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

Abstract

This paper covers an extension of the study of Doctors et al. (J Ship Res 52(4):263-273, 2008) on oscillations in wave resistance during the constant-velocity phase of a towing-tank resistance test on a ship model to the case of relatively shallow water. We demonstrate here that the unsteady effects are very prominent and that it is essentially impossible to achieve a steady-state resistance curve in a towing tank of typical proportions for a water-depth-tomodel-length ratio of 0.25. This statement is particularly true in the speed region near a depth Froude number of unity. However, on the positive side, we show here that an application of unsteady linearized wave-resistance theory provides an excellent prediction of the measured total resistance, when one accounts for the form factor in the usual manner. Finally, a simple application of the results to the planning and analysis of towing-tank tests is presented

Full-scale resistance prediction in finite waters: A study using computational fluid dynamics simulations, model test experiments and sea trial measurements

“…It was shown that the amplitude of this wave decays at a certain rate. 6 More recently, Day et al 7 investigated the effect of shallow water on the oscillation period and…”

On the scale effects of resistance model tests of high-speed monohulls

“…The tank length along which the model can travel in the constant speed phase of the test would be considerably less for larger model sizes. It is evident that all of these issues would result in inaccuracy of model test results in the case of employing large high-speed craft models [2]. Scale effects of the high-speed monohulls' dynamic lift in transom regions is another source of error especially for the planing hulls [3].…”

Energy-efficient large medium-speed catamarans: Hull form design by full-scale CFD simulations

“…It was shown that the amplitude of this wave decays at a certain rate (Wehausen, 1964). More recently, Day et al (2009) investigated the effect of shallow water on the oscillation period and derived a correction for the motion period from model test experiments using a Wigley hull that depends on the depth Froude number (F r h ) which is applicable for F r h > 0.2:…”

“…The motion period increases in shallow water with increasing F r h and reaches infinity at F r h = 1, because the model speed and the phase velocity of the wave created by the disturbance are identical. Day et al (2009) advised that this pitch motion may influence the resistance prediction in a towing tank of finite length, because an integer number of motion cycles need to be resolved to determine a reliable average of the measured resistance force. Furthermore they point out that the decay rate of the oscillations in shallow water is significantly smaller than that in deep water.…”

“…However, unsteady finite water effects lead to an increase in the period of oscillations in resistance and running attitude, namely sinkage and trim. They are known to occur in model testing and when mathematically describing the transient flow around the vessel (Wehausen, 1964;Day et al, 2009). The oscillation period is dictated by the towing speed, but it can grow in shallow water so that less than one oscillation cycle may be recorded within one run which leads to inaccurate results when averaging the transient data record.…”