Rudder -Dry dock inspection and repairs

 Rudder

The rudder is the most important part of the ship. If the rudder becomes defective, the ship can no longer operate, even though the condition of the hull and machinery is satisfactory. The  rudder being normally under water, its condition can be observed only during a bottom inspection when the ship is docked.

Drydock inspection

Detecting ingress of water into rudder
If we find some wet area in the rudder platet, it is likely that cracks have occurred in the rudder plate and sea water has ingressed. Even if water has entered into the rudder, only the buoyancy of the rudder is lost and no major casualty will occur. However, internal parts of the rudder might corrode, therefore, the plug in the bottom plate of the rudder should be opened and water should be drained out.

Measurement and allowable values of bearing and jumping clearances
Measurement of clearances of all bearings are to be carried out during rudder inspection. Therefore, clearances of the sleeve and the bush in the longitudinal direction (F～A) and the transverse direction (P～S) of the rudder should be measured. The two methods described below may be used for measurement.

Standard Clearance
i) Pintle
For a newly built ship, the standard clearance is 1.5 mm.
For a ship in service, Maximum allowable clearances between pintle and bush is 6 mm.
IF the actual clearance exceeeds 6mm, the bush should be renewed.

2) Neck bearing
 The rudder is generally supported at three points; In case o f a hanging rudder, the supporting point is two. However, in rare cases, some ships have rudders supported at four points, with an additional intermediate bearing below the uppermost support, namely the rudder carrier
Clearance in the neck bearing can be measured after the rudder is overhauled. Unless other wise the measurement is carried out using a feeler gauge.The standard clearance is 4.0 mm, If the clearance exceeds 5.0mm, the bush should be replaced. Actually the wear down of the neck bearing bush is smaller than the pintle.

3 ) Jumping Clearance:

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. The clearances can be taken from rudder trunk – between the stopper and carrier bearing.

Wear to heel disk

Generally two hard, semi-circular steel disk is fitted in the shoe piece and the bottom of pintle, one above the other so that a point contact is obtained; however, the upper heel disk is sometimes part of the pintle. In this case, the lower part of the pintle is semi-circular. This heel disk rotates together with the rudder and has a box-shaped spigot. The semi-circular shape of the heel disk becomes flat when it wears out, causing the rudder comimg downwards. If the heel disk becomes thin due to wear, it should be renewed. The designd clearance between rudder bottom and　shue piece　 is 20 mm to 30 mm. If the clearance is between 0 and 10 mm, the heel disk should be renewed, or the rudder carrier should be opened up and the surface of bearing disk

Rudder carrier
Though there is no relationship to bottom inspection, the rudder carrier is an important part connecting the rudder and the steering gear in the steering gear room. The points for inspecting the rudder carrier are listed below.

(1) Looseness of bolts connecting rudder carrier to deck are to be examined with the test hammer.
(2) Cracks in deck connection part

In the construction shown in the figure on the left, crack will not apear in the deck. But　in the figure right, cracks might appear in the welded joint at the inserted liner to the deck. when the thicker liner plate is welded to deck. Sometimes　circumference cracks might be appeared in the weld joining to the deck.
(3) Looseness of wedge
Where reamer bolts are not used but a wedge is used for securing the rudder carrier to the deck, if the wedge becomes loose, or the direction in which the wedge is driven is incorrect, the carrier might turn; therefore, confirm that the wedge has been secured correctly.

Deformations and Fractures

Rudder blade, rudder stock, rudder horn and propeller boss/brackets have to be checked for deformations.

Indications 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.

welds of movable part to the rudder blade, and 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.

Fractures should be looked for at weld connection between rudder horn, Propeller boss and propeller shaft brackets, and stern frame.

Fractures should be looked for at the upper and lower corners in way of the pintle recess in case of semi-spade rudders.

Fractures should be looked 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. Typical fractures are shown 

 Fatigue fractures should be looked for at the palm plate itself in case of

loosened or lost coupling bolts.

Fractures should be looked for in the rudder plating in way of the internal stiffening structures since (resonant) vibrations of the plating may have occurred.

If the rudder stock is deformed, fractures should be looked for in rudder stock by nondestructive examinations before commencing repair measures, in particular in and around the keyway.

