The influence of rudder resistance on overall ship resistance is a well covered topic, and there is very good regression data that enable us to quickly assess it. However, once the propeller is present, the rudder can have a significantly different influence on the total propulsion power, since it is often located in the jet of the propeller upstream. This creates a self-reinforcing mechanism: the rudder increases the resistance of the ship at given speed, the propeller needs to give more thrust to compensate, this creates a stronger wash, the stronger wash increases the rudder resistance, and the cycle continues until a balanced state is reached.

Pressure field around the rudder downstream of the propeller

In this study we will try to shed some light on the topic by quantifying the increase in power that the rudder is responsible for. Self-propulsion CFD simulations with and without the rudder are conducted, allowing direct comparison of total power.

Test vessel

The KRISO Container Vessel was selected for this study due to its relatively high speed, and easy access to geometry and propeller data at https://simman2014.dk/ship-data/moeri-container-ship/geometry-and-conditions-moeri-container-ship/. All simulations are performed in full scale, using our actuator disc propeller model. The table below shows the main characteristics of the vessel, while the propeller characteristics can be found at the link above.

KRISO Container Vessel

Results and comparison

The differences in the pressure field behind the propeller are quite obvious in the two simulations. Presence of rudder increases the pressure downstream of the propeller, and it also increases the velocity of the jet coming from the propeller. Below are images showing the difference in the pressure and velocity field behind the stern with and without the rudder.

What we really want to see is how much propulsive power the rudder is costing us. The table below shows the propulsion characteristics calculated with our self-propulsion CFD simulations with and without the rudder, together with differences in percentages. We can see that the total increase in propulsion power is 2.2 percent, while the thrust force increase is 4.1 percent. The efficiency of the propeller dropped by 0.6 percent.

Note a few interesting facts:

• The thrust of the propeller has increased, but the rotation rate has decreased,
• This indicates that the propeller operates at lower inflow velocity when the rudder is present, due to the blockage to the flow that the rudder imposes,
• The increase in thrust is relatively larger than the increase in power, i.e. torque of the propeller.

Comparison of propulsion characteristics without and with the rudder

These results indicate that a significant amount of power can be spent on the rudder, and this is why rudder design is important. Hopefully this will be useful for you in future design. Coming soon are similar studies for planing hulls, where the relative importance of appendages increases dramatically.