on the resistance evaluation (Table III). In this connection, it is hard to accept the overestimated resistance coefficient in shallow water case. The authors' second reason (2 under prediction of blockage effect…), if corrected by taking the effect of free surface depression into account, may work out a further resistance increment. So we need a more careful approach.
Propulsion test simulation is, I believe, a challenging problem which seems to be yet at the beginning stage. In this case, to check the dipping at AP (usually measured at this station) seems to be a good index whether the simulation is done well or not. I appreciate very much your comments about this including measured results.
The considered depth Froude number does not yet lead to a considerable widening of the wave contour angle. However, the method is capable of capturing this phenomenon as demonstrated previously for an inland water vessel at depth Froude number 0.9 .
The sinkage is not very strongly affected by either viscous effects or the towing force. However, the remarks of Prof. Kajitani concerning their influence on trim are qualitatively correct. The towing force may be incorporated in the RSM but no information for the experimental condition was available. So we did not include this option. Tentative initial computations for the deep-water case showed only small influence of changing the towing force between still-water line and propeller height. The effect on trim is expected to increase with shallow water. However, as far as pressures on the channel bottom are concerned, the main influence will be sinkage. As the computations are already within the margin of uncertainty of the experiments for sinkage and trim, we did not focus our efforts on a further improvement of the method in the ability to predict trim and sinkage.
We agree with Prof. Kajitani that we should focus instead on improving the resistance prediction which at present does not satisfy our expectations. Small differences of trim and sinkage affect the accuracy of the resistance prediction, especially for shallow water. So, in order to improve the resistance prediction to the required degree, we may need a free-surface RANSE solver with sinkage and trim effect. As the problem of insufficient resistance prediction is shared by many other colleagues, it appears that we will need considerable more shared research worldwide before we see consistently accurate resistance predictions for real ship geometries, especially for the more complicated shallow-water hydrodynamics.
The propulsion test simulations predicted trim and sinkage using the RSM without propeller action, i.e., the same trim and sinkage as for the resistance test. Admittedly this is crude, but it is better than the usual practice of taking the zero-speed design floating condition. We do not have measured results in addition to the ones published with the kind permission of the Duisburg Towing Tank. We share Prof. Kajitani's wish for further details to validate our computational procedures and hope that maybe ITTC may provide in the future such data.