Shallow-Water Performance Prediction - Hybrid Approach of Model Testing and CFD Calculations

There are limitations in the performance predictions for shallow-water ships that normally operate at relatively high speeds, using towing tank tests only. These limitations are the result of the limited dimensions and the limiting critical channel speed of the tank. Therefore, in order to provide a complete performance prediction for a ferry, designed to operate in shallow-water at relatively high speed, MARIN has performed both tank tests and CFD calculations. In order to be able to use the CFD model, it was firstly validated by comparing it with the towing tank results. Hereafter, CFD calculations have been performed at full scale with and without a restriction in channel width, in order to provide insights on the scaling and channel width effects. The results of the CFD calculations with an unrestricted channel width were then compared to the tank tests results, which were corrected for the tank wall effects and extrapolated to full scale. Doing so, a good agreement was found for operating speeds below the critical tank speed. Above critical tank speed, a convergence of the CFD calculations for shallow water of unrestricted width could be obtained, as long as no local critical flows occurred. These critical flows did not only occur at a critical Froude depth number FnH = 1, but also just before, thus limiting the maximum operating speed of the ferry. It appeared that the hull shape contributed substantially to these speed limitations, triggering local critical flows before the critical Froude depth number was reached.