When a ship is sailing, the propeller blades bite more in their lowermost position than in their uppermost position. The resulting side-thrust effect is larger the more shallow the water is (as during harbour manoeuvres). Therefore, a clockwise (looking from aft to fore) rotating propeller will tend to push the stern in the starboard direction, i.e. pushing the stem to port, during ahead running.
When reversing the propeller to astern running, the side-thrust effect is also reversed. Thus, the pilot has to know how the ship reacts in a given situation. It is, therefore, an unwritten law that on a ship fitted with a fixed pitch propeller the propeller is always designed for clockwise rotation when running ahead.
In order to obtain the same side-thrust on ships fitted with a controllable pitch propeller, when reversing to astern, CP propellers are normally designed for anti-clockwise rotation when sailing ahead.