DNV is currently running a major research programme to investigate the effects of hull flexibility on various ships types.

The purpose of this research project is to identify and quantify influence factors, which are not normally accounted for in classical shaft alignment analyses, and to develop analytical methods capable of accurately predicting these effects. The project has been running since 2001 and is due for completion in 2008.

During the design and class phase of a vessel, the hull and the machinery is usually treated as two separate disciplines, where no interaction between the disciplines is considered. In the past, consideration of this interaction was perhaps not relevant. The hull’s stiffness leads to small hull deflections due to changing vessel operating conditions. However, as more advanced calculation tools and new materials have become available, ship designers have optimised the plate thickness of the hulls.

More flexible hulls lead to large hull deflections due to changing vessel operating conditions. In many cases, the propulsion shafting of a vessel is extremely sensitive to variations in the position of the shaft bearings (shaft alignment). The position of a bearing is determined by the shape of the hull. For alignment-sensitive propulsion shafting, which is installed in a flexible hull, failure to account for these effects during the design phase may lead to damages in the propulsion line. New calculation methods have also allowed the ship designers to optimise the hull shape with respect to hydro - dynamics. The shape of the hull will determine the wake field experienced by the propeller and therefore also the propeller- generated forces.

These forces will have a significant effect on the shaft/bearing interaction of the aft-most bearings, and the magnitude and direction of the propeller forces will depend on the operating condition of the vessel. Failure to account for the hull shape dependent propeller forces may therefore also lead to damages in the propulsion line. For a safe and reliable design of the propulsion line, it is no longer possible to ignore the influence of the hull flexibility and shape in the design phase.