can be improved by modifying the watertight integrity. Even small changes can help, such as adding a sill on a staircase opening going down from the bulkhead deck. Today, regulations allow for flooding simulations to replace traditional probability calculations. A computer simulation alone can document improvements in stability margins, without having to make physical modifications.
One method often used to enhance stability is by reducing the free surface effect. If the ship has large and wide tanks, splitting them longitudinally will help. Some ships’ tanks can be difficult to empty or fill completely; fixing this will help to reduce the free surface effect.
The steps above can be effective, but the gains are usually quite small. If more significant improvements are needed, the ship’s hull itself can be modified. There are two ways of doing this: add sponsons to the ship’s sides or a sponson-ducktail to the stern. Side-mounted sponsons provide an effective way to improve stability but they also make the ship wider, which can be a problem when entering ports or canals. Sponsons on the side are also visible, which might be undesirable.
The sponson-ducktail may be a better option as it does not affect the width or aesthetic appearance of the ship. A large steel structure is added to the stern which extends below and above the waterline. The addition significantly increases the GM and is environmentally friendly, which means there is no fuel cost penalty. And it can even increase the top speed due to a longer waterline. If an architect is involved in its design, a sponson-ducktail can also make the ship look better. Of course, this type of project is vast and needs proper planning as it affects stability calculations, longitudinal strength, the bilge system, shell penetrations, gross tonnage and more.
Ever-increasing ship weights can make stability maintenance a headache and conversion plans must often be scrapped due to insufficient stability margins. However, as shown above, there are ways to improve stability which do not negatively impact a vessel’s performance.
Markus Aarnio is the chief naval architect at Foreship
This article was first published in the Spring/Summer 2020 issue of Cruise & Ferry Review. All information was correct at the time of printing, but may since have changed.
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