Hydroelastic Inflatable Boats: A Possible Design Methodology

1. Fluid Structure Interactions Research Group Hydroelastic Inflatable Boats: A Possible Design Methodology P.K.Halswell1, P.A.Wilson1, D.J.Taunton1 and S.Austen2 1 Faculty of Engineering and the Environment; 2 Royal National Lifeboat Institution; ph3e09@soton.ac.uk • 1. Introduction • The Royal National Lifeboat Institution (RNLI) use a 5 m inflatable boat (IB), called the IB1, for search and rescues in littoral waters. Anecdotal evidence has indicated, but not proved, that the hydroelasticity of the IB1 improves the performance, especially in waves or surf. The RNLI require scientifically-based design guidelines for the IB. Current designs are based on designers’ experience and trial and error for a rigid vessel. Project Aims • Divide the entirely hydroelastic boat into manageable hydroelastic problems • Create a design methodology for the manageable hydroelastic problems • Develop design tools for each hydroelastic problem 2. Current Aspects of Hydroelasticity The majority of ships are considered rigid when they are designed. If hydroelasticity is considered during the design stage it is primarily used to calculate the stresses and strain within the structure so that the structural weight can be minimised [Hirdaris and Temarel (2009)]. Hydroelasticity consists of two main research areas: global hydroelasticity and slamming (or local) hydroelasticity. Global hydroelasticity studies the longitudinal bending and torsional twisting within the boat [Bishop and Price (1979)], see figure 2. Hydroelastic slamming investigates the local deformation caused by a wedge impacting a free surface [Faltinsen (1997)], see figure 3. These provide two manageable hydroelastic problems. Figure 1: In the foreground is the RNLI IB1 and in the background is the RNLI Atlantic 85 RIB. [Website] Figure 2: Global hydroelasticity Figure 3: Hydroelastic Slamming 3. New Aspect of Hydroelasticity The IB1 has a fabric hull which as able to deform when the vessel is planing, see figure 4. This defines a practical hydroelastic problem called a hydroelastic planing surface. Rigid structure Elastic structure Water surface Figure 4: An underwater photograph of the hull deforming at 19 knots [Dand et al (2008)] References: BISHOP, R. E. D., PRICE, W. G., 1979, Hydroelasticity of ships, Cambridge University Press DAND, I. W., AUSTEN, S., BARNES, J., The speed of fast Inflatable Lifeboats, Int. Journal of Small Craft Technology Vol. 150 Part B2, pp. 23-32, 2008 FALTINSEN, O. M., 1997, The effect of hydroelasticity on ship slamming, Phil. Trans. of the Royal Soc. A: Mathematical, Physical and Engineering Sciences 355, (1724), pp. 575-591 HIRDARIS, S. E., TEMAREL, P., 2009, Hydroelasticity of ships: recent advances and future trends, Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment 223, (3), pp. 305-330 WEBSITE viewed on 22/08/2011, image of IB1 and Atlantic 85. http://www.rnlisunderland.org/information/lifeboats/pg18.html Hydroelastic Slamming 4. Design Methodology Three practical hydroelastic problems have now been described (global hydroelasticity, hydroelasticity slamming and hydroelastic planing surface) but how do they link them together? A key design tool in naval architecture is strip theory. Strip theory involves solving the forces on transverse slices of the boat and then integrating over the length to find the forces and moments over the length of the boat. Strip theory provides an order to studied the aspects of hydroelasticity. This is because the hydroelastic slamming aspect can be compared to the transverse slices of strip theory. Then the forces on the hydroelastic transverse slices can be integrated to find the forces on a hydroelastic planing surface. Finally, wave loads can be applied to the forces on a hydroelastic planing surface to solve the global hydroelastic forces. 5. Conclusion An entirely hydroelastic boat has been subdivided into manageable hydroelastic problems. The hydroelastic problems have been linked together using strip theory. Further work will include the development of design tools for each hydroelastic aspect. Hydroelastic Planing Surface Global Hydroelasticity Strip Theory Acknowledgement: This project is supported and funded by the University of Southampton, EPSRC and the RNLI FSI Away Day 2012