3.8.4 Change of thruster propeller blade design The original propeller blades were designed in 1994 by the company KaMeWa AB that has now been acquired by Rolls-Royce. The standard production method for propeller blades was milling of the blade by CNC machines (Computer Numerical Control). However, the blade foot area was ground and polished by hand (figure 52 on page 46). In 2006/2007, the production methods were changed to increase the efficiency and accuracy in propeller blade manufacturing. With the new production methods, the blade foot area was also milled by means of CNC machines. During the preparation for the new production technology, all previous propeller blade designs had to be transitioned from 2D drawings into 3D models by CAD (Computer Aided Design). It was discovered that the radius of the transition between the blade base and the blade itself was too small. This applied to several blade designs and hence Rolls Royce Marine decided to change the design and immediately informed the relevant foundry to modify the relevant patterns. The propeller blades of the new design differ from the original design by weighing 4.5 kg less and having a considerably larger radius of the transition between the blade base and the blade. During the change of production method, it was also discovered that two of the Rolls Royce Marine manufacturing plants had been using different design philosophies with respect to indicating the radius of the transition between the blade base and the blade. This radius, if indicated on a drawing, is only an indication. The 3D shape of the blade is complex and radiuses will vary depending on which section of the root/blade is measured. It is hard to make a standardized and exact measure of the radius of the transition between the blade base and the blade and there are no requirements for this in neither ISO484-22, nor in the relevant Class Society rules. Bearing this in mind, the manufacturer found that the relevant radius was below what was indicated on the drawing. This is explained as a mistake made during the pattern production process, or as the method of grinding by hand in the blade foot area (prior to 2006/07) leading to excessive material removal; a deviation that was not captured by the manufacturer’s internal quality system. The new propeller blade on the aft stern thruster that was fitted as a replacement in October 2012 was of the new design from after 2006/2007 and its radius of the transition was 35 mm. The damaged forward stern thruster that caused the leakage had a propeller design of the older design with a smaller radius.
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ISO484-2 “Shipbuilding – Ship screw propellers – Manufacturing tolerances – Part 2: Propellers of diameters between 0.80 m and 250 m inclusive”.
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