Propeller and rudder as an interacting system

The propeller and rudder perform different functions: the propeller generates thrust, while the rudder deflects water sideways and thereby influences the vessel’s direction. In a conventional stern arrangement, the propeller’s slipstream strikes the rudder. The two components therefore act on the same flow field and the forces it generates.

The propeller generates thrust

The rotating blades accelerate water aft. At the same time, swirl and vortices form in the propeller wake, so the flow reaching the rudder has already been shaped by the propulsion system. The incoming flow therefore differs from still, unaccelerated water. The rudder sits in this stream and cuts across it, meaning the propeller’s effect and the rudder’s position jointly influence the forces at the stern.

The rudder influences direction

When the rudder is turned, it deflects water sideways. The resulting forces and moments contribute to the vessel’s rotational motion. During a manoeuvre, the propeller also exerts lateral forces, so a change of course cannot be explained by the rudder angle alone. The rudder, in turn, affects the propeller flow because its surface extends into the stream and partially interrupts it.

This creates a two-way interaction: the propeller flow changes the water reaching the rudder, while the rudder influences the flow and the forces acting on the components at the stern. The extent of this interaction depends on the specific arrangement; the number of rudders and the position of the stern appendages shape the flow and turning ability.

The arrangement determines how the components interact

Tests on a twin-screw tanker model compared stern configurations with a single or twin rudder and modified skegs. The configurations tested differed substantially in their turning ability. Hydrodynamic studies also examine the forces and moments on the rudder and its interaction with the propeller during manoeuvres. This example shows why the number of propellers, rudder arrangement and other stern appendages need to be assessed together, rather than attributing the effect to a single component.

Related components add to this picture: pod drives with steerable propeller pods and bow thrusters and transverse-thrust systems are other propulsion elements; hull shape and water resistance form another part of the hydrodynamic picture.