 A key ability for a ship-handler is a good understanding of how the wind, tide and swell, the passage of other vessels, as well as the shape of the seabed, will affect a vessel's movement, which, together with an understanding of a specific vessel's performance, should allow that vessel a safe passage.Fundamental to low-speed maneuvering is an understanding of the configuration and handedness of the propeller(s).An effect known as propeller walk will kick the stern of the vessel to port or starboard depending on the configuration and the type of propeller when large variations on propeller rotation speed or changes of propeller rotation direction take place.