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You as C/E are requested to inform your company with respect to inspection of propeller in drydock during which you noticed a surface crack on one of the blades. State the steps taken by you as C/E for successful handling of the situation. Also if some surface cracks are noticed on the keyway of a tail shaft, state the steps taken by you for tackling the problem. Explain the effects in case the cracks, as stated are not dealt with at the proper time?

Surface cracks have been noticed on one of the propeller blades:-
As soon as a crack has been observed on the propeller blade, the head office as well as the Classification Society must be notified. The services of a propeller expert would also be needed in certain cases.

Cracks on a propeller surface may be due to:-

  1. Physical damage
  2. Faults during casting 
Physical Damage:- These types of cracks generally occur on the tips and the leading edge of the propeller. Pitting may occur near the tips on the driving face and on the whole of the fore side due to cavitation. Propeller blades are sometimes damaged by floating debris which is drawn into the propeller stream.
Such damage must be made good as it reduces the propeller efficiency, while the performance is improved by polishing the blade surface. If a built propeller is fitted, it is necessary to ensure that the blades are tight and the pitch should be checked at the same time. Small surface cracks can be ground out. Cracks at the blade tips may require the cropping of the blade section. If this is carried out, a matching section on the opposite blade would also have to be cropped in order to maintain balance. Since thrust developed by such a cropped propeller would be reduced, the main engine rpm will have to be adjusted to compensate for the loss. All actions must be taken in consultation with the classification society and
propeller experts. It must be noted that most classification societies do not permit any welding on the propeller as heat treatment and annealing is an extremely complicated process. Therefore in extreme cases of cracks, it may be necessary to replace the entire propeller

Cracks due to Faulty Casting:- These are extremely small cracks of the order of length 3mm and depth 1mm. the region of the blade most prone to such cracks is within 0.2 – 0.4 of the radius of the propeller. These cracks are extremely difficult to spot when the propeller is stationery. The cracks only open when the propeller develops thrust in operation due to the bending moment induced. The only lasting solution to such a faulty fabricated propeller is to
change it with a new one. 

Surface cracks on the keyway of a tail-end shaft:- The area of the cracks is to be inspected visually as well as ultrasonically (or with dye penetrant) to detect all the cracks. Causes of this are quoted as an inadequate force fit between propeller and tailshaft causing loss of peripheral grip which allows the propeller to move and make contact with the key. This causes excessive dynamic load to fall on key and shaft adjacent to keyway. This causes incipient cracks (small and superficial) which usually begin at high stress concentration areas i.e. around the leading edge of the keyway. 

Fatigue failures may occur due to corrosion and temperature variations in seawater, altering the force fits. The key is also to be inspected for any damage. Also, the dimensions of the key must be accurately measured to ensure no play is present between key
and keyway. Abrupt changes in shape of the section cause stress concentration to build-up due to interruption of the stress flow lines. This build up in stress causes cracks to develop and supports crack propagation. With this in mind, it can be seen that shapes or sections which may be subject to great stresses; should be well rounded or gradually tapered off to give smooth stress flow.
Round end keys should be used, and the keyway in propeller boss and cone of the tailshaft are to be provided with a smooth fillet at bottom of keyways, fillet radius at least 0.0125 of shaft diameter at top of cone. Sharp edges at top of keyway to be removed. Two screw pins should secure key in the keyway and forward pin should be at least ⅓ of key length from forward end. Pin holes should have a depth not exceeding pin diameter. Hole edges to be bevelled.
The cracks in the keyway can be ground depending on the depth of the cracks. Large cracks would require welding and machining. Welding will only be carried out after appropriate heat treatment and annealing. The annealing period required for a tail end shaft of a large ship may extend up to 7 days. It must be noted that in a tailshaft with key and keyed propeller, the thrust of the propeller is not absorbed by the key but by the shrink fit between propeller hub
and shaft. Therefore cracks in the keyway of the tail end shaft do not pose as much a problem as cracks on the shaft itself or cracks on the propeller.


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