That means if there is a crest at the stern, the water velocity will generally have a movement to the forward particle velocity. In a trough, it will be just the other way. The water particle will move like this. Therefore, the velocity will be backwards. Now, suppose there was not a crest, but there was a trough at the this thing. I have just drawn a
single wave, wave, length equal to ship length, but suppose it was not like that. Suppose the second wave appeared here and the trough appeared there, then a wake would have been created which would have been supporting the movement of water.