18-19.1 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



231 



ON THE PADDLES OF STEAMERS. 



On the Paddles of Steamers — their Figure^ Dip, ndckness, Mate- 

 rial, Number, SfC. By Thomas Ewbank, Esq., City of New York. 

 [From the Journal of the Franklin Institute.'] 

 {Concluded from page 213.) 



The foregoing experiments and remarks relate chiefly to the 

 figure and dip of paddles. Other traits next solicited investiga- 

 tion; and, though neither prominent, nor promising any adequate 

 i-eward for the requisite lahour, they were thought worth attending 

 to, since engineers will certainly he urged shortly to cast about 

 for every means of adding, though ever so little, to the speed of 

 steamers. 



Buoyant or Displacing Paddles. 



It had been imagined, that the resistance which fluids oppose to 

 the sinking of bulky bodies in them, might be employed as an ele- 

 ment of propulsion — that if close barrels, for example, were fas- 

 tened to the arms of a wheel, their ends would act as paddles, and 

 the force required to plunge them (equal to 62 lb. for each cubic 

 foot of water displaced), also react favourably on the boat. To 

 test this idea, eight square and tight boxes, 7 inches by 7, and 



6 inches deep, were secured to the arms of one wheel, and set to 

 work against the eight blades, No. 1, (fig. 3), on the other. The 

 boxes required, very sensibly, more power to carry them round 

 than any other tried, and were miserably deficient in pushing the 

 vessel forward with it — certainly not equalling four of the com- 

 peting blades. They produced quite a commotion in the water, 

 carried large quantities over with them, and, could we have com- 

 municated sufficient velocity, would probably have formed a verti- 

 cal ring of it. These boxes were, and should be, considered simply 

 as unusually thick blades. All paddles are buoyant in proportion 

 to their thickness. 



Thickness of Paddles. 



But though worthless in one respect, they were valuable in 

 another, for they led us to the fact, or the law, that the propelling 

 1-irtue of blades expands and contracts with their thickness. 

 Thicken them till they touch each other, and they form a perfect 

 drum, which could exert no more propelling power than a revolv- 

 ing grindstone; — reduce them to the thinnest plates, consistent 

 with the strains they have to oppose, and in the same ratio that 

 property is augmented in them. 



The boxes nere removed, and hoards, gths of an inch thick, and 



7 inches square put in their places. These represented common 

 plank paddles, and were found sensibly inferior to their metalline 

 competitors, whose thickness was slightly less than xct'^ inch. 

 We next took away two of the latter, when no very obvious change 

 in the boat's direction occurred. When two moi-e were taken off, 

 the remaining four were unable to contend with the wooden ones. 

 These, it will be remembered, were |th the thickness of the boxes, 

 and consequently inherited that proportion of their defects. 



It was also very observable how much more water was raised by 

 the boards than by the plates. It could not easily be cast oif their 

 blunt boundaries, but kept running over them, from one side to 

 another — a fact rendered more distinct in the boxes. Nothing 

 could declare plainer, that the sharper the dipping edges of paddles 

 are made, the more back-water they throw off at the point where 

 its departure is most beneficial: that is, when the re-action favours 

 the vessel's progress — and, consequently, less is carried higher 

 than the axis. A very little labour would impart this feature — in 

 other words, would make their section a wedge. The resulting 

 benefit would repay the expenditure a hundred-fold. 



Compared to metal, wood approaches in its nature to sponge; 

 water clings to it; its pores are absorbing vessels, that suck it in, 

 and assist to retain it on the surface. 



Here nature also confirms the positions arrived at. Extreme 

 tenuity of blade is stamped with perfection by her. Hence we see 

 it strengthened by reticulated bars in the wings of insects — by 

 radial, angular, and tapering ribs in the fins and tails of fishes. 

 An uniformly thick, and unsupported slab, like our paddles, is 

 nowhere met with. We cannot imagine natatory or soaring 

 organs, formed after such a pattern, without feeling the absurdity. 



The caudal propellers of fishes are necessarily thick where they 

 join the bodies, but how rapidly is the substance diminished, and 

 to a mere film, at their extremities, so much so, that they are often 

 there torn and jagged, by accident or wear, as fishermen well know. 

 There must, therefore, be some powerful reason for withholding 

 the material — one that overbalances all inconveniences resulting 

 from its absence; and what can it be but the thinner the blade, the 

 more efficient as a proi)eller it is — the longer is its stroke, and the 

 more effectual is the power that wields it. The same law prevails 



in the wings of birds; their outward boundaries are feathered off 

 to almost nothing. 



