1840.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



117 



AMERICAN STEAM BOATS. 



The following comparison of the power of the engines employed in 

 tlic steaiii bonis navigating the river? of North America and those 

 running here on the Thames may not be nninteresting to many of our 

 readers, particnlarly tliose who are engaged in steam navigation. 



We have taken as the basis of om- calculations the following parti- 

 culars of the " Rochester," a steam boat plying on the river Hudson, 

 between New York and Albany, which have been furnished by Mr. 

 David Stevenson in his " Sketch of the Civil Engineering of North 

 America." 



The Rochester is 209 feet 10 inches in length, and 24 feet beam, the 

 depth of her hold is 8 feet 6 inches, and she draws, with an average 

 number of passengers, 4 feet of water. The diameter of the paddle 

 wheels is 24 feet, and the length of the floats, which are 24 in mnnber 

 on each wheel, is 10 feet, and their dip, under the above circumstance, 

 2 feet 6 inches. The vessel is propelled by one engine, liaving a 

 cylinder 43 inches in diameter, with a 10 feet stroke. The steam, 

 which is generated under a high pressure, is cut oft" at half stroke, 

 and condensed. 



Under ordinary circumstances the engine is worked by steam of 

 from 25 to 30 lbs. pressure, and in this case the piston makes about 25 

 double strokes per min\ite ; but when the Rochester is pitched against 

 another \'essel, and at her full speed, the steam is often carried as 

 high as 45 lbs. on the square inch in the boiler, and the piston then 

 makes 27 double strokes, or in other words, moves through a space of 

 540 feet per minute, or 6' 13 miles an hour. In this case the circum- 

 ference of the paddle wheels moves at the rate of 23-13 miles an hour. 



It was under these circumstances that Mr. Stevenson made a passage 

 in this vessel, during which he informs us she attained a speed of llJ-55 

 miles an hour, her piston then making 27 double strokes per minute, 

 and the tide being just on the turn; by which we judge the pressure 

 of the steam in the boiler to have been 45 lbs. on the square inch. Mr. 

 Stevenson remarks that " at that time the vessel could not be far from 

 having attained the maximum speed at which her engines are capable 

 of propeUing her through the water." What the precise signification 

 of this observation may be we do not exactly comprehend : the only 

 way in which we can account for it is either that hard firing was carried 

 nearly to the greatest extent possible in the furnaces, or that 45 lbs. 

 on the square inch was not far from the highest pressure which the 

 boiler was capable of sustaining without damage. 



Allowing that at this great speed the steam is wire-drawn to such a 

 degree as to lose 4-7 1 lbs. of its pressure (which is a much greater loss 

 than is probably experienced in reality), we will assume tlie initial 

 pressvu'e of the steam in the cylinder to have been (including the pres- 

 sure of the atmosphere) 55 lbs. on the square inch. The relative 

 volume of steam of this pressure is 507'3, and as it is cut oft' at half 

 stroke, its mean pressure through the stroke, reckoning the waste space 

 at the end of the cylinder at ^ of the contents of the cylinder, was 

 40*47 lbs. From this must be deducted the pressure in the conilenser, 

 which Mr. Stevenson estimates at 5 lbs. per square inch. This leaves 

 a mean eftective pressure of 41'47 lbs. per square inch, which multi- 

 plied by the area of the piston, which is 1452-2 square inches, gives 

 00222"731bs. for the total effective pressure on the surface of the pis- 

 ton, and nudtiplying this by its velocity 540, and dividing by 33000, 

 we find the gross power to be 985-463 horse ])ower. If we considered 

 the pressure in the condenser as a ]5art of the load of the engine, 

 which would be the fairest way to sliow the comparative merits of 

 diflerent engines, since it is a defect when the pressure in the con- 

 denser is considerable, we should find the gross power of the Roches- 

 ter's engines to be 1 104-3 horse power. 



Supposing the above data to be correct, the quantity of water boiled 

 oft" to supply the engine must have been 5-9041 cubic feet per minute, 

 or 354-2413 cubic feet per hour. 



