I've seen this question popping up from time to time, especially on various internet public forums. It is rather intriguing because there have been many reports of such an effect. This website tries to explain how this may appear to occur, especially in cases where the boat makes its own "wind". However, the explanation is a bit lacking, so I found another that at least had some figures in it. Still another website that was made for kids may have the best and simplest explanation if you just one a quick and dirty answer.