Prediction of Hydrodynamic Impact Pressure Acting on Bow in Rough Sea

Abstract

The characteristics of hydrodynamic impact pressure acting on bow have been investigated. in detail by many researchers. However, since the impact pressure is a nonlinear and complicated phenomenon of very short duration, it is difficult to obtain reproducible data even if the experiment is carried out with extreme care. Accordingly, the quantitative analyses of the effects due to bow flare shape, advance speed, steepness of incident wave and other factors have not been executed with sufficient accuracy.In the meanwhile, the studies on the stochastic properties of the impact pressure have been started recently, and only a few papers were reported. It is because the measurements of impact pressure, impact velocity (relative normal velocity at the moment of impact) and impact angle (relative angle between the hull surface and the wave surface) are difficult, and also the considerably long time is required for the experiment and analysis work in order to collect a large number of data.Therefore, the author carried out the experiments in both regular and irregular waves to clarify the problem mentioned above, applying the skillful technique introduced by S. Takezavva and S. Hasegawa. The displacement effect on bow wave steepness, the frequency of occurrence of impact pressure and the distribution of random variables such as impact pressure, impact velocity and impact angle were determined successfully.Summarizing this study, the following conclusions are obtained.1) The slope of local wave surface at the moment when the impact pressure acts on bow is proportional to the product of maximum slope of incident wave and the bow wave interference factor γ. The interference factor γ was obtained from the test result in short regular waves.2) The frequency of occurrence of impact pressure was determined experimentally as a function of effective wave height, advance speed, relative vertical motion at bow and stem angle to a horizontal plane.3) The probability density of the impact pressure can be represented by the log-normal distibution. The impact velocity and the impact angle can also be explained by the log-normal distribution and by the normal distribution respectively. The relationship among three random variables are made clear theoretically and experimentally.The prediction of impact pressure in rough sea based on the obtained experiment results will be reported in the near future.