This incident refers to cracks identified in a vessel’s propeller hub. When carrying out hull renewal survey in drydock, two open cracks were found during the visual inspection of the propeller hub. The propeller was a 4 bladed mono-block fixed pitch manganese aluminium bronze propeller fitted with cone and key and with diameter: 5205 mm.
The image above shows to the left One of the cracks found during survey and after penetrant testing (PT) and to the right the Finite element model of the hub showing the location of the groove and the keyway.
The cracks were located in between the blades in way of the forward part of the hub. After grinding, the following extent of damage was established:
- Crack 1: length 360 mm, max depth 45 mm
- Crack 2: length 500 mm, max depth 75 mm
This was considered to be a very severe damage as up to 36% of the hub thickness had cracked.
Most likely the cracks had appeared as a result of misapplication of heat to facilitate removal of the propeller from the shaft or in order to obtain a tighter fit. This type of boss-cracking is normally associated with the use of high intensity flames such as oxyacetylene. The boss will appear as to be in order just after such treatment but cracks will appear after an incubation period of several months, dependent on the level of stress and the alloy involved, normally at subsequent drydocking.
This type of failure is known as stress corrosion, a formation of brittle cracks in a normally sound material through the simultaneous action of tensile stress and a corrosive environment of seawater. In this case, the initiating stresses were resulting from steep thermal gradients produced by the use of high intensity flames, which created high residual stresses in the hub.
Propellers made of high tensile brass are the ones most often affected, but this kind of cracking has also occasionally occurred with propellers of manganese aluminium bronze. Shallow cracks can be removed by chipping and grinding, provided that the crack is completely removed. If welds are made it is normally necessary subsequently to carry out stress relieving. In this particular case the deepest crack was unfortunately located in line with the keyway.
After stress calculations had been done by the Class it was decided that the propeller had to be condemned due to the high stress levels.
Lessons to be learned for similar incidents such as the one mentioned above:
- Application of high intensity flames when removing the propeller from the shaft may cause cracks in the hub after several months. This is normally due to the fact that the workers actually doing the job do not realise or have never been told how small temperature increase is actually required to expand the propeller hub sufficiently for this purpose. The surface temperature should not at any point exceed 150ºC for an copper alloy. The hub should be heated by the hot gases, not the flames.
- Alternatively, propeller bosses may be heated by the use of steam, or by heavily insulated, low powered electric blanket controlled to give a maximum temperature of 100ºC to 150ºC at the boss surface.
Source: DNV
Officer of the Watch 
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