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IN THE ENGINE ROOM. PROPORTIONS AND. EFFICIENCY OF PRO- PELLER WHEELS. BY CAPT. H. C. PEARSONS. When we see among the magnificent vessels on the lakes two large vessels of similar proportions and of about the same displacement, making what we call good time, without material difference in their speed, though one of them has less than 40 per cent of the power of the other, we are led instinctively to ask the cause of so much difference in the cost and power of the proportion when there is so little difference in the results. Again, we hear from those reported to be good en- gineers such expressions as the following: é “A wheel is of little account that does not show a liberal slip.”’ ‘‘Some wheels run too near to the pitch.”’ Such expressions would indicate that ‘slip’? was at a premium. In such a case why not use a farmer’s post auger for a propeller wheel? It would afford all the slip desired, and at far less expense than the modern wheel. Moreover, it would be less exposed to injury. Such conditions and remarks show a lamentable lack of system or method, or knowledge of; fundamental principles concerning the propeller wheel. It is rare that any two engineers, high in the confidence of ship- owners, will design machinery to fit out a vessel for any given purpose that will give the same results, or even approximately the same results. There is no doubt but this diversity of re- sults comes from the want of a knowledge of © one or two of the principal factors that enter into the problem. : It does not appear that the ideathat a certain amount of water per unit of power, must be acted on by the wheel fin order to afford..the engine sufficient inertia to work upon has yet found its way into the minds of more than a few engineers, or we would not see such di- versity of size of wheels for working off the same amount of power that our lake marine affords. And there.is another important factor that, so far as I know, has not been brought to use, nor even mooted by engineers, and that is the pitch angle at the center of effort of the blade of the wheel. ; It is this angle that determines the portion or per cent of the total work delivered to the wheel that is available for propulsion. And it seems to have escaped the notice of engineers that, this angle being known, the efficiency of the wheel becomes determined at once and readily found. oO This is of the first importance in designing the motive plant for a steamship, as the place of the shaft cannot be known till the diameter of wheel is known, and this diameter must be known from its own efficiency, and the amount of power to be employed. But this pitch angle cannot be known till its place or distance from the center of wheel is known, because the angle of pitch varies from the end of the blade, where it is minimum, to the center, where it is a right angle. In ‘King’s Notes on Steam’ we are informed that this center of effort is at the circumference of a circle having one half the area of a circle whose diameter is : that of the wheel, or the square root of % =about 70 per cent of the radius from center of wheel. This would be the case only for the motion of wheel, which quickly removes this question from the realm of statics to that of dynamics, where the conditions are very dif- ferent. For the present we will merely say that this - point is at 90 per cent of radius of wheel from the cen-— ter, not 70 per cent. It would be foreign to the object of this paper to give the analysis from which the formula was derived for finding this point—but I give the formula. The distance from center of wheel to center of effort equals : Voz plus 3% R= 90%R where R represents length of blade measured from cen- ter of wheel. The method of finding the minimum volume of water ‘to be acted on by the wheel perl. H. P. to afford the THE MARINE RECORD. “requisite amount of inertia for engine to work upon, is that of observation—simple and pure. In this discussion the helix of the wheel face is sup- posed to be that of the true screw, the most efficient of all the multitudinous forms known. And it will be ob- served, too, that a unit of disc area in one wheel may have a very different holding power from that of the same unit in another wheel, which may have a differ- ent weight of water on it, thus: A ten-foot wheel will afford twice as much inertia per unit of disc area as a five-foot wheel, for the reason that there is twice as much weight of water over its center of effort. We will show the use of these two factors in the de- signing of the propeller wheel: ; Suppose we have found that a vessel must have a traction or pull on the towline of 4,000 I. H. P. to propel it through the water at a given rate, and that we wish to apply that power by means of twin screws—required the diameter of the screws. Rule for the power.— Divide the direct traction—4,000 Z. H. P. —by the efficiency of the screw to be used, The quotient is the total H. P. that must be provided to produce the re- quired traction or direct pull. “But,’? says one, ‘show are we to know the efficiency of the wheel before it is made and tried?’’ That is precisely the question we are talking about, and which we propose to illustrate. It will be remembered that we have said that the efficiency depends on the pitch angle at the center of effort of the blade. This angleI have computed and SECTION OF CRIB, READY FOR LAUNCHING. given in the following table for wheels whose pitch varies by one-tenth the diameter, from one to two diameters. ‘The efficiency is the square of the cosine of the pitch angles: TABLE OF PITCH-ANGLES ANDYEFFICIENCY OF PROPELLER WHEELS. Pitch in Pitch-angle at | Per Cent of Use- | Per Cent of Lost Diameter. |Center of Effort. » ful Work. Work. 1.0 19,.° 29 89.9 11.0 1.1 21. 16 86.8 13.2 1.2 23: 00 84.7 15.3 3 24. 42 $2.6 17.5 1.4 26. 20 803 19.7 1,5 QT: BT, 78.0 22.0 1.6 29. 30 75.8 24.2 1.7 31. 01 73.5 26.5 1.8 32. 29° Wait 28.9 19 33. 54 68.9 31.1 2.0 35. 17 66.6 33.4 In view of the high piston speed and high tension of steam in current use, we will select a wheel of say 1.3 diameter for the pitch having an efficiency 82.5 percent. This, it must:be remembered, is the per cent of that part of the total power delivered to the wheel, and in no way Can we get any more power with that proportioned wheel. It shows what would be utilized if the wheel could work, as in.a nut, without slip, and hence must be regarded as the /heoretica/, and not the net or ultimate efficiency, which depends largely on the skill of the en- gineer in adapting size of wheel to amount of work to “be done. Dividingiour 4,000 1. H. P. by the efficiency, 82.5 per cent, and by the per cent of.total work delivered to wheel, say 82 per cent, we have: ii 4,000 divided by (82.5% X 82%) = 5,913,103, Pe ae To this add say 20 per cent for a reserve power for emergencies, making a total of 7,095 I. H. P. that must be provided in order that we may depend at all times on a shaft thrust of 4,000 1. H. P. ‘ The cube root of the half of this, or say 3,500, will be the required diameter of wheel, thus: ; 3A 3.500 = 15,15 feet, or say 15. feet. “But,” says one, “that is more than the depth of water that the ship can draw. ”’ Never mind if the wheel does run with its ‘‘back-fin”’ a little ont of water. The damage to the wheel by run- ning slightly out of the water will be nothing, while the damage to the coal bunkers from working with a wheel a foot too small would be a serious matter. Multiplying this diameter by our tabulated ratio 1.3, and we have the pitch = 19.5, or say 19 feet—giving us a wheel 15 x 19 feet. — I give another example from actual practice, showing that we have found about the minimum amount of water to be acted on to afford the requisite inertia for the engine to work upon, and that we have found the place of the center of effort on blade of wheel correctly. See ‘Test of the Steamer City of Lowell,’’ as re- ported in Engineering News for Nov. 14, 1895, by Prof. James E. Denton, of Stevens Institute of Technology. He informs us that the wheels are of 11.1 feet diameter, and 16.63 feet pitch, giving a ratio 1% diameter for the pitch; that with 111.2 revolutions per minute, _ making a speed of 20 statute miles per hour, the “ordinary service speed, the engines developed 3,000 I. H. P., or 1,500 per wheel. ‘Then divid- ing the cube of the diameter of the wheel by the H. P. developed we have BS; 11.12 divided by 1,500 = 1.1 cylindric feet. That is to say, each unit of H. P. has slightly more than one cylindric foot of water to act upon. And deducing the slip from the above _ data, we find it to be 4.7 per cent, which is evidently near its minimum limit. é : Again, wheel making 126.86 revolutions per minute, and 22.19 miles per hour, and engines Z developing about 4,350 H. P., or 2,175 per : wheel. We find the inertia for H. P. 1.11% divided by 2,175 = 69% of 1 cylindric foot and the slip we find to be 7.4 per cent. So that by putting on the extra power the inertia has fallen off from 1.1c. ft. to.69 of one c. ft. per H. P., and the slip has risen from .048 to .074, or more than SO per cent. This shows us that about one cylindric foot is the minimum volume of water per H. P. that can afford the requisite amount of inertia for engine to work upon. Furthermore, Prof. Denton informs us that the efficiency of the screws has reached the unusually high value of 78.3 per cent. Our table shows that a wheel of 1% diameter for the pitch gives 78 per cent of useful work—agreeing with his results within a small frac- tion of 1 per cent. He also gives 81.7 per cent as the part of the total power delivered to wheel, and the ‘‘power of tow hull at 64 per cent,”’ as efficiency of wheel. Multiplying the per cent delivered to wheel, .817 by our tabular efficiency .78, we have .817 X .78 = 63.73 % for efficiency of wheel, thus agreeing again within a small fraction of 1 per cent with Mr. Denton’s figures, and thereby showing that we have found the correct | place of the pitch-angle or center of effort. (CONCLUDED NEXT’ WEEK.) sb8 Rus Sey aseton = Wp re ence Reese en Ta TY d PROPOSED NEW DRY-DOCK AT BROOKLYN. A. large new dry-dock is under contemplation at present by the John N. Robbins Company, the agents of the Erie Basin dry-docks, Brooklyn, New York. The company is now in possession of two docks, one : of which is 520 feet long and 105 feet wide. Thé new dock is to be located to the right of these and will b 800 feet long and 150 feet wide. It will be deep enough to take in the largest war or merchant vessel afloat an will have a draft of 28 feet in the clear. Mr. Rob- bins, the president of the company, says that the plans” have not been completed and that it is not decided when the work will actually begin. phish PRON A, PEN RT a a