Incident Information on Broken Rudderstock due to Corrosion Fatigue

This incident refers to a broken rudderstock due to corrosion fatigue. While at sea, the steering failed to respond. Investigations revealed that the rudder was not ίn the position as indicated by the rudder angle indicator on the bridge and in the steering gear room. An underwater inspection was carried out and the rudder was found to have an angle of 90 degrees to port. The rudder was temporarily secured, and the vessel was subsequent!y towed to the port for close-up inspection in dry-dock and for permanent repairs.

2013.08.16 - Incident Information on Broken Rudderstock due to Corrosion Fatigue Figure 1

The rudderstock was found broken, having sheared off just above the top end of the lower taper where the rudder is secured to the rudderstock by a hydraulic nut. As a result the rudder had dropped and was resting on the step for the pintle below  (See figure below).

2013.08.16 - Incident Information on Broken Rudderstock due to Corrosion Fatigue Figure 2

The sealing at the end of the stainless steel liner proved to be insufficient to prevent ingress of seawater to the tapered surface of the rudderstock. Consequently, a galvanic element was built, the stainless steel being the cathode and the carbon steel the anode. Thus the suffering part in this case was the rudderstock.

After dismantling, the sheared surface at the break of the rudderstock indicated a typical fatigue fracture. Ιt had started at the corrosion groove and appeared to have been working for some time (See figures below).

2013.08.16 - Incident Information on Broken Rudderstock due to Corrosion Fatigue Figure 3

Α new rudderstock was machined according to original approved drawing, and the tapers were machined to suit the existing steering gear rotary vane and rudder blade tapers.

Similar damage is experienced from time to time, for rudderstocks and pintles in particular. In order to prevent such damage it is essential to avoid creation of a galvanic element, therefore:

  • Boundaries between normal cast/forged steel and stainless steel are to be protected against ingress of seawater.
  • At every opportunity, the sealing of such boundaries is to be confirmed in good condition and fit for purpose.
  • It is good practice to renew this type of sealing every time the rudder is taken down.
  • Emergency steering is to be readily available and the crew must be familiar with the arrangement, just in case.

Source: DNV

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