Starting from rest when going ahead. The transverse thrust effect will tend to swing the stern to starboard and the bow to port. The engines working ahead cause the propeller to drive a spiral flow of water towards the rudder (screw race) so the helm can be used to counteract the effect of transverse thrust.
Moving at steady speed ahead. The transverse thrust of the propeller will tend to swing the ships head to port. The wake current reduces the steering power but the helm can be used very effectively to counteract the effect of transverse thrust as there is a strong flow of water against the rudder due to the screw race and the vessels headway.
Moving ahead with engines stopped.
There is no transverse thrust effect from the propeller (constant pitch). The helm is less effective when the engines are stopped as there is no flow of water from the propeller against the rudder. If the vessel has good headway the rudder will have some effect but this is reduced by the wake current.
Starting from rest when going astern. The transverse thrust effect of the propeller and the effect of the propeller slipstream on the hull causes the stern to swing to port. The helm cannot be used effectively to counteract the swing as there is no flow of water against the rudder from the propeller, or from the movement of the ship.
Moving at steady speed astern.
The transverse thrust effect is reduced as the ship gathers sternway so the tendency to turn is less than when starting from rest. The rudder may become effective if the vessel moves astern rapidly but, for many ships, the steering effect is unpredictable.
