Answer MEP Question 26

Q. Your ship after having been accidentally grounded was taken to a dry-dock for inspection and necessary repairs. What defects would you look for in the following parts, that may have sustained damage due to grounding and suggest methods of repairs and tests that may be required to be carried out to the defects noticed:

A. Propeller and tail-end shaft

B. Main engine crankshaft.

Defects to look for:

- The Rudder may be aground/damaged. 

- Structural damage to the double bottom structure in the machinery space as it may not be possible to see any structural damage clearly while afloat. 

- There may be bend, breakage, twist or crack on the propeller shaft and tail-end shaft. 

- In most cases, the C/shaft alignment will deface. 

- Main engine crankshaft may have slipped through the Pitch of the propeller could have changed. 

- Stern tube seal & bearing portion might have been damaged 

- If the keyed propeller is fitted then due to grounding along the key and keyways cracks will be there 

A. Propeller and tail-end shaft 

- In some cases, the propeller is completely lost due to severe grounding. 

- Coupling bolts are changed following a grounding on most cases due to heave stress and loss of its property. 

- Propeller shaft alignment needs to be re-done and the complete operation is witnessed by the class surveyor. 

- In case of a CPP operating mechanism can be damaged push-pull rod can get bent. Cracks on the boss, if it is tapered, will get disturbed. 

REPAIR: 

Roughening 

- There will be significant losses inefficiency caused by blade roughening and the same can be avoided by the application of a foul release coating with a surface finish equivalent to a new or well-polished propeller. Roughing may lead to rapid degradation. Very small pits (less than 1mm) can be ground out and polished. Deeper pits can be repaired by welding, grinding and polishing. The temporary repair can be carried out using resin fillers. 

Deformation 

- Mi-or edge deformation can be corrected in situ using hand operated. Straightening levers and "soft" flame heating torch (not oxy-fuel). Major straightening and pitch checking would be carried out in a shore facility. 

Edge Cracks 

- Mi-or edge cracks can quickly become more serious, especially at the leading edge. If the cracks are less than 10mm in length, they are ground out and the edge." faired". Larger cracks may need to be repaired by drilling a small. Hole at the root the crack, gouging out, welding, grinding and polishing (not in the inner third of their blade). Temporary arresting of propagation has been carried by drilling a small hole at the root of the crack, plugging the hole, grinding and polishing. 

Serious Edge Damage

- Propellers if appreciably damaged, should be repaired in a shipyard rather than in place on the ship. When extensive propeller repairs are contemplating or when repairs are of a nature that requires machining or chipping to restore to designed dimension, a yard estimate should be obtained before proceeding with the work, and approval of the A.S.S. If a propeller blade is bent to the extent that it must be repaired quickly so as to keep the vessel in service and it is not possible to return it to a qualified propeller manufactures for repair great care should be exercised in the application of heat. At no time, should the repaired section be worked at a temperature higher than cherry red? As soon as this condition changes to black heat, work should stop and the worked portion reheated. 

Application of heat 

This can be used to assist the fitting/removal of some propellers, but great care must be taken to avoid high residual stresses being introduced when cooling. The consequences of residual stresses could be "stress corrosion cracking" of the boss which may not occur until some weeks or months after the application of the heat. Stress corrosion cracking in the boss due to the Incorrect use of heat for fitting/removal usually results in a scrapped propeller. The heating process has been successfully carried out using steam, electric blanket or soft flames such as produced by paraffin, or propane and air. A fierce flame such as oxygen and fuel should not be used. Under no circumstances should heat be used with shrink-fit bosses since the resin bond to the insert is destroyed. 

B. Main engine crankshaft- 

Crankshaft alignment issues; buses: Propeller damage directly causes heavy damage to the crankshaft and running gear, Bottom end bolts may also fail due to sudden stoppage while the ship was running which leads to c/shaft damages or similar. 

How to identify 

Displacement of witness marks at pins/web interface. Timing altered over part of the engine. Vibration from the engine at normal running. 

Result: Timing of units on another side of slip to camshaft drive altered. Possible reduction in grip of a shrink fit. 

Remedy: For minor slippage timing adjustment with regular checks may be sufficient. For major slippage, say greater than 4°, then it is necessary to return to original position by the arrangement of jacks, strong backs and preferably liquid nitrogen. Only gentle heating should be applied to avoid undue thermal stress. Whenever possible and convenient, the repair should be carried out at a ship repair facility if this is not possible; there are several recorded precedents for recovery procedures. 

1. Chill as far as possible the crankpin (dry ice etc.).  

2. After several hours of cooling, warm up the web by applying a broad flame over a wide area. 

3. Lock the journal against rotation. (Remove shims and harden down the keep). 

4. Jack the web back into position (mount the jack on wooden battens to protect the bedplate structure from a high and localized load). 

5. Erect a stop above the web, to prevent overshooting the desired positions, before applying jacking force. 

6. The fitting of dowels is not recommended, they upset the hoop stresses mentioned earlier and, being driven in also lift the surface apart and thereby reduce the frictional grip essential to the shrink fit. 

7. Record the affected unit in the engine log, ensuring that the witness marks are regularly checked initially, after just a few running hours and then after every voyage or extremely heavy weather, where possible. Adequate precautions must be taken against fire, contamination of lubricating oil and unexpected engine movement and so on. Such a repair should not be undertaken by the ship's staff without referring to the engine builders and the superintendent engineer.