Study on the Wave-Induced Steady Force and Moment

Abstract

Recently calculation formulae for the wave-induced steady sway force and yaw moment were derived by Kashiwagi in the same fashion as Maruo's added-resistance formula ; the only unknown in these formulae is the Kochin function equivalent to the ship-generated progressive waves far from the ship. This paper presents a new method of computing the Kochin function. It is based on the unified slender-ship theory and thus able to incorporate rationally the 3-D and forward-speed effects on the longitudinal source distribution representing ship's disturbance in the far field. Thus there is no need to displace downward the vertical position of source distribution ; the pragmatic treatment of which has been used in a conventional strip method to weaken the two-dimensionally determined singularities. The depthward displacement has also been said to ensure the convergence of the infinite integral in the added-resistance formula. This paper shows that the infinite integral can be evaluated precisely with singularities placed on the free surface in terms of only the Fourier-series coefficients of the longitudinal source distribution.Computations are performed and compared with existing data for SR108 container ship and a blunt ship. Except for the underprediction of the added resistance in shorter wavelength region of head-sea conditions, the new method agrees well with experiments for all angles of incidence, and thus improves considerably upon the strip method.