Providing the crucial steering and manoeuvrability of any vessel – the fitted rudder or rudders are an essential part of continued operation.

The forces generated around the rudder are proportional to the size and speed of the vessel – with enormous loads being supported through bearings to the ship structure in the case of large container of cruise vessels. Given the high loads and almost continuous rotation in one direction or the other – good lubrication is essential

Rudders

Rudders are most usually sited at the aft end of a vessel and depending on design may be a single blade aft of a propeller, or twin blades mirrored along the centreline of the vessel.

Effort to maintain a steering course in heavy seas or rotate the vessel while moving is large and the majority of rudder systems are hydraulically actuated and controlled. Rudderstock diameters can vary from only a few centimetres on a leisure vessel up to well in in excess of 1.5m in the case of larger ships.

The rudder loads are supported through bearings taking the weight of the rudder as well as the rotational and side loads at the top and the bottom of the rudderstock.

Fitted bearing

Fitted bearings can be water or grease lubricated – with various types of seal fitted both to prevent seawater or abrasive ingress to the bearings or grease lubricant escaping into the sea.

Two main types of rudder seal are fitted in the majority of installations:

  • Face seal – where the sealing interface is in an axial direction along the rudderstock, and the rotating part of the seal contacts a static seat housed within the rudder trunk.
  • Lip seal – where the sealing interface is in a radial direction with the rotating rudderstock liner rotating relative to lip seals contained within the rudder trunk.

In both cases, good performance and longevity rely on correct lubrication, alignment and regular inspection.

Maintaining

Maintaining good lubrication of both the bearings and seals is essential to maximise life of the equipment. Most issues where excessive wear or premature failure are seen is due to lack of correct lubrication – grease lubricated systems rely on seals to retain grease around the bearings. 

Classification Society Surveyors will want to see records of rudder bearing clearance values. A rapid increase or values above acceptable limits will trigger conditions being imposed and require docking.

Through the life of the vessel, bearing wear will occur naturally but in cases where bearing wear within acceptable limits will affect seal function – grease can be lost contributing to pollution and accelerated wear. In such cases we have previously supported customers with bespoke higher tolerance seals to maximise remaining bearing life.

Rudder Seal Survey

Within a rudder seal survey, AtZ would gather all available operating information from the vessel, before planning a more detailed inspection. This would include:

  • Onboard monitoring data and drawing review
  • Hydraulic and control system inspection
  • Checking bearing clearance (where possible afloat / in dock)
  • Checking lubrication log and estimating any leakage rates
  • Draw conclusions on system condition and remaining life
  • Support required parts, service or upgrade proposal

A full and detailed survey of the rudderstock bearings and rudderstock liners could usually only be completed in dry dock with partial or full removal of the rudder.

In the case rudders are removed, we would always recommend a full bearing inspection with replacement as required and replacement of all seals.


FAQ’s

Grease consumption is high but we have acceptable clearances – is this a problem?

Yes – grease is escaping into the sea, and whether EAL or mineral based this is unacceptable.
Grease escaping is evidence that the seals have failed, are damaged, or are incapable of operating with the bearing clearances imposed.
Seals must be replaced with new items and in the case of increased bearing clearance, should be with a seal capable of operating with the full range of rudderstock movement. 
This can usually be completed in active service alongside, but a review of the system design and access would be required to offer a full proposal from AtZ.

We want to change to an EAL lubricating grease and hydraulic oil – is this a straight swap?

No – each part of the rudder system needs to be considered for compatibility first. A system flush may be required and some seals and other equipment might need to be changed for a compatible material.
AtZ can review your system and advise any required steps for upgrade.

Our vessel is newly constructed and while all appears in order, we have some unusual noises in heavy seas from the rudder?

We have seen many cases in which equipment has been installed with no or poor alignment accounting for loaded parameters of the vessel.
An urgent review of the system together with an alignment check in service would be the best course of action to identify root cause. In some cases, it may also be possible to conduct a bearing clearance survey in service.

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