SALT WATER ACTION.

Auckland Star, Volume LXIII, Issue 194, 17 August 1932, Page 3

SALT WATER ACTION.

WEAR ON PROPELLERS. SCARRED AND PITTED. FERRY STEAMER BLADES. Another application of the old proverb, "constant dripping wears away a stone," is to be (seen in the wear and tear of water on the propellers of a vessel continually in use. The Devonport Ferry Company have recently had occasion to discard some of the propellers which have been used on one of their vessels for some years past, the reason being that the blades have been pitted by the csalt water almost beyond recognition. The blades were iitted to the Toroa in 1925, and ware among- the 12 blades sent out from England with the engine. The engineer to the company said that they were supposed to bo of gunmeta'l, which should give a much longer life than seven years. He pointed out that the wear had taken place on the driving surface of the blades. One theory advanced for the wear, besides that of an inferior metal, which would obviously be one contributing canec, was that millions of molecules of air were sucked down by the twisting action of the blades, and the concussion of them en the metal caused the wearing. That, however, was open to debate. The better and more expensive propellers, said the engineer, were made o_ bull metal, which resisted the action of sea water much more vigorously. He cited the case of the propellers of an old tug which was well-known on the harbour some years ago, the Young Bungaree. All the necessary engineering particulars were given to a firm at Home, and the propellers which were sent out, made of finst-claes metal, showed very little apparent wear after 17 years' constant use. For a vessel that was all the time starting and stopping, an efficient propeller was most important, for the number of revolutions started to increase with the wear and the loee of surface, and that meant in turn more wear on the engines, and a less ability on the part of the vessel to pull up at the wharves. The slip of the screw was also increased. For one turn of the propeller, the vessel was supposed to move Bft Din. It actually moved less, owing to the resistance of the hull. The difference between the speed of the hull and the moving of the propeller was the slip of the propeller. That had been calculated as higli as 15 to 17 per cent.

"Well, they're not much good now," the, engineer said, and anyone seeing the irregularity of the edge, and the wearing even of the inner surface, had no difficulty in agreeing with him.