Propeller Repair for Marine Vessels
Repairs to propellers can be done by argon-arc welding. The normal procedure followed by the workshop, which is subject to approval, is:
- Removal of dirt and oxides is carried out by grinding. Cracks are to be drilled out and ground, or machined to a Vee or double Vee joint
- The metal to be welded is preheated to the required temperature (>200 Degree Celsius)
- Appropriate filler rods are utilized, compatible with the parent metal. After the welding is complete, the metal should be stress-relieved by heating it to 500 Degree Celsius, and cooling at a controlled rate. Care to be taken to avoid warping of the blades during the heating and repair work. Serious cracks in the blade will require careful consideration, with respect to repair methods, cost of repair Vs cost of replacement, time available, availability of spares and dry dock time.
There are various options, which may be examined:
- Drill out the crack arresting holes, at the roots of the crack, and monitor for further spread, only if the time available is insufficient for proper repair, and the crack is limited in its spread
- Possibility of carrying out metal-locking
- Cropping of the blades
If the extent of the crack is large, then drilling holes could lead to pieces of the blades breaking away. This may cause further damage, vibration and reduced reliability.
Cropping may be a solution to vibration problems, as removal of the diagonally opposite blade tip will increase no static imbalance. Effect on the performance of the vessel will not be excessive (usually there is less than 10% reduction in speed). Cavitation is to be expected, due to the cropping of the tips. Also a change in the ‘barred speed’ (critical speed) range, with possible rising of exhaust temperatures and specific fuel consumption of the engine may be experienced.
Inspection of Propellers
Cracks, in most cases, can be detected only by carrying out a dye penetrant test; sometimes they are not detected even after carrying out this test. This test need not be carried out for the entire surface of the blade. Usually only the area from root to 0.4R of the blade is checked (R is the radius of the circle that the propeller draws when it rotates). A stop-hole is drilled at the tip of the crack and the hole has to be plugged, using a wooden plug. An entry is made in the Survey Record.