Propeller walk
Propeller walk is the term for a propellers tendency to rotate a boat as well as accelerating it forwards or backwards.
A right-handed propeller (which rotates clockwise when in forward gear) will tend to push the aft of the boat to starboard. When in reverse gear, the effect will be much greater and opposite. A right-handed propeller will now push the aft of the boat to port.
Knowing of and understanding propeller walk is important when maneuvering in small spaces. It can be used to ones advantage while mooring off, or it can complicate a maneuver if the effect works against you.
Propeller walk is caused by a different thrust delivered by each half of the propeller. If the driveshaft is placed in the water under an angle, the tips of the blades will travel unequal distances through the water, depending on if they are on the port or starboard side of the propeller. The difference in distance leads to a difference in thrust delivered and thus to a net force, perpendicular to the movement of the boat. The effect is usually most notable when the propeller is in reverse gear and it is influenced by the angle of the driveshaft and the size of the propeller.
Other terms for propeller walk are propeller effect, asymmetric thrust and asymmetric blade effect