In-depth study of shallow water

A number of studies have cast a new light on ship resistance in shallow water. While previously, tests in shallow-water basins were essentially analysed in the same way as those in deep water, it is now understood that there are important differences. A more complete understanding of shallow water ship hydrodynamics has been gained, and new procedures have been developed.

While the Shallow Water Basin has a substantial width of 16 m, this still affects the measurements far more than in deep water. No method was available to correct for this, i.e. to translate the model resistance in the tank to that in a waterway of unlimited width and equal depth. By analysing the flow field from several computations, the nature of the tank wall effect was established, and a new theoretical method developed [1]

A . It requires Hoyte Raven

making a single potential-flow computation; evaluation of some fluxes from the result, and the solution of an algebraic equation to obtain corrected model speeds. Thus the measured resistance points are shifted to a slightly higher speed by an amount that depends on water depth, speed and hull form. It then appears that the limited tank width exaggerated the apparent water depth dependence: after the correction, the true water depth effect appears to be a lot smaller. The second figure, an example of older model tests for a ferry, illustrates the significance of the correction.

first step we introduced in 2011, was to correct for the effect of the limited width of the model basin.

Viscous resistance increase But there is another important aspect. Model tests are ‘extrapolated’ to full scale to derive a ship performance prediction. The straightforward application of common model-to-ship extrapolation methods would include the shallow-water resistance increase entirely in the ‘wave’ or ‘residual’ resistance component, which is assumed equal for model and ship. But much of the resistance increase in shallow water is actually viscous resistance. Computational studies [2]

have indicated that this viscous

resistance increase is in most cases a similar percentage for model and ship, and should be included in an increase of the form factor. This is the method now applied at MARIN, and also this reduces the assumed water-depth dependence of the ship resistance. Both steps have substantially improved the power predictions for ships in shallow water.

Speed trials Incipient shallow-water effects may also occur in ship speed trials. Usually, a contract speed at a given power is specified for deep water. But for large or fast ships, the actual water depth at the

22 report

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