Answer MEP Question 21

Q. A. Outline the procedure for the inspection of the rudder in a dry dock, 

B. What are the requirements with respect to steering gear as per SOLAS 74, as amended for the following:

i. Relief valve; 

ii. Steering gear control; 

iii. Electrical power circuit

Inspection and repair on a rudder in a dry dock require staging to be erected. Rudder drop is to be measure at steering flat. Various clearances are to be checked. Top & Bottom pintle key & keyways to check.

Open rudder bottom plug, check for ingress of water, and subsequently perform the air test to detect cracks if any and carry out repair welding and retest as per the classification society's advice.

The rudder may be required to be removed to carry out a repair on the rudder or on rudder stock.

Rudder, Rudder Stock, and pintle

• Excessive clearance between sleeve and bush of the rudder stock/pintle beyond the allowable limit specified by the classification society.
• Condition of the sleeve. If the sleeve is loose. Ingress of water may have caused corrosion.
• Deep pitting corrosion in the rudder stock and pintle adjacent to the stainless steel sleeve.
• Slipping of the rudder stock cone coupling. For a vertical cone coupling with a hydraulic pressure connection, sliding of the rudder stock cone in the cast piece may cause surface damages.
• Where a stainless steel liner/sleeve/cladding for the pintle/rudder stock is fitted into a stainless steel bush, an additional check should be made for crevice corrosion.

Checking tor any deformations

• The rudder blade, rudder stock, rudder horn, and propeller boss/brackets have to be checked for deformation. 
• Indication of deformation of rudder stock/rudder horn could be found by excessive clearance.
• Possible twisting deformation or slipping of cone connection can be observed by the difference in angle between rudder and tiller.
• If bending or twisting deformation is found, the rudder has to be dismounted for further inspection.

Checking for any fractures

• Fractures in rudder plating should be looked for at slot welds, welds of removable parts to the rudder blade, and d welds of the access plate in case of vertical cone coupling between rudder blade, and rudder stock and/or pintle.
• Such welds may have latent defects due to the limited applicable welding procedure. Serious fractures in rudder plating may cause loss of rudder.
• Look for, at weld connection between rudder horn, propeller boss and propeller shaft brackets and stern frame.
• Look for, at the transition radius between rudder stock and horizontal coupling (palm) plate, and the connection between horizontal coupling plate and rudder blade in case of horizontal coupling.
• Look for, in the rudder plating in way of the internal stiffening structures since (resonant) vibrations of the plating may have occurred.

Rudder clearances

Dimension (1) must be greater than jump clearance (4) to protect steering gear from damage in the event of grounding of the skeg or rudder.

Dimension (2) should be sufficient to cater for wear in carrier bearing and substantially greater than riding washer clearance (5).

Usually, (1) and (2) are of the order 20mm/25mm on a small/medium size vessel.

If the riding washer clearance has reduced then the carrier bearing is wearing or the skeg is set up. Check for the reduction in steering gear crosshead dimension (2) to verify downward displacement.

Ensure no drydock keel blocks in way of the skeg. Always apply the wear limits given by the manufacturer. The following table gives recommended maximum limits of clearances.

These limits do not only take into consideration the functioning of the bearing but also the effects of “hammering” and damage to the rudder itself as clearances increase. 

See manufacturer’s recommendations for special rudder designs or the use of synthetic bearing materials. To avoid jamming and premature failure renewed bearings should have adequate clearance. Special care is required with those synthetic materials which swell. The rudder should always be swung in drydock when possible after repair to check free movement.

In general initial clearances should be around 0.050” for a 6” pintle to around 0.70” for a 14” pintle. Far in service acceptable clearance, we feel about 1/64" clearance per 1" of the diameter of pintle up to 1% of diameter would be acceptable for another two years of service.

1/64" = 0.015625" = 0.397 m/m

Jumping stopper and jumping clearance

If the rudder is lifted when underway due to the wave impact or contact with floating objects, and/or bottom contact, the steering gear may be

damaged. To prevent such damage, a jumping stopper is provided. The jumping stopper, as shown in the figures below may be fitted over the gudgeon

or assembled in the rudder carrier. The designed clearance is 2.0 mm maximum.

There are no instances of damage or corrosion to the jumping stopper itself. However, if the clearance measured is found to be large, it can be

concluded that the rudder has moved down. Because a hanging rudder does not have a shoe piece, one does not know whether the rudder has moved

down or not. Therefore, it is recommended to enter the transom space for approaching the rudder trunk and measure the clearance

between the base of the rudder carrier and the jumping stopper. If access is not available, the clearance is measured from outside the ship on top of the

rudder at the upper pintle where the jumping stopper piece is fitted.

Rudder wear down measurements 

Rudder wear-down refers to the measurements taken generally during a docking period to indicate excessive wear in the steering gear system particularly the rudder carrier. 

This wear-down or rudder drop is measured using a special L-shaped instrument called a "trammel" gauge. When the vessel is built, a distinct center-punch mark is made onto the rudder stock and on to a corresponding, suitable, location on the vessel's structure. In the diagram below it is shown as a deck-head girder, which is typical. The trammel is originally manufactured to suit these marks and acts as the "reference". As the carrier wears the upper pointer will fall below the center punch mark by an amount equal to the extent of the wear down. 

(i) Relief valve:- Regulation 29

Relief valves shall be fitted to any part of the hydraulic system which can be isolated and in which pressure can be generated from the power source or from external forces. The setting of the relief valves shall not exceed the design pressure. The valves shall be of adequate size and so arranged as to avoid an undue rise in pressure above the design pressure. 

(ii) Steering gear control

Steering gear control shall be provided for the main steering gear, both on the navigation bridge and in the steering gear compartment

where there are two main steering gear provided,, by two independent control systems, both operable from the navigation bridge. This does not require duplication of the steering wheel or steering lever. Where the control system consists of a hydraulic telemotor, a second independent system need not be fitted, except in a tanker, chemical tanker or gas carrier of 10,000 gross tonnage and upwards;

For the auxiliary steering gear, in the steering gear compartment and, if power-operated, it shall also be operable from the navigation bridge and shall be independent of the control system for the main steering gear.

A means of communication shall be provided between the navigation bridge and steering gear compartment.

(iii) Electric power circuit

Any main and auxiliary steering gear control system operable from the navigation bridge shall comply with the following:

If electric, it shall be served by its own separate circuit supplied from a steering gear power circuit from a point within the steering gear compartment, or directly from switchboard busbars supplying that steering gear power circuit at a point on the switchboard adjacent to the supply to the steering gear power circuit;

means shall be provided in the steering gear compartment for disconnecting any control system operable from the navigation bridge from the steering gear it serves;

The system shall be capable of being brought into operation from a position on the navigation bridge;

In the event of a failure of electrical power supply to the control system, an audible and visual alarm shall be given on the navigation bridge; and short circuit protection only shall be provided for steering gear control supply circuits.

The electrical power circuits and the steering gear control systems with their associated components, cables and pipes required by this regulation and by regulation 30 shall be separated as far as is practicable throughout their length.