배의 표면이 사포처럼 거칠면 물이 선체에 더 잘 들러붙을 것이고 그러면 경계층이 두꺼워져서 배는 움직임에 더 많은 저항을 마주할 것이다.
If the surface of a ship were rough like sandpaper water would stick better to the hull and the boundary layer would be thicker and the boat would face more resistance to motion.