This technical article gives the reader an overview of the activities carried out during a dry dock, explicitly focused around the stern mechanical equipment; stern shaft, propeller, rudder and associated ancillary equipment. A typical oil-lubricated stern tube arrangement is considered in this example. Separately identified is a list of items a class surveyor may wish to include as part of their stern area survey.
Classification Society requirements
During a dry dock, a classification society surveyor will be on-site and will carry out the survey of the equipment as broadly described below:
Tail shaft survey
The propeller and stern tube assembly is an independent survey from the docking survey. The survey would typically include the complete withdrawal of the propeller and tail shaft. It could consist of an examination of the following, as applicable:
- Visual inspection of the propellor
- Fasteners and leak tightness of the hub and blades would be carried out on the controllable pitch propellers
- Dismantling of the CPP may not be carried out unless considered necessary by the surveyor
- Wear down measurement and recording of the aft bearing
- Aft stern tube seal leakage testing and recording
- Propellor nut and threaded end of the tail shaft
- Propellor cone, key and keyway
- The use of an efficient crack detection method on the aft end of the cylindrical part of the tail shaft and part or all of the tail shaft taper
- Where the propellor fitting is by solid flange coupling, crack detection to be carried out on the flange fillet area of the tail shaft
- Tail shaft in the way of stern tube bearings
- Stern tube bearing(s) clearance measurements.
On re-assembly, the following is noted:
- Propellor push up distance and hydraulic force
- Wear down measurement and recording
- Aft stern tube seal leakage performance
- Refitting of any rope guards/cutters.
Rudder survey
The survey can be carried out with the rudder attached but may be removed anyway as part of the tail shaft survey. The examination could include the following, as applicable:
- Visual inspection of the rudder
- Visual inspection of the pintle(s)
- If considered a necessity by the surveyor, the rudder is lifted or inspection plates removed to allow access for assessment of the pintle
- Visual inspection of the rudder stock
- Visual inspection of the stern frame
- Ascertain and record clearances in the rudder bearings
- Pressure test the rudder as deemed by the attending surveyor.
Dry Dock Activities
A wide variety of tail shaft and rudder arrangements exists across different sizes and types of vessels. The sections below describe activities for typical dry dock procedures.
Tail shaft dry dock procedure
- Check the stern tube for oil leakage
- Cut off the rope guard and note any fouling by fishing nets or cables/ropes
- Measure the wear down, ensuring a mark is made on the liner to identify where the wear down measurement was taken. Ensure the relative position between the gauge and seal housing is marked so that the gauge is tightened to the same extent on the next measurement
- Drain the stern tube oil
- Partially disassemble aft seal
- Remove propellor cap, propellor nut, then propellor
- Inspect blade surfaces for erosion, pitting, impact damage, and bending. Inspection should be focused on areas in line with classification society requirements
- If deemed necessary, the propellor should be repaired by an appropriate method
- Take measurements of the propeller and compare with previous records
- For controllable pitch propellers, check the blade movements and zero pitch settings concerning the wheelhouse. Check for hydraulic oil leakages and security of fasteners on the blades and carriers
- If tail shaft removal is required, disassemble the aft seal, forward seal and coupling to the intermediate shaft
- Withdraw the tail shaft
- Perform visual inspection and NDT (as required) on the tail shaft, stern tube and stern tube bearings
- Check for fractures at the weld connections on the propellor boss, tail shaft brackets and stern frame
- Refit the tail shaft and re-couple at forward end according to procedure
- Refit the propellor
- According to the procedure, after a correct and recorded propellor push–up (fitting) has concluded, tighten the hydraulic propellor nut. Ensure that the hydraulic hose connection bores on the hub and nut are suitably sealed and fitted with their plugs. The plugs should be locked in position
- Fit propellor cap with gasket and ensure the void space within the gap has been filled with grease or tallow. Finish with cement in the securing bolt recesses to ensure a smooth finish. It is common for the propellor to be polished
- Secure the aft seal housing and liner, ensuring the bolt heads are locked by a suitable method locking two bolts at a time.
- Following the seal makers’ recommendations, adjust as necessary, the concentricity between liner and shaft and concentricity between the liner and seal housing. Complete this for the forward seal also
- Complete the build of both forward and aft seals and test for functionality and leaks following the maker’s instructions
- Ensuring the liner is the same position as previously marked take the wear down readings and record. The liner must be in the same position as some eccentricity may exist in the liner that can affect the outcome of the measurements
- Before attaching the wear down gauge ensure the hole is clean and the gauge is tightened to the same level as the previous measurement, by aligning the previously made marks
- Securely weld the rope guard in position and attach the rope cutters in line with the maker’s instructions
- If the vessel is fitted with and Impressed Current Cathodic Protection system (ICCP), ensure the epoxy shields around the anodes are done. Conduct an insulation test
- In addition to the ICCP system, zinc anodes are fitted around the stern area. They should be checked for wastage and renewed as necessary. It is advised not to fit anodes around the stern boss directly in front of the propellor so as not to create turbulence in the water flow to the propellor
Rudder dry dock procedure
- When the dock water is initially pumped out, check to see if water is coming out of the rudder, thereby identifying if the rudder has been breached
- If the rudder appears breached internal parts of the rudder may be corroded. Open the bottom plug and drain the rudder
- Pressure test the rudder according to specification
- Dependent on levels of corrosions/pitting, thickness gauging may be required
- Zinc anodes are fitted on the rudder blade. They should be checked for wastage and renewed as necessary
- Measure the clearances of all bearings. Check longitudinally (Forward/Aft) and transversely (Port/Starboard). It is generally found that a pintle bearing wears quicker than a neck bearing. Pay attention to the amount of wear and consider a bearing replacement if necessary.
- Standard clearances for both pintle and neck positions is 1 – 2mm
- Check the Rudder jumping clearance. The average clearance between the jumping stopper and the rudder is 2-3 mm and is measured using a thickness gauge. If the clearance is large, it can be assumed that the rudder has dropped down. This must be considered against the maker’s limits in the steering gear machinery
- At the rudder carrier, check the bolt torques connecting the carrier to the deck and check for cracks in the deck adjoining fastening positions
- Check the true position of the rudder when compared to the rudder angle indicator to check for any differences resulting from bending or deformation
- The rudder blade, rudder horn and rudder stock must be checked for deformations. Excessive deformation may manifest itself as excessive clearances
- Check for fractures at the weld connections on the rudder horn.
- Inspect corrosion and erosion levels in the rudder and rudder horn plating, paying attention to the welds
- The rudder may be removed for remedial repairs or sometimes to enable removal of the propellor
- If the vessel is fitted with and Impressed Current Cathodic Protection system (ICCP), ensure the epoxy shields around the anodes are done. Conduct an insulation test
- In addition to the ICCP system, zinc anodes are fitted on the rudder. They should be checked for wastage and renewed as necessary.