Showing papers in "Journal of Ship Research in 1964"

On Testing Materials for Cavitation Damage Resistance

Abstract: : Recent experiments with a magnetostriction apparatus show that cavitation damage rate is time dependent. This is confirmed by an analysis of the experimental data obtained in various earlier investigations. There are four zones of damage rate with respect to testing time namely: (1) Incubation, (2) Accumulation, (3) Attenuation, and (4) Steady State. In the fourth, or Steady State Zone, the damage rate varies as the square of the amplitude of oscillation within the range tested for water at 80 F. The damage rate increases with frequency and then decreases with increasing frequency. The depth of liquid in the beaker, the beaker diameter and the depth of immersion of the specimen do not seem to affect the damage rate substantially. The average depth of erosion is independent of the diameter of the specimen. Based on these experimental results, certain recommendations are made for testing materials for cavitation damage resistance. (Author)

Estimation of stability derivatives and indices of various ship forms, and comparison with experimental results

Abstract: The analytical method of reference for estimating stability derivatives, and hence stability on course, which combines Albring's empirical modifications of simplified flow theory with low aspect-ratio wing theory, is extended to take into consideration the effects on course stability of higher aspect-ratio fins as well. The method, which has been applied in the earlier report to a family of eight hulls of 0.5 block coefficient, is tested further by application to eight Series 60 forms differing in block coefficient as well as in beam, draft, and displacement--with and without rudders; to an extreme vee modification of a Series 60 model; and to three other forms--a Mariner Class model, a destroyer, and a hopper dredge. Comparison with experimental results shows that the values of stability derivatives and indices determined by the analytical method are of the right orders of magnitude and indicate correct trends. Application to a variety of ship forms has demonstrated that the method can predict relative effects of changes in the geometry of a ship form, as well as the effects of changes in skeg and rudder area.

Supercavitating Propellers—Momentum Theory

Abstract: : Based entirely on momentum and other simple considerations a reasonably complete picture of the one dimensional pressure and velocity fields associated with heavily supercavitating propellers and drag discs was constructed. It is shown that the flow is very often retarded while approaching a heavily supercavitating propeller, so that the so-called ideal efficiency of such a propeller takes on values in excess of unity. The ideal efficiency actually increases with decreasing blade cavitational efficiency; however, the net efficiency at the same time decreases. This retardation causes a reduction of thrust deduction, or even a change in its sign. Charts are provided from which the inflow speed may readily be estimated. The effect of water tunnel boundaries are discussed, particularly the phenomena of choking.