The choice of tank width depends on the proposed model tests. The most straightforward tank is a single paddle in a narrow flume that represents a 2D slice, with the model fully blocking the width of the tank. This type of model is relatively easy to analyse because the waves and flow act in a plane. Visibility is excellent and models are readily accessible. It is a very good and economic tank for early investigations. A slightly wider tank with a single paddle can have a 3D model subjected to long-crested waves that pass round the sides so that 3D edge effects can be observed. The main difficulty is that as the width increases the frequency of the resonant cross wave becomes very close to the working frequency of the tank. For example a 0.7m deep tank 1.2m wide will have a cross wave of 0.78Hz. Having multiple force-absorbing paddles in such a tank can help to prevent cross waves forming by absorbing the unwanted wave energy. The most realistic mixed seas, in the open ocean, have to be modeled in a wide tank with multiple individually controlled paddles. Software control of the paddles will allow a full range of waves and wave spectra to be generated.
The width of the tank depends on model width and the angle of waves required on either side. For a line of paddles the angular spread is limited by the angle from the model to the tip of the line of straight paddles. One way to increase the angular capability of a tank is to build the bank of paddles in a curve, or on multiple sides of the tank. Another is to use the Dalrymple method to use reflected waves from the tank sides to generate angled waves anywhere along the length of the tank. The Dalrymple method works well when the tank width is at least one and a half times the wavelength of the required wave.
The tank has to have enough length to allow for three distinct areas. First there is the paddle and enough space for the evanescent waves to decay. Waves from a well-controlled paddle need to travel approximately twice the hinge depth of the paddle to become fully developed. The model zone depends on the size and motion of the model. Towing tanks are the extreme example where the length has to be sufficient to allow the carriage to accelerate, run and then slow down. For wide tanks the combination of width and length determines the angle of waves that approach the model. Finally there is the wave absorbing beach which has to be at least half the length of the design wavelength to achieve 90% absorption.