Model scale prediction of seakeeping and global bending moment on a high speed craft

Abstract

Estimating the forces acting on a high speed craft for structural assessment purposes is challenging due to the complex combination of hydrodynamic loads. These include wave pressures, slamming pressures and their load effects at a global ‘hull girder’ level. An experimental approach to validate the use of established strip theory methods for predicting the hydrodynamic wave pressure loads is presented here. A series of regular wave seakeeping tests on a small scale segmented hull model of a high speed craft were completed and compared to equivalent seakeeping calculations. Experiments were conducted with a 1:13.5 scale hull model of a 17-metre all weather lifeboat, which is typical of a high speed watercraft capable of operating in severe weather at exposed locations. The model is segmented into four sections, rigidly held together by an instrumented backbone beam which can measure shear force and bending moments at the segmentation points. The model was tested over a full range of operating speeds and wave encounter frequencies. Results are compared with two strip theory calculations, the first using a zero speed Green function and the second using a Rankine source method with a forward speed correction term. It is shown that the Green function strip theory provides better correlation at lower speeds (up to a Froude number of 0.4) and the Rankine source method is more appropriate at higher speeds. The experiments generally show good equivalence to calculations and demonstrate that strip theory gives reasonable predictions of the global bending moment assuming linearity with the wave height. Equivalently, this suggests that strip theory is an appropriate method to predict the hydrodynamic wave induced load component in displacement and semi-planing regimes. This unlocks the potential for strip theory to be used as part of a complete assessment of the seakeeping loads on a high speed craft by combining the wave induced loads with the other significant load components, which include the local hydrodynamic wave pressures and the transient loads caused by slamming impacts. This may be applied as part of an analysis to determine a craft’s safe operating envelope or for interpreting component loads within full scale trial data.