SPEED SECRETS.

Ashburton Guardian, Volume 56, Issue 114, 25 February 1936, Page 8

SPEED SECRETS.

THE LINER QUEEN MARY. Four spare propellers are to be kept at .Southampton for the use of the liner Queen Mary when she goes into service. The first of them has just left the foundry at Millwall for the Canard stores depot, states a writer in the “Daily Telegraph.” Each weighs 3b tons, is 20 feet wide, and has blades of a maximum breadth of over six feet. They are the largest ever made and, for technical reasons, their size is not likely to he surpassed. From experts of the Manganese, Bronzo and Brass Company, the makers, I learnt something of the calculated power represented in the four massive blades of each screw. Every minute, as the Queen Mary travels on her voyages, 6,500,000 gallons of water will be “expelled” by each of the four screws. The tips of the blades, whirling through the water at 150 miles an hour, will each travel a distance of about 30,000 miles on tin' round trip. •So great is the strain imposed upon the equipment that the propeller-shaft in action is twisted round several degrees from the normal. Each blade yields elestically to the terrific pressure although at its root the special manganese bronze of its construction is a foot thick. A mild electric current is actually set up between the ship’s hull and the churning screws as she travels through the water. The great liner will be “sucked.” rather than “pushed” through the water. Actually two-thirds of the propulsive effort is exerted by the back, or suction, face of the blades, and only one-third by the so-called driving face. A dozenexperts have worked for two years on the design of the propellers—they cannot be mass produced. Weather conditions, speed requirements, clearances, characteristics of each radial section of the blades—based to some extent on aeronautical principles—and choice ol materials, were all factors to lie considered. Cavitation the formation of a vacuum on part of the blades at certain speeds of revolution, of cardinal importance to the lilt* of the propeller and the vessel’s speed and erosion had to be overcome, in relation to the Queen Mary’s specifications. The final design was the result of many tank experiments, and the study of efficiency curves of whole ‘ families” of other propellers.