The Effects of Seiche on Tank Tests.

Abstract

In the towing tank, seiche with the amplitude of several millimeters is frequently observed. In this report, some characteristics of the seiche concerning its development and decay, the effect of flow and inclination of surface due to seiche on the resistance and propulsion tests are discussed. Seiche in the towing tank is generated from many causes. Model running and launching into the trimming tank, in the ordinary testing practice, are the most remarkable causes. The maximum double amplitude of seiche in the ordinary tests is possible to increase to about 5 millimeters. Seiche in the towing tank is usually composed of one-node basic oscillation and other multi-node oscillations which decay rapidly. The one-node oscillation remains for a long time. Half-time period of the decay of the seiche of lengthwise one node is about 6 hours in this tank. The fluctuation of resistance due to seiche is calculated and compared with the experiments. The agreement between the calculation and experiment is fairly good. The fluctuation is composed of a part proportional to the model displacement and the change of the relative speed. It becomes about 50 gr per 1 mm of double amplitutude of seiche. This order of the fluctuation should not be neglected from the view point of accuracy. In the propulsion tests, the calculated speed change of the model is the order of the speed of water particle due to seiche. The maximum speed change is about 1.5 mm/sec per 1 mm of double amplitude of seiche.