STEAMERS' MANGANESE PROPELLERS.

Te Aroha News, Volume VII, Issue 418, 9 November 1889, Page 6

STEAMERS' MANGANESE PROPELLERS.

The escape of the warship Calliope from Apia harbour, when all other men-of-war were either sunk or beached, has been attributed by^some experts in England to the fact that her propeller was made of manganoFo bronze. The matter is referred to not merely lor the sake of demonstrating Ihe advantage which it is thought the Calliope derived from the. character of her propeller, but also to show that a sqrew ot this description really oilers a substantial gain in the mat,ter of speed. The Calliope made herwayagainstthe storm at the rate of one-halt knot per, hour, which represented the difference between what horcngineswero capable of doing when forced to their maximum, and the power of the storm, and' it very rarely happens that an extra half knot is all that there is between safety and destruction. Yeb'hh halt knot is not to be despised at any time, fair weather or foul, particularly should the .voyage be a long one. The extra speed is acceptable, and the saving in coal, should it not be requiied, ii> by no means inconsiderable. Tho steel screw blados of several steamers plying between England and the west coast of South America were exchanged ior others of manganese bronze, and, although the gain in one instance was only 9 quarter of a' knot per hour, the effect was eeen in the saving of between eight and nine tons per dayinbhecoalaccount. In two ships running to Australia, in which a change of propellers was made from steel to manganese bronze, ot exactly the &amo suriac'o and pitch, tho speed was increased almost a kn6t an hour. Eight ships of the White Star line have propellers of manganese bronze,*as> well as tho City of New York and the City of Paris and two of the Cunarders. Other largo companies have adopted thesamo material, and have been rewarded by increased speed and coal economy. Outside the circlo of the mercantile maiine theie are numerous instances in the British navy, where ti saving in the weight of the propeller of 20 to 25 per cent, is claimed. The French have also adopted the tame metal for some of their fastest vessels, notably the Forbin, of nineteen and three-quarter knots, one of the fastest cruisers in the world. These examples have- likewise been followed by Russia in some of her fast vessels. A peculiar kind of manganese bronze is used for this purpose, which possesses great strength and toughness, said to be about equal to that of the best cast steel, and as compared with gun metal, of which propellers used to be quite generally made, it possesses twice the strength, so that a greater reduction can be made~ in the thickness of the blades, which therefore become finer and sharper. There is also a peculiar smoothness of surface, which lesults in a les&ened skin friction, a very important consideration \vhere high rates ot speed are to be maintained, and the competition for first place among the ocean greyhounds is as keen and active as at preseut. Manganese bronze castings are claimed to be smoother and less liable to warp than cast steel. Freedom from pitting and corrosion preserves the blados for a- long time in their original form, so that the -life of the propeller will bo fully equal to that of the vessel. This metal is considerably more expensive than steel, and weight for weight. It is about a quarter more expensive than gun metal, but the saving in weight over the latter and the fact ot the pitting which steel is subjected to from the action of salt water makes a very strong argument in favour of manganese bronze. ' '