CHAPTER 7
SHAFTING AND PROPELLERS TEXT SHAFTING AND PROPELLERS WORDS AND EXPRESSIONS
TEXT The transmission system on a ship transmits power from the engine to the propeller.It includes shafts, bearings and finally the propeller itself. The thrust from the propeller is transferred to the ship through the transmission system The system includes the thrust shaft, one or more intermediate shafts and the tailshaft These shafts are supported by the thrust block, intermediate bearings and the stemtube bearing. There is a sealing arrangeme at either end of the tailshaft with the propeller and cone completing the arrangement. The thrust block transfers the thrust from the propeller to the hull of the ship. It must therefore be solidly constructed and the construction must be strong enough to withstand normal and shock loads. One of the two types of shaft bearings is the aftermost tunnel bearing. The aftermost tunnel bearing has a top and bottom bearing shell because it must counteract the propeller mass and take a vertical upward thrust at the forward end of the tailshaft. The other shaft bearings only support the shaft weight and thus have only lower half bearing shells. The sterntube bearing supports the tailshaft and a considerable proportion of the propeller weight. It also acts as a gland to prevent the entry of sea water. There may be one or more sections of intermediate shafting between the thrust shaft and the tailshaft, depending upon the machinery space location. The intermediate shafting has flanges at each end and may be increased in diameter where it is supported by bearings. The propeller consists of a boss with several blades. When rotated it thrusts its way through the water. The thrust is transmitted along the shafting to the thrust block and finally to the ship's structure A propeller which turns clockwise when viewed from aft is considered right-handed. A twin-screw ship usually has a right-handed starboard propeller and left-handed port propeller. Cavitation, the forming and bursting of vapour-filled cavities or bubbles, can occur as a result of pressure variations on the back of a propeller blade. The results are a loss of thrust, erosion of the blade surface, vibrations in the afterbody of the ship and noise. When a ship is in dry - dock the propeller should be thoroughly examined and repaired if necessary. Even the smallest cracks should not be ignored as they can result in the loss of a blade Edge cracks should be welded up with suitable electrodes. Bent blades should be made good by heat treatment, surface roughness caused by slight pitting can be lightly ground out and the area polished.
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