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16 MN May 2011 TECHNICAL There are several terms used to classi- fy propeller damage and we will cover two of them here: incorrect handling and extreme heating of the hub or blades. Propeller blade tips or edges can be damaged during transport and this should be prevented by using wood, rub- ber, or any other soft protection to avoid direct contact with hard surfaces. When handling the propeller out of its package, leaning on the blades should be avoided as much as possible, as any deformation can move the pitch or even unbalance the wheel resulting in extra cost to the owner when the propeller is re-checked. When heat is applied in an uncontrolled manner to the propeller hub or blades, it can create and concentrate stresses in the material. This occurs when a specific area is heated and tries to expand while the metal around it does not. These stresses can be relieved when plasticity starts at a red hot point where all stresses vanish. When it starts to cool-down the heated area contracts and any remaining stresses are relieved by the plastic flow which stops at around 480°F, allowing a tensile stress field to be built up in the metal. The typical manner local heating abuse comes about is the incorrect heating of the hub when installing or removing it from the shaft, and during blade repair by inappropriate methods. Stress corrosion cracks are usually caused by concentrated heat, like oxy-acetylene or oxy-propane used to heat the propeller hub, thus pro- ducing high tensile residual stresses that will surely lead to cracking after several weeks or months of operation. Alternative methods to heat the boss with great care include moving multi-soft flame evenly to avoid hot spots, steam, or electric blankets, and avoiding exceeding the metal’s limits by using temperature sticks or pyrometers. These processes will take longer to reach the desired expansion, but will definitely be safer and assure a longer life for your propeller. Leonardo Montoya is a mechanical engineer with Rice Propulsion. He has 14 years experience in marine propulsion, propeller and nozzle design, production, repairs and sales. Contact him at leonardom@ricepropulsion.com or 877-239- 6304. Avoiding Propeller Damage Due to Inappropriate Handling & Heating By Leonardo Montoya
