Unsteady effects during resistance tests on a ship model in a towing tank

It is known that there are oscillations in the wave resistance during the constantvelocity phase of a towing-tank resistance test on a ship model. In this work, the unsteady thin-ship resistance theory has been applied to this case. The results have been compared with experiment data obtained using a towing carriage the velocity history of which can be programmed. It is demonstrated here that generally excellent correlation exists between the theory and the experiments. In particular, one can predict the influence of Froude number, rate of acceleration, and type of smoothing of the acceleration on the characteristics of the oscillations. These characteristics include the amplitude, rate of decay, frequency, and phasing of the oscillations in the curve of wave resistance versus time

Hydrodynamics of High-Speed Small Craft

These notes cover the hydrodynamic aspects of air-cushion vehicles, hydrofoil boats, planing craft, and water propellers. In each case, the emphasis has been placed on a physical description of the problem, together with a theoretical development of the results. Comparison of the available theories with experiments has been supplied where possible.http://deepblue.lib.umich.edu/bitstream/2027.42/91685/1/Publication_No_292.pd

Reanalysis of the Sydney Harbor RiverCat Ferry

In this paper, we revisit the hydrodynamics supporting the design and development of the RiverCat class of catamaran ferries operating in Sydney Harbor since 1991. More advanced software is used here. This software accounts for the hydrodynamics of the transom demisterns that experience partial or full ventilation, depending on the vessel speed. This ventilation gives rise to the hydrostatic drag, which adds to the total drag of the vessel. The presence of the transom also creates a hollow in the water. This hollow causes an effective hydrodynamic lengthening of the vessel, which leads to a reduction in the wave resistance. Hence, a detailed analysis is required in order to optimize the size of the transom. It is demonstrated that the drag of the vessel and the wave generation can be predicted with good accuracy. Finally, the software is also used to optimize the vessel further by means of affine transformations of the hull geometry

The Wave Resistance of an Air-Cushion Vehicle in Accelerated Motion

This report is concerned with the theoretical wave resistance of an air_cushion vehicle (ACV) traveling over water of uniform finite or infinite depth, in steady or unsteady motion. Referring first to steady motion, it is shown that the unrealistic oscillations in the wave resistance curve at low Froude numbers found by previous workers can be eliminated by using a smoothed out pressure distribution rather than one with sharp edges studied exclusively in the past. The main result of unsteady motion calculations is that the peak wave resistance in shallow water, even in moderately accelerated motion, is appreciably less than the corresponding steady-state value. In fact, cases have been found where an ACV starting from rest under the action of a constant thrust would seem to be unable to cross the critical depth Froude number on the basis of quasi_steady estimates of wave resistance, while the more elaborate unsteady calculations show that it has sufficient power to reach its final supercritical cruising speed. An interesting feature of unsteady motion is that besides wave resistance there is another mechanism transferring energy to the free surface which is here called the dynamic sustention power. Contrary to intuition, the wave resistance in unsteady motion over finite depth sometimes becomes negative at supercritical Froude numbers before finally approaching zero at infinite speed.http://deepblue.lib.umich.edu/bitstream/2027.42/91630/1/Publication_No_099.pd

Wave generation of a compartmented surface-effect ship

A series of carefully controlled experiments on the wave-generation characteristics of a model of a compartmented surface-effect ship has been conducted in a towing tank. Configurations of the model included cases encompassing one subcushion and two subcushions, as well as differing values of the pressures in the subcushions. It was shown that a reduced wave generation in the appropriate Froude number range could be achieved in this manner. Furthermore, a previously developed theory for the wave generation of marine vessels was verified for the model for a Froude number greater than 0.40

Resistance of a ship undergoing oscillatory motion

In this paper, we describe extensions to the research of Doctors, Day and Clelland (2008) and Day, Clelland and Doctors (2008), in which the oscillations in the wave resistance during the constant-velocity phase of a towing-tank resistance test on a ship model were measured and predicted, in the cases of relatively deep and relatively shallow water. In the current study, the ship model was towed with a harmonic velocity component superimposed on the usual constant forward velocity. This work constitutes a ¯rst step in the understanding of the unsteady hydrodynamics of a racing shell (rowing boat). We show here that the unsteady wave resistance varies considerably from the traditional (steady) average value. Indeed, the wave resistance is frequently negative during part of the oscillatory cycle. However, the general effect is an increase in the temporal mean value of the wave resistance; this suggests that every effort should be made to reduce the unsteadiness of the motion. We also demonstrate that the unsteady wave-resistance theory provides an excellent prediction of the measured effects summarized here. These predictions are often within a few percent of the measured values of the resistance