On the Structural Design of Bilge Keels

The reduction of response motion experienced by monohull marine structures through energy dissipation performed by passive stabilisers such as bilge keels is an inexpensive system, when compared to the active motion reduction systems such as active stabilisers or active stabilising fluid tanks. This is the main reason why, despite the obvious advances in motion reduction and control, bilge keels are still designed, produced and fitted to the most recent ships and other marine structures produced. The design philosophy of passive roll stabilisers such as bilge keels has been, and still is, often based on empirical or design codes that define a standard structural arrangement, which is then sized in accordance with the specific characteristics of the vessel. This paper discusses the main internal structural arrangement configurations which may be adopted for bilge keel design, highlights the critical details which particular attention has to be paid to, presents the main steps involved in the sizing of the structural elements, and evaluates the more advantageous configurations with respect to performance vs. production cost. In what respects internal arrangement, the widest possible range of possibilities is considered, from the simple and fairly inefficient flat bar configuration, to the highly complex high performance diaphragm based designs. The advantages, disadvantages, estimated design and production efforts are presented and discussed. This work aims at providing a comprehensive review and a systematisation of the design principles applied for these structural components, envisaging to providing a simple and effective guide for the global and detailed design of their structures. Conclusions and further research work are drawn regarding the results obtained and the best solutions proposed.