Cruise and Ferry Review - Summer/Spring 2020

1 5 4 V IEWPOINT Solving cruise ship stability A cruise ship inevitably gains weight over her lifetime but there are ways to improve stability without impeding vessel performance I n 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% 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 Markus Aarnio: Foreship