Corrosion/Erosion/Abrasion

Corrosion/erosion (such as deep pitting corrosion) should be looked for in rudder/rudder horn plating, especially in welds. In extreme cases the corrosion /erosion may cause a large fracture

Pintle damage

The cylindrical sleeve is expanded by heating, and when the inner diameter becomes large, the pintle is inserted by shrinkage-fitting. The two  members are only held against each other physically; therefore, the sleeve might become slack due to vibrations or ingress of sea water between the members. The bush retainer and support are a comparatively thinner welded rings made of steel plate. If some part of this ring is corroded, bush might loosen. When they become excessively thin, the bush retainer or support should be replaced, If the bush has been shrinkage-fitted into the shoe piece, it will not fall off; however, there are instances of the bush turned into a fine pieces, which in turn found its way between the pintle and the shoe piece then disappeared.If the slack is excessive, the sleeve drops.

The copper alloy / stainless steel 316 sleeve is shrinkage-fitted on the bearing surface of the forged steel pintle, therefore, the ends of the sleeve are likely to be subjected to galvanic action. Sometimes the tapered end of the pintle corrodes circumferentially and its thickness is reduced only at the corroded part. Moreover, the tapered part of the pintle is in metallic contact with the cast parts of the pintle. If the sea water enters into the small clearance, the tapered part corrodes; therefore, 0-rings are generally fitted at both ends of the sleeve. If 0-rings cease to exist, the tapered part gradually corrodes due to the effects of the sea water, and finally, the hair crack appears around the taper end of the pintle. Pintle can break on a long run, if crack is allowed to propagate.

When the rudder is lifted, strike the sleeve with a test hammer and check for slack. If we press the sleeve lightly with a finger while striking it with a test hammer, we might feel a slack of sleeve. If we find some slack, strike all around the sleeve with the hammer and record the slacked locations. If the slack is found over 2/3 rd of all surface , the sleeve should be replaced.

Twist in Rudder Stock
Among the damages of rudder the most troublesome damage is twisting of the rudder stock. In furthermore, In most cases twisting is accompanied with by bend of the rudder stock.　
When we watch the rudder just aft in the dry dock and the rudder is found to have swung to any P or S side, then the rudder stock is likely to have twisted., since in dry dock always the rudder is kept in midship position.
Twisting is caused due to the external force to the rudder plate in case of grounding, touching with mud, rock or floating objects. During sailing , if the rudder suddenly responds strangely and becomes heavier than usual, the rudder stock has probably twisted. If the angle of twist is small, there is practically no effect on steering. When the twisting angle is less than two degrees, there is no problem. When the ship heavily stranded, the twisting combined with bending of rudder stock can be the results.

Position of twist
Not the same as dents and cracks, It is very difficult to find the position of twist .
The rudder stock above the neck bearing is slender, so the most cases it may be assumed that this part of the rudder stock will be twisted, But it is very difficult to check a position correctly.
The twisting angle is measured after the rudder stock is overhauled and placed on the level block. The difference of the position of key way on the top of rudder stock and the position of rudder flange. The rules of the Germanischer Lloyd ( the German classification society) prescribe the replacement of the rudder stock when the angle of twist is greater than 10 degrees.

Repairing twist

If the rudder stock is twisted due to excessive forces such as contact or grounding and has no additional damage (fractures, significant bending etc.), the stock may be reused.

Consideration will be given on a case by case basis.

Normally the key way, if any, has to be milled in a new position.

The following table, giving criteria in relation to the degree of twisting, is for guidance only.

A = Angle of twist in degrees.

L = Length of the stock over which the twist appears uniform.

d = diameter of twisted portion of stock.

(1) For A < L/D

Stock acceptable for further service without any heat treatment subject to stock being free of cracks or other defects.

For small angles of twist a “stepped” or larger key may be considered as an alternative to a new keyway. Proposal is to be submitted for class approval.

(2) For L/D < A < 5L/D:

The stock is to be stress relieved. Proposal is to be submitted for class approval.

(3) For A > 5L/D

Stock is to be fully annealed or normalized. Proposal is to be submitted for class approval.

 The entire rudder stock is subjected to twisting forces, therefore, examine the entire surface of the rudder stock for very small flaws.

