Cruise and Ferry Review - Summer/Spring 2020

1 5 5 A sponson-ducktail can improve a ship’s stability and its environmental impact 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. CFR Markus Aarnio is the chief naval architect at Foreship “As the lightweight rises, the centre of gravity increases, which makes it difficult to maintain adequate stability”