# PERTURBATION FORCES ACTING ON A PITCHING SHIP IN REGULAR SHOAL WAVES

Formulas are given for calculating perturbation forces and moments and phase shift with respect to oncoming waves acting regularly lengthwise on a ship in shoal waters. These formulas are derived making the following assumptions: byflow is planar; characteristic velocity is that of wave contour change in the vertical; planar progressive waves strike the ship dead in the water. Linearity of the problem assumes that the total velocity potential is the sum of the potentials of the wave and the velocities caused by the presence of the ship; in calculating the correction factor for extreme vessel settling based on waterline levels, the correction factor is given as a parabola; velocity in the rib cross-section is assumed to be constant over the height; in calculating the apparent mass, the ribs are assumed to be rectangular with a beam/draft ratio of two; effect of the free surface on the apparent mass size is taken from results of solving the wave problem of forced vertical oscillations of a round cylinder on the calm surface of a liquid of limited depth; the effect of the difference between planar and three-dimensional byflow on apparent mass in vertical oscillations is approximated by a coefficient as a function of the vessel length/width ratio. It is noted that the above assumptions are not too coarse, particularly since it is known that in shoal water rib byflow approximates planar byflow, and wave-contour change in the vertical decreases as settling depth increases. The formulas derived give figures for a Todd Series 60 ship 60 percent laden for depth/draft ratios of 4 and 6. Computational results are graphed and compared with experimental data for a basin of unlimited depth. The computations result in the conclusions that 1) in long waves, the pertubation forces and moments in shoal water are greater than in deep water; these forces and moments decrease at wavelengths equalling ship length, and increase again at shorter lengths; 2) in both shoal and deep water, ship-length waves account for 50--70 percent of total perturbation forces and moments; 3) computation of hydrodynamic forces and moments by Krylovs method exaggerates their strength above total.

• Corporate Authors:

• Authors:
• Ankudinov, V K
• Publication Date: 1966

## Filing Info

• Accession Number: 00014980
• Record Type: Publication
• Source Agency: Joint Publications Research Service
• Files: TRIS
• Created Date: May 7 1973 12:00AM