Towed Ship Motion Test in Directional Spectrum Waves in a Long Tank (Part 2)

Abstract

As a continuation to our previuos report on tank testing results of ship motions in directional spectrum waves this paper presents further test results under 'following sea' condition. Uniform irregular waves with arbitrary directional spectrum have been generated in the whole area of towing tank of Yokohama National University by the newly installed snake type wave maker and running tests have been carried out at Froude numbers 0.2 and 0.275. The experimental data have been analyzed at encounter frequency base using both Fast Fourier Transformation (FFT) and Maximum Entropy Method (MEM) to obtain the power spectrum of motions and the results are presented along with the theoretical estimates. Directional transfer functions of motions have been calculated using well known New Strip Method (NSM) and analyzed directional wave spectra obtained from experimental data by Maximum Likelihood Method (MLM) have been used to estimate theorectical motion spectra. The compared results show fairly good agreement. This paper also presents short term prediction of motions calculated for both 'head sea' and 'following sea' conditions over venous range of wave periods.