Selecting the right options for propeller blade modifications
Shiprepair & Maintenance: 3rd Quarter 2019
When confronted with a bent or broken propeller, the preferred option for Belgium-based Hydrex is to utilise its in-house developed cold straightening technique. This procedure enables the company to straighten damaged blades in-water, allowing commercial operations to continue without the need to drydock.
On occasions, however, the damage to the propeller blades is too great for cold straightening and, in such cases, cropping is often the only option. Employing propeller blade cutting equipment developed by its own research department where there is an even number of blades, an identical piece will be cropped from the opposite blade to restore the hydrodynamic stability of the propeller.
A recent example of this technique involved a 190m bulk carrier on which four propeller blades were severely bent. A Hydrex team was mobilised to Cam Pha, Vietnam, to bring the damaged blades back as close as possible to their original condition.
An on-site inspection revealed that the four blades were bent over by between 90 and almost 180 degrees. The team then used the information acquired during the inspection to calculate and determine the exact measurements needed to modify the trailing edges of the propeller blades. Next, the divers cropped the blades and ground their edges to give them the correct radius. When the cropping was complete, they polished the blades to make sure that any remaining loss of efficiency would be minimal.
During another project, this time in Singapore, it was found that only two of the four blades of a 183m tanker needed to be cropped using this technique. The other two propeller blades had suffered small cracks and dents along their trailing edges, but because the damage to these blades was limited, cropping was not required. Hydrex divers ground away the cracks and polished the edges of the blades to prevent further cracking.
Propeller cropping was required on a 220m tanker in Amsterdam, which had lost the tip of one of its propeller blades. The opposite blade had not been damaged, but it was also cropped to keep the propeller’s balance.
Hydrex has, however, also had the chance to use its favoured straightening technique on a bulk carrier in the port of Rotterdam. Four out of five propeller blades had small bent areas on the tip and trailing edge. These were straightened one by one, allowing the bulk carrier to continue its voyage without going off-hire to drydock and with the performance of the propeller restored.
Hydrex says its R&D department is seeking ways of further enhancing propeller repair techniques. New types of both the straightening and cutting machines have recently been put into service and these are said to have allowed the company to straighten blades that could previously only be cropped, and to crop extremely damaged blades with only a minimal loss of efficiency for the propeller.