International Shipbuilding Progress - Volume 52, issue 3

Authors: Garme, Karl

Article Type: Research Article

Abstract: Time-domain simulation, based on 2-dimensional high-frequency added mass formulation of the sectional forces, is a computationally attractive approach to the non-linear behaviour of the planing craft in waves. Nevertheless, the 2-dimensional strip approach, demands for corrections of the lift distribution in order to correctly model the running attitude which is significant for the loads and motions for planing craft in waves. This has traditionally been made by a reduction of the hydrostatic lift and moment with …reference to the geometrical fact that the transom stern is dry. This action improves the simulation of the running attitude in calm water but not necessarily of the dynamic behaviour in waves. The paper proposes a correction operating on both the hydrostatic and the hydrodynamic terms of the load distribution. A reduction function is chosen on the basis of model test measurements of the near-transom pressure, and on published model data on running attitude. The approach is satisfactory validated in calm water, regular and irregular waves. Simulations of model experiments and comparison with measurement time series are presented. The correction improves the simulation in both calm water and in waves for a wider speed range compared to a correction limited to the hydrostatic terms. Show more

Citation: International Shipbuilding Progress, vol. 52, no. 3, pp. 201-230, 2005

Authors: Millward, A. | Brown, G.L.

Article Type: Research Article

Abstract: The High Speed Marine Vehicle Committee of the International Towing Tank Conference has recommended that in tests on models of high speed marine vehicles the actual wetted surface area should be used to calculate the frictional resistance component in the scaling procedure rather than the static wetted surface area used for low speed ships. This recommendation has been made since it has been shown that the wetted surface area can vary significantly from the static wetted …area, particularly at higher speeds and therefore affect the predicted power requirement. The Committee has also recommended that, since there is no universally agreed method of measuring this wetted surface area in model tests, the method of measuring the area should also be given. A new method of measuring the actual wetted surface area has been developed. The method is based on capacitance where the model hull has been given a metallic coating and then an insulating coating so that it effectively becomes one plate of a capacitor with the water of the towing tank or water channel becoming the other plate. The experiments have shown it to be a viable technique. It has the advantage that it can be undertaken at the same time as the standard resistance tests while not adding to the manpower requirement such as that necessary with visual observation of the wetted surface area. Show more

Citation: International Shipbuilding Progress, vol. 52, no. 3, pp. 231-244, 2005

Authors: Brockett, Terry E.

Article Type: Research Article

Abstract: Two aspects of modeling the impact of a characteristic thick boundary layer developing along the inner duct surface with inviscid elements are explored. First, it is argued that the trajectory of shed-vortex filaments adjacent to the walls depends on the boundary-layer speed reduction and the duct contraction selected to achieve decelerating through-flow control. Self induction associated with the forced contraction carries the tip-vortex filaments toward the duct surface, producing locally large surface speeds. This …interaction calculation is combined with other components of a simple inviscid model of ducted-propulsor performance: axisymmetric duct and hub boundary-element approximations coupled with a lifting-line representation of each blade row. Parameters of the overall model are selected to bring about correlation with the circumferential-mean pressure distribution on the duct and component loads acting on the blades of the ERG Pumpjet, a strongly decelerating ducted propulsor. A second impact of the developing axial flow-speed reduction near the passage walls is a local mismatch between the flow angle and rotor surface slope. Parametric numerical estimates using an inviscid lifting-surface design code illustrate the degree of change in the rotor blade-tip geometry needed to achieve flow alignment. The rotor for the ERG Pumpjet has a large-diameter hub with significant crosssection area. An adjustment in shaft thrust to compensate for the increased pressure acting in a hub gap aft of the rotor produces a corrected axial force level more in line with other decelerating ducted propulsors. Show more

Citation: International Shipbuilding Progress, vol. 52, no. 3, pp. 245-271, 2005

Authors: Varyani, K.S. | Pham, X.P. | Olsen, E.O.

Article Type: Research Article

Abstract: This paper briefly reviews the phenomenon of green water faced by container ships and a time-domain method based on strip theory for predicting the occurrence of green water. Verification of this method with experimental data in regular waves is also carried out showing good agreement. A simple CFD hydrodynamic model is developed to represent the green water flow on deck and the parameters of this model are based on experimental data. In order to test the …appropriateness of this model, comparison in terms of horizontal green water load (acting on a vertical structure) and vertical deck pressure is carried out with test data. Having achieved good agreement from this comparison, the research looks into the application of breakwaters in reducing the loading effects of green water. Double-skin breakwaters with and without perforation are simulated facing similar green water condition. Since protected structures can withstand a certain level of load without suffering detrimental effects, perforation is introduced to lessen the green water load that breakwater is to bear. Three systematically varied sizes of perforated holes are simulated and effects are evaluated. The research finally concludes on the performance of double-skin breakwater with/without perforation in reducing green water load and the overall effects of the perforation in balancing this load onto the structure and also the breakwater itself. Show more

Citation: International Shipbuilding Progress, vol. 52, no. 3, pp. 273-292, 2005