Considering the Rochester's midship section as a rectangle, its area 

 cannot exceed 90 square feet, and the power employed in propelling 

 her at any given speed must bear some proportion to that area, de- 

 pending on the form of her body. The power is also admitted to vary 

 as the cube of the velocity; therefore the total power employed in 

 propelling a certain vessel at a given speed may be represented by 

 the expression 



KAV, 

 in which K is a coefficient depending on-the form of the vessel, A the 

 area of her immersed midship section, and V her velocity. 



The Gravesend steam boat " Ruby," belonging to the Diamond 

 Company, is 155 feet in length, and her beam 19 feet. Her draught 

 of water with 300 passengers on board was 4 feet 4 inclies forward, 

 and 4 feet 8 inches aft, mean 4 feet inches, and the area of her mid- 

 ship section immersed 65-6 square feet. The diameter of her paddle 



wheels is 17 feet 2 inches, the number of floats on each wheel 11' 

 their length 9 feet, depth 18 inches, and their dip under the above 

 circumstances '20 inches. 



The vessel is propelled by a pair of engines of 50 horso power each. 

 The diameter of the cylinders is 40 inches, the length of stroke 3 I'ect 

 inches, pressure of steam in the boilers 3i lbs. above the atrnospliere, 

 vacuum in the condensers 28i inches, number of revolutions per minute 

 3I5, and speed of the vessel 13-5 miles per hour. 



The area of the two pistons taken together is 2513-28 square inches, 

 and the effective pressure of the steam on each square uich of the 

 pistons is 3-5 + 13-852 lbs. = 17-352 lbs. which multiplied by the area 

 of the pistons gives the total eft'ective pressure = 43010-43 lbs., and 

 multiplying this by the velocity of the pistons, which is -220-5 loot, 

 and dividing by 33000, we find the gross power = 291-4 horse power. 

 Or, considering the pressure in the condenser as a part of the load, as 

 we did for the Rochester, the pressure on each square inch of the 

 pistons being 3-5 -(- 14-71 = 18-21 lbs., we should find the gross 

 power = 305-81 horse power. Of this gross power, wdiich we will 

 call P, a certain portion is employed in overcoming the friction of the 

 engine and the resistance of the steam in the condenser, owing to the 

 vacuum not being perfect ; and we may assume this portion, in engines 

 of the same construction and working on the same system, to bear a 

 constant proporti(m to the gross power P. The remainder, which is 

 employed solely in propelling the vessel, may therefore be represented 

 by the expression k P, in which k is a constant coefficient. 



We have shown above that this quantity may also be represented by 

 K' A' V'-', 

 K' being the coefficient of resistance of the Ruby, A' her immersed 

 midship section, and V her velocity. We must therefore have 

 kP = K' A' V'. 



If P' be the gross power required to propel a vessel of the same 

 form as the Ruby, but whose midship section innnersed is equal to 

 that of the Rochester, or A, at the velocity V, which is that of the 

 Rochester, we shall have 



„ AV^ 

 kP' = kP- 



P' = P 



A^" 



AV' . 

 A'V'3 



Substituting the values of all the known quantities, we obtain 



90X10-55^ „,, ,, 

 P' = 305-81 .TT-r-<7T;rrr — 824-54, 



05-0X13-5' 



which is less than three quarters of the gross power of the Rochester's 

 engine. 



The eft"ective power of the Ruby's engines, that is, the power ap- 

 plied to the paddle wheels, calculated from the resistance to the floats 

 by Mr. Mornay's method given in Tredgold's Treatise on the steam 

 engine, page 132 of the Appendix, but with double the coefficient, 

 (Mr. Mornay having found since the publication of the above mention- 

 ed work, that the resistance to a body moving through a fluid should 

 be double what the generally received theory makes it), is found lo 

 be equal to 2(i7-80 horse power ; but if we calculate their etfectivc 

 power by M. de Pambour's rule, which is