Boat handling expertly under power requires an understanding the propeller effect called [ prop walk or propeller walk ] . To explain how prop walk occurs, the lower blade of a turning propeller rotates in denser water than the upper blade (as water density increases with depth) thus creating a paddle wheel effect. This has a tendency to push the stern sideways in the direction or the propeller rotation. A right-handed or clockwise-rotating propeller tends to "walk" the stern to starboard when moving forwards. The direction of rotation reverses in reverse gear when the yacht is going astern, with the propeller "walking" the stern to port.