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CAVITATION. In very high-speed vessels such as de- stroyers, the - troublesome phenomenon termed "cavitation" is likely to occur if the blade surface be too' small for the thrust developed. Cavitation is failure in the supply of water .to the propeller, and results in the formation of a vacuum or cavity at the forward faces of the blades owing to the water being unable to fol- low the blades fast enough. It was first discovered by Mr. Barnaby during the trials of H. M. destroyer Daring in 1894. It may be likened to what would occur in the case of a pump plunger which rises so quickly that it exerts a pull on the water beneath it of more than 15 lbs. per square inch. In that case the pressure of the atmosphere is insuf- ficient to cause the water to follow the plinger, and a vacuum. or cavity. is formed. In the case of propellers, a cavity is formed on the forward side of the blade if the peripheral speed is too high, on account of the head of water being in- sufficient to cause it to follow the blade; and the consequence is a reduced thrust. At the surface of the water, cavitation . 'would take place when the pull of the blade on the water exceeds the pressure of the atmosphere; and below the sur- face, when the pull exceeds the pressure of the atmosphere plus the pressure due to the head of. water. This, however, leaves out of consideration, for the sake of simplicity, the fact that, owing to the water giving off vapor when the pressure is reduced, cavitation actually takes place when the pull is somewhat less than this. . It commences to take place at the tips of the blades (where the circumferential velocity is highest) as they pass the high- est point of their path where the pressure of water is least. In destroyers and other high-speed ves- sels, the projected blade area should not be less than that which will ensure the thrust per square inch being less than about .12 Ibs., otherwise cav.tation may ensue. : Cavitation may be.detected by a sharp ascent in the slip curve above a certain speed. The thrust exerted by a propeller de- pends on the difference in pressure on 'the after and forward sides of the blades. It is therefore desirable to have as great a pressure on the after side and as small a pressure on the forward side as pos- sible. Reduction of pressure on the for- ward side of the blade is equivalent to increase of thrust, afid 's to be sought for. It would therefore seem, on the face of it, that if the propeller revolved so rapidly that the water was unable to follow the blades, so causing the pressure on the forward side to fall to zero, a highly satisfactory state of things would be arrived at. But further consideration 'sketch of the paddle wheel. "TAE Marine REVIEW will show that if the water does not fol- low the blades, the pressure on the after side will also fall, as there would. be a lack of water there also, so causing a greatly diminished thrust. It is a sine qua non that to obtain efficiency the wa- ter must be in contact with the forward side of the blade. Cavitation is more easily grasped by considering the paddle-wheel. Referring to the sketch which shows a hypothetical case for the sake of simplicity, we will suppose we are dealing with only two floats A and B. The float A enters the water so rapidly that a cavity is formed So far this float is con- satisfactory state of forward Of 1b cerned, a highly RE gives rise to the greatest difference of opinion. But the author feels sure that, in time, the apparent vagaries of the screw will be entirely mastered by theory and reduced to mathematical formulae; and then we may expect marine engineer- ing to progress by leaps and bounds. SYMBOLS USED IN THIS PAPER, F--Force or thrust exerted by the pro- peller in noundals. m--Mass of water acted on by the propeller per second in pounds. v--Speed of the race relatively to the ship in feet per second. V--Speed of the ship in feet per sec- ond, , things is arrived at;-for we have a great pressure of water on the after face of the float and no pressure on the forward face, tending to create a great amount of thrust to drive the vessel ahead. But it is apparent that the float B has no water to engage with, and a loss in thrust is therefore inevitable. (What would ac- tually happen in a real case, with the full number of floats and the vessel moving ahead, is that the water line in the vicin- ity of the wheel would probably take the form shown by the dotted line, that is to say, all the floats would be only partially but equally immersed). ; Almost the same sort of thing happens to the screw-propeller; you cannot have great pressure on the aftersides of the blades while there is a cavity on the for- ward sides, owing to an insufficient quan- tity of water flowing past them. Cavitation is due to the slip of the propeller; if there were no slip there could be no -cavitation. This will be more easily seen by referring to the In this pa- per the author has touched upon a few of the leading features only..of screw- propellers and propulsion; to deal with the subject at all fully would be impos- sible in a short' paper. It is the most recondite and difficult subject that mar- ine engineers have to deal with, and it W--Weight of a cubic foot of sea wa- ter' in poundals. g--Force of gravity in poundals. A--Area of cross section of the race in square feet. w--Speed of the wake in feet per sec- ond. oe Vi--Speed of the propeller through the water in which it works in feet per sec- ond. ' P--Pitch of propeller in feet. R--Revolutions of propeller per min- ute. M--Momentum of the race per second. D--Diameter of propeller in feet. . The naval cruiser Newark has been turned over to the first naval battal- ion, New York state militia, and will be fitted up at once for her new duties. The naval militia will in the meantime retain its old headquarters on the Granité State, formerly the New Hampshire. The New York & Porto Rico Steam- ship Co. has purchased the steamers Hugoma and Massapequa for service between New Orleans and Porto Rico. These steamers will be operated in conjunction with those already on the route, namely, the Arkadia, Berwind and Santurce. |