A wide range of rudder or steerable duct actuating force, cavitation and integrity problems can occur from time to time. In the case of rudders, these may take the form of bearing failures or erosion of the rudder plating, the latter due either to cavitation developed by the rudder itself or from the cavitation entrained in the helical slipstream from the propeller. Steerable ducts can also have accentuated vertical seaway loadings and, in common with fixed ducts, can suffer from erosion on their inner plating.

Actuating force problems can occur with either form of steering device.
Typical of such an investigation was the full-scale measurement with strain gauges of rudder
stock moments on the semibalanced spade rudder of a container vessel. This assignment was undertaken in
order to establish the reasons and to provide a basis for remedial action for both an under powering of the steering
gear and a poor response to the helm from the ship.
Rudder torque and stock bending strain measurements were made and the results of this trial’s programme were compared
with computed loadings.
These were derived from the surface pressure distribution over the rudder horn and blade by means of one of Lloyd’s

Register’s lifting body vortex panel codes.

For this computation, the incident flow to the rudder was evaluated from a lifting line propeller analysis procedure.
The rudder model was verified initially using the model test data and, subsequently, calculations were performed
for the container ship rudder in the slipstream of the propeller for comparison with the measured full scale data.
It was found that good agreement between the measured and predicted loading was obtained and that the form of the rudder
stock torque relationship for a 35 to 35 degree rudder angle zigzag manoeuvre was well represented.

The same modelling procedure was then used to predict the effect of increasing the rudder chord length by 20%.
The predicted maximum torque increased by more than 150%, while the side force increased by approximately 10%.
A similar percentage increase in rudder torque was subsequently remeasured on board the ship when the rudder extension was fitted.

LESSON LEARNED
Rudder problems can be solved by a combination of fullscale measurement and theoretical approaches. Small
changes in dimension can lead to large changes in applied 
 
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