The reflection is irresistable. With what nicety and care nature 

 perfects her propellers, and how clumsy and untinished are ours; 

 as if, forsooth, a vessel's progress did not depend upon them! 



The last two experiments demonstrate, that the less water a 

 paddle displaces by its volume, the more efficient it is; that all ac- 

 cumulation of material behind its acting face, beyond what is ab- 

 solutely necessary to strengthen it, is injurious, and ought to he 

 avoided. But how does this accord with the current practice? 

 Oaken planks are universally employed, and I have heard more 

 than one engineer assert, that the thicker they are the better! 

 Because, said they, if their propelling property be not enhanced, 

 it is not diminished, and their additional weight is a positive ad- 

 vantage, since the heavier the wheels are, the easier they work — 

 the more uniform are their movements. 



The Gorgon, an English steamer, had "large wheels and little 

 power," so she used oak or pine scantlings, 5 inches by 6, or 6 by 

 8, for paddles. Had her managers been aware of the true effect 

 of thick blades, they never would have adopted them with the 

 view of economising power. 



Paddle planks vary in thickness from 1^ to 3 inches. No sea 

 steamers have them less than 2 inches. In the English vessels 

 they are 2-}; in others, as the Franklin, they are 2i; in some of 

 the largest class they are 3. The Atlantic and the Pacific, each of 

 3,000 tons, now building for the Collins' Line, are to have them 

 3 inches. The former is to have 28 blades; hence, united, they 

 will form a solid mass, seven feet thick, in each wheel— just one- 

 fifth of its diameter! They are to be 12i feet long, by 34 inclies; 

 those of both wheels will, therefore, contain nearly 500 cubic "feet 

 of timber, and must displace that enormous volume of water at 

 every revolution, by their submersion alone! — and, as we have 

 seen, not only uselessly, but with a serious retardation of the ves- 

 sel's headway, and waste of her motive power. 



The wheels of the Pacific are to be 36 feet in diameter; each 

 will have 30 blades, 11 J feet, by 3 feet; the solid contents of her 

 paddles will, therefore, equal 517 cubic feet. Her loss from the 

 same source will, therefore, be greater. In every revolution of 

 each wheel, her paddles will lose 7* feet of effective stroke, and 

 those of the Athintic 7 feet! Those of the ocean steamer United 

 States are 2^ or 2J inches thick; they are 36 in number, but as 

 they are "split," and attached on both sides of the arms, there are 

 really 72. They certainly diminish the effective strokes of her 

 blades, from 10 to 15 feet, in every turn of her wheels, startling as 

 the assertion is. 



Has the attention of engineers ever been turned this way? Or 

 have they forgotten, that a \olume of water equal to that of a 

 boat's paddles, and every inch of material submerged with them, 

 is neutralised as a resisting medium, as often as it is displaced by 

 their immersion; — that water is to them what steam is to pistons 

 — the more space the latter occupy in cylinders, the shorter be- 

 comes their stroke, because metal then takes the place of steam; 

 the object to be moved crowds out the mover. Thicken a piston 

 till it fills its cylinder, and the motive agent being wholly kept 

 out, all motion ceases. 



It is much the same with the paddles of a wheel. Let them fill 

 up ^, ^, ir, or 4, of the circles they describe, and in those propor- 

 tions they lose their virtue, because in the same proportion they dis- 

 place, or push aside, the fluid agent on which their worth depends. 



The Atlantic will lose seven feet stroke in every turn of her 

 wheels. I leave to mathematicians to determine, how many more 

 miles an hour she would make, if the loss were reduced to seven 

 inches, by using j-inch iron, in place of 3-inch plank. 



There are several interesting questions about paddles that yet 

 require solutions, but as respects their thickness, there is no tnean 

 to seek; the thinnest is the best under all circumstances— thin, 

 were it possible, as a lamina of mica. The only question is, 

 AV^hat material will supply the thinnest sheets to resist the pres- 

 sure they are to oppose? Plates of steel, I opine, will yet be 

 adopted. 



Number of Paddles. 



The experiments of each day convinced us that, so far as pro- 

 pulsion is concerned, the fewer the paddles, the faster went the 

 boat, so long as one at each wheel, or an area equal to the face of 

 one, was kept in full play. A greater number in the water merely 

 cuts it into slices, throws them into commotion and diminishes the 

 resistance they should oppose to the blades. As a further elucida- 

 tion of this fact, we tried, at the suggestion of Mr. B., four blades, 

 7 X 11', against the eight test ones, 7x7. The smaller number 

 had a decided advantage over the greater, and the cause was visi- 

 ble: they had a fuU sweep, through an unbroken, undisturbed 



