Ocean Magazine : 2 July August 2005
The two worst enemies of the propeller are corrosion and fouling from marine growth. There is much that can be done to arrest both of these problems. Cavitation It may be argued that cavitation is not a form of corrosion, but it is still one of the main causes of propeller damage. Cavitation has many causes, some of which should be eliminated during the design phase. Cavitation occurs when the water pressure exceeds the localised water vapour pressure. This results in water cavities or bubbles in the water. The bubbles frequently collapse on the surface of the propeller blade, this implosion is so violent that the surface is mechanically attached and some of the metal is plucked from the surface, resulting in a pit on the surface. Cavitation can be a very serious problem with propellers, as they are frequently made to work in less than optimal conditions. In addition to the characteristic blade pitting that results from cavitation, other side effects include; additional noise, vibration and in extreme cases intermittent engine racing. Propeller cavitation usually starts around the blade tips. Some metals have a greater resistance to attack from cavitation than others. The more noble metals are much more resistant to cavitation attack than metals such as mild steel. As a general guide, cavitation resistance is increased with the hardness of the metal. Cavitation issues should be addressed by a naval architect or a cavitation specialist as this is a complex subject. Electrolysis and Galvanic Corrosion Electrolysis and galvanic corrosion are frequently confused. Galvanic corrosion is caused by two dissimilar metals being in contact with each other, in the presence of an electrolyte, such as sea water. Electrolysis is caused when an external current, called a stray current finds a path between two metals in the presence of an electrolyte. The two metals may be of exactly the same type or different types. Propellers are subject to both types of attack and preventative measures such as the correct installation of sacrificial anodes and proper earthing systems are imperative to prevent damage. Antifouling Paints With the introduction of new environmental protection laws and the responsibility of paint companies to find new solutions to the issues associated with fouling, there are few antifouling paints currently available that can withstand the rigours of this harsh working environment for a sustained period of time. Therefore regular cleaning is one way of ensuring the blades operate at peak performance. ENGINE ROOM Propeller Care and Maintenance Propellers are a precise piece of engineering. They must live and work in a harsh environment. Some basic maintenance and care will ensure they last for a long time.
3 September October 2005
1 May June 2005