In case you cannot take the ship out and you have to do this in shallow water. Then you must be able to find out to calculate and show that since the water depth is low, we get a particular speed, and if the water was deep, we would have got so and so speed. That extrapolated speed to deep water should be satisfying the contractual speed. The problem there comes that if the water is really low, then the extrapolation will not give the speed because of the uncertainties involved which I have just mentioned. So, sometimes it may be necessary to do model tests in shallow water.