But in this case now it is repeating after 8 seconds or whatever seconds, because you are now going into it therefore, the ship gives a kind of if I think a ship getting a push is earlier it was getting a push at every 10 seconds, but now it is going to get a push at every 8 seconds.
This is what we call encounter frequency, you take another case of opposite case the waves are moving and you are also moving. In fact, if you know I have just draw this outside, say if may happen that the wave speed and your speed is exactly same. If you stand here you will you will not feel that you are moving there is any wave, you will think that you are stationary because, you look at outside and you see that the same crest is right here I am moving along with that.