When the engines are going either ahead or astern the rotation of the propeller tends to turn the ship, even when the rudder is amidships. If the ship is in light condition the upper blades of the propeller are likely to churn and break up the water near the surface with the result that the lower blades have to overcome greater resistance than the upper ones. As most vessels have a single right handed propeller the lower blades turn to port when going ahead causing the stern to swing to starboard. On going astern the lower blades turn to starboard and the stern swings to port. The transverse thrust effect may still be present when the propeller is immersed well below the surface, in which case it cannot be attributed to the greater resistance encountered by the lower blades. The influx of water to the propeller is not parallel to the axis and the thrust of the rotating propeller is consequently distorted. The amount of transverse thrust is related to the shape of the hull in the vicinity of the propeller. A bluff stern vessel will have more bias due to this effect than a fine lined vessel. When a controllable pitch propeller is fitted the bias will always be towards the same side, as the direction of rotation is not changed when going astern.
