A bent, damaged or cracked propeller blade can produce severe vibrations on the vessel. The resulting unevenly pressure distribution loads on the propeller will increase loads on the shaft and can even damage the engine and transmission system. The Classification society may require repairs to be made to the propeller before the vessel is allowed to continue. Even If there is no significant increase in vibration the uneven flow along the blade surfaces will result in reduced propeller efficiency and possible further damage due to cavitation.
Having to drydock a vessel to repair a propeller blade could be costly and time consuming but there are now a number of well established companies offering underwater propeller repair and maintenance services. Such work can often be undertaken within the vessel’s normal schedule without causing significant delays or interruption. In many cases these repairs can be accepted as Class Approved permanent repairs.
The range of underwater propeller repair and maintenance services being offered can include:
Propeller blade straightening
Removal of damaged blade sections
Edge restoration or modification, including anti-sing correction
Contour modification to eliminate cavitation
Static and hydrodynamic balancing
Crack repair and propagation control
Replacement of C.P. blades and blade seals
Dave Bleyenberg, Hydrex Production Executive, explained: “While winter months usually result in an increase in propeller repairs, this normally occurs between January and March. But we have already attended a number of vessel inspections on propeller blades broken or damaged by ice or debris.”
Underwater cold straightening machines has been in use for quite some time. In 2002, Hydrex originally teamed up with UK Stone Manganese Marine Shipcare to develop an underwater propeller straightening service without the need for major cropping. The process involves many cycles of hydraulic cold static loading applied in sequence throughout the bend. During the process, the subtle changes in geometry are measured and evaluated.
Hydrex’s research department has now been looking into ways to enhance the technique even further to improve their services. A new model of the straightening machine was recently put into service. It is compatible with the existing model and is used to restore more severely bent propeller blades.
If straightening is not an option, the affected area on the blade will be cropped to restore the hydrodynamic balance. This kind of repair is carried out with the propeller blade cutting equipment that was also developed by the Hydrex research department. First a detailed underwater inspection is performed by a Hydrex diver/technician team to obtain the exact parameters of the damage which are then used for a detailed calculation of the ideal cutting line.