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October, 1914 the salient features of this document, but perhaps enough has been said to indicate the value of its contents to manufacturers. Supplementing this in- formation and similar matter concern- ing other articles, the department has arrange a postal and cable service of urgent commercial news from the consular officers all over the world, and this information will also be placed at the disposal of British trade with the least possible delay. Screw Propellers In a recent address before the gradu- ate section of the North East Coast In- stitution of Engineering and Shipbuild- ige on "Screw. Propellers' "Mr. P. Y. Brimblecombe remarked that screw pro- pellers being a factor of great import- ance in the propulsion of ships, innumer- able calculations had been made and ex- periments performed with a view to de- termining their most efficient form. He went very exhaustively in the course of his remarks, into the theory of propeller - construction and work, and gave his au- dience the benefit of many useful forma- lae in connection therewith. He then proceeded to refer to the phenomenon known as cavitation. If, he said, a ship's propeller was worked at gradually increasing revolutions and producing gradually increased thrust, it was found that, beyond a certain point, there must be a marked increase in the number of revolutions required to pro- duce a very moderate increase of thrust --i.c., the apparent slip showed a sudden increase. That had been attributed to the formation of cavities, principally at the back of the propeller blade, so that the propeller was working to some ex- tent in a partial vacuum. { Mr. Barnaby considered that the phe- nomenon was due to an attempt to pro- duce too large a thrust for the area of the blades, the maximum possible being about 1114 pounds per square inch, while Naval Constructor Taylor, of the American Navy, considered that Barna- by's explanation was incomplete, and suggested that the two main factors in cavitation were the speed of the blade through the water and the shape of the blade section. The former could be con- veniently measured by tip speed, and that should be kept as low as possible, consistent with the other factors having reasonable values. The limit of tip speed was about 12,000 feet per minute. Taylor's view was that the controlling factor in cavitation was the formation of cavities on the front of the face of the leading edge, due to the driving face, Which increased with the increase of speed. He, therefore, suggested that the best way to avoid or delay cavitation Was to make the blade as wide as Pos- sible, so that, if cascading occurred over THE MARINE REVIEW the leading portion, the water might get back on the trailing portion and deliver its energy as thrust; and the leading edge of the blades should be made as thin as possible, so as to delay or avoid cascading, and, in order to obtain the necessary strength for leading edge, he suggested casting ribs across the blade. Deeper immersion of the propeller would also assist in eliminating cavita- tion, as, of course, the pressure of water would be greater. It was very important that a propeller should have sufficient immersion, since, if it broke the surface of the water, its efficiency would be re- duced considerably, and the greater the depth of the screw below the surface the less was the chance of its being drawn out of the water by pitching or rolling. Blade Material The materials of which propeller blades were made included cast iron, cast steel, forged steel, and manganese or some other strong bronze. Cast iron was used for the blades of propellers which worked under conditions rendering them very liable to strike against obstruc- tions. When so striking, the cast iron being weak, the blade broke, and by so breaking saved the shafting of the en- gine. Its disadvantages were extreme corrosion in seawater, heavy blade sec- tions and blunt edges. Cast steel was stronger than cast iron, but had the same disadvantages, although in a lesser degree. Manganese bronze and _ other strong bronzes appeared to be all that might be wished for in propeller ma- terial. They were of high strength, per- mitting a low ratio of thickness to width of blade, while they could be brought to a sharp edge, and were sub- ject to comparatively little corrosion. They were also cast without difficulty, giving blades free from porosity and blow-holes. They would, however, exer- cise a strong corrosive action on a steel hull, unless zinc plates were fitted in their vicinity. Hubs were usually made of cast iron or semi-steel for cast iron or cast steel propellers, and of mangan- ese bronze for manganese bronze blades. Propeller E ficiency The efficiency of a propeller varied in- versely as the number of blades--i.e., a propeller with two blades was more effi- cient than a propeller with three identi- cal blades, but the total thrust and torque increased as the number of blades was increased, although the thrust and torque per blade fell off. In practice, the number of blades was decided by considering the efficiency of resulting propellers, it being remembered that a four-bladed propeller might be smaller than a three-bladed, and hence have a 387 pitch ratio more favorable to efficiency than the pitch ratio of the corresponding three-bladed propeller. Though there was little direct infor- mation on the subject, it was probable that single screws were more efficient than twin screws, and that there was a progressive disadvantage in using triple and quadruple screws. The differences were not large, however, and any type under favorable conditions might be more efficient than others for which favorable condition ¢ould not be secur- ed. The single engine was, of course, simpler and cheaper than two engines with the same power, and, in like man- ner, two were cheaper than three or four. For moderate powers and speeds, a single screw would be chosen, unless there were distinct advantages, such as handiness or greater security from breakdown. For example, warships all had two screws or more, and turbine steamers had two, three, or four screws, for the better ace otnosene of the turbine. Due to Propeller Action A topic which was much discussed among laymen was that of a' vessel's deviation from a straight course while the rudder was kept in a central posi- tion. That circumstance was due to pro- peller. action. The speed of wake at the surface of the water was greater than that at the keel--consequently the upper blade experienced more. resistance than the lower, and tended to drive the stem round. If the screw was right-handed and did not draw down air, it would tend to cause the vessel to carry a star--- board helm in order to maintain a course. If there was air in the wake, caused, for example, by the vessel being at a light draught of water, the effect was reversed, the lower blade predom- inated, and a port helm must be carried. The odds in the present naval strug- gle may probably be expressed pretty accurately--so far as material goes--in terms of weight of heavy gunfire. In- cluding the two battleships just taken over, the British forces in the North -- Sea on the declaration of war mounted 142 13.5-inch guns and 306 12-inch guns. These weapons have an aggregate fire delivery respectively of 297 tons and 116 tons of metal. The German High Sea fleet on the declaration of war mounted 108 12-inch guns and 126 11-inch guns, respectively discharging 39.3 tons and 30 tons of projectiles. The Bureau Veritas reports that dur- ing the month of June 287 steamers and 69 sailing vessels met with accidents, and that the number of vessels lost were 15 steamers of 25,273 tons and 22 eae vessels of 7,156 tons.