Hydrodynamic impact response, a flexible view

The popularity of high-speed craft is steadily increasing. Until now, much attention has been focussed on the hydrodynamic aspects of these craft. The structural design of these vessels is usually considered in a quasi static sense. However, due to the requirement of light ship structures, fast ship tend to be flexible. This flexibility combined with high service speeds and high propulsion thrusts, causes a dynamic response to occur rather than a static. Moreover due to high speeds and high propulsion powers, the probability of impact by waves (slamming and wave slap) increases significantly. Therefore the consequences in terms of both global and local huil damage become more serious. A European consortium of shipyards, engineering companies, classification societies and research organisations has carried out the project SEAWORTH, aiming at a reliable prediction of loads due to slamming and the consequential structural response. In SEAWORTH, the probability of slamming as well as the subsequent loading and response are investigated. One of the contributions of TNO in this project has been to carry out full-scale slamming tests and to develop a prototype of a slam gauge to be used in model tests. In the adopted approach, a slam is seen as an impact introducing vibrations in the ship structure. The response of a shell subjected to a slam can be described by a set of natural vibration modes. It is shown that the natural frequencies yield a base for damage prediction. Based on this approach, a model-scale transducer is designed and built to measure the impact on a model in a seaway. It is argued that impact velocity is an adequate parameter for quantifying impact. In this paper, the approach is explained, including the results of full-scale slam tests and the results of scaled slam tests with the model-scale transducer.