Through-Life Solutions for Cruise Ship Stability
By Markus Aarnio
Ship stability means two things. First, the ship must float and be safe enough in all weathers and in all the operational conditions it will encounter; this is called intact stability. Second, the ship has to float and enable safe evacuation if there is hull damage; this is called damage stability.
In practice, adequate stability is maintained by the ship’s officers taking care that the metacentric height (GM) is always above the intact stability and damage stability limit curves. Today, a loading computer is used to calculate this.
When a ship is delivered from a shipyard, its stability margin depends on the accuracy of the shipyard’s weight calculation and on the owner’s requirements, among other things.
However, as the ship ages, the situation changes: Ships continuously gain weight. In fact, maritime regulations require that a ship is weighed every five years. This is called the lightweight survey, and if the results show that the lightweight (the weight of the ship when empty and without any tank contents, passengers, crew, stores, etc.) has increased by more than 2 percent, then the center of gravity also has to be measured in what is called an inclining test.
There are several reasons for the increasing weight. Usually, the first lightweight survey is based on tests done on the ship when it has not yet been fully completed and all of the owner’s materials are not yet on board; these missing weights need to be estimated, but, commonly, this estimation tends toward a lightweight that is too low. Other reasons for weight gains include conversion, modifications and technical upgrades done during the ship’s life. Painting, dust and dirt accumulation in inaccessible locations, deposits in the ship’s system and tanks, and even humidity retained in the insulation materials can also add to heaviness. The paint alone on a 15-year-old cruise ship, for example, can easily add 100 tons to the weight of the ship on delivery.
In addition, lightweight surveys and inclining tests are subject to inaccuracy and are often done in less than optimal conditions: 1-cm error in a draft reading–which is easily possible when draft is not being read in calm water–can mean a 100-ton inaccuracy in lightweight.
When the ship’s lightweight continuously rises and, as is often the case, the center of gravity increases at the same time, maintaining adequate stability may become a challenge. As sailing below stability limits is not an option and reducing weight is usually impossible, something will have to be done.
The traditional way to improve the situation is to use permanent ballast; this would mean either adding something heavy low down on the ship (for example, concrete or some other high-density material, or filling double bottom spaces permanently with freshwater) or maintaining high deadweight continuously (for example, by maintaining fuel or potable water in tanks at high levels). Whichever of these options is chosen, fixed ballast is not a good solution: Adding permanent ballast can increase the draft by too much; adding weight also increases fuel consumption and CO2 emissions, while continuously high fuel levels imply shorter bunkering intervals and a shorter operating range.
Fortunately, there are better ways to improve the situation than simply adding weight to the ship’s bottom. One option is to lower the stability limit curves. Damage stability can be improved, for example, by modifying the watertight integrity: This can be achieved by adding watertight doors and bulkheads or cross- and down-flooding arrangements. Even small changes, like adding a sill on a staircase opening going down from the bulkhead deck, can help. Today, regulations allow for flooding simulations to replace traditional standard probability calculations; a computer simulation alone can document improvements in stability margins, without having to make real vessel modifications.
One method often used to improve stability is to improve the GM by reducing free surface effect: If the ship has large and wide tanks, splitting these longitudinally will help. In some ships, for example, there can be tanks which are for some reason difficult to empty or fill completely; fixing this will help to reduce the free surface effect as well.
The steps above can be effective, but the gains are usually quite small. If more significant improvements are needed, for example, in the case of a larger conversion project, the ship’s hull itself will need to be modified. There are two ways of doing this: A sponson-ducktail can be added in the stern, or sponsons can be added on the ship sides. Sponsons added to the side of the vessel provide a very effective way of improving stability but they also make the ship wider, which can be a problem if port or canal limits constrain the beam of the ship. In this case, the sponson is also very visible, which might not be to the liking of the shipowner.
The alternative sponson-ducktail does not make the ship wider and is also a better option from the aesthetic viewpoint. Here, a large–typically some hundreds of tons–steel structure is added to the stern, which extends both below and above the waterline. The addition has the effect of significantly increasing the GM; even if the stability limit curves usually increase at the same time, the net gain in stability will typically provide adequate margins for the rest of the ship’s service life.
A properly designed sponson-ducktail is also environmentally friendly; in practice, there is no fuel cost penalty, and the top speed of the vessel can sometimes even be increased due to the ship’s longer waterline.
If an architect is involved in its design, a sponson-ducktail can even make the ship better looking.
Of course, adding a sponson-ducktail is a big project that needs to be planned properly: It affects stability calculations, longitudinal strength, the bilge system, shell penetrations, gross tonnage, etc.
Ever-increasing ship weights can sometimes make maintaining stability a headache, and conversion plans to add to a ship’s earning potential can sometimes be scrapped due to insufficient stability margins. However, as shown above, there are ways to improve stability, even by significant margins, which do not have negative effects on vessel performance.