Bent rudder stocks

Bent rudder stocks without any fractures may be repaired, depending on the size of the deformation, either by heating or by cold straightening in an approved workshop according to an approved procedure. Repair proposal should be submitted.

In case of warm straightening, as a guideline, the temperature should usually not exceed the heat treatment temperature of 530-580oC.

- In case of fractures on a rudder stock with deformations, the stock may be used again depending on the nature and extent of the fractures. If a welding repair is considered acceptable, the fractures are to be removed by machining/grinding and the welding is to be based on an approved welding procedure together with post weld heat treatment as required by the Classification society.

Wastage of Rudder stocks / Pintles

Rudder stocks and/or pintles may be repaired by welding replacing wasted material by similar weld material provided its chemical composition is suitable for welding, i.e. the carbon content must usually not exceed 0.25%. The welding procedures are to be identified in function of the carbon equivalent (Ceq). After removal of the wasted area (corrosion, scratches, etc.) by machining and/or grinding the build-up welding has to be carried out by an automatic spiral welding according to an approved welding procedure. The welding has to be extended over the area of large bending moments (rudder stocks). In special cases post weld heat treatment has to be carried out according to the requirements of the Classification society. After final machining, a sufficient number of layers of welding material have to remain on the rudder stock/pintle.

Rudder plates

Fractures in welding seams can be gouged out and re-welded with proper root penetration. If vibration may be the possible cause an alternative repair may have to be considered.
Fractures in the lower and/or upper corners of the pintle recess of semi-spade rudders that do not propagate into vertical or horizontal stiffening structures may be repaired by gouging out and welding. For longer fractures internal structure should be checked, and repair proposal should be submitted.
As a guide, the preheating temperature can be obtained from the diagram below using the plate thickness and carbon equivalent of the thicker structure.
For fractures at the connection between plating and cast pieces an adequate preheating is necessary. The preheating temperature is to be determined taking into account the following parameters:

• Chemical composition (carbon equivalent Ceq)
• Thickness of the structur
• Hydrogen content in the welding consumables 
• Heat input
•  
• Measures when cracks are detected
  (1) Open the plug at the bottom of rudder plate and drain the sea water from the rudder.
  (2) After close the plug fill the rudder with air to perform the air test (0.15 bar) and check the cracks.
  (3) Re-weld the crack , with proper heat treatment procedures or as approved by class.
  (4 ) Welding work first should be tested by Non-Destructive Testing means X-rays, Ultrasonic testing.
  (5) After welding, carry out the air test(0.15 bar) again to confirm that the repair has been completed correctly.
  (6)Rudder should be hydraulically pressure tested upto 2.45 m of water head for confirming water-tight integrity from the top surface of rudder - after all repairs .
  

  
  
  
  Lifting and Removing Rudder
  At first the rudder bearing clearance (Between inner diameter of bush and rudder stock or pintle) should be measured when inspecting the condition of the bearing. If an abnormality is found, the rudder should be lifted or removed, depending on its construction.Rudders with upper and lower pintles or lower pintle only, the rudder has to be lifted.
  Mariner type rudder, the rudder should be lowered;
  Simplex rudder, the rudder post should be removed
   In any case, the tiller of the steering gear should be overhauled and removed, in such a way that the rudder and steering gear should be disconnected, and the jumping stopper removed.
  An example of the sequence for lifting the rudder is shown 
  
  
  

  
  
  
  
  
  
  
  
  
  
  If the rudder is lifted by a jack, the jack should be positioned under the vertical frame of the rudder, otherwise it might dent the bottom plate of the rudder. If the sequence is not followed correctly, the rudder might drop and break the shoe piece; therefore, work should be carried out with much care.
  
  
  Additional surveys of the repair (including non-destructive fracture) to be undertaken as per class requirements and condition.
  Before returning the rudder in service
  - Rudder should be hydraulically pressure tested upto 2.45 m of water head for confirming water-tight integrity from the top surface of rudder.
  
  
  - Rudder drop to be checked to confirm load on the pintle bearings, condition of the pintle bearings, free movement of rudder 35 deg. on either side, external rudder stoppers before rudder is put back into service.
  Proper fitment of sacrificial anodes. rudder closing plates and bottom drain to be checked for proper fitment. Rudder stock palm nuts to be covered with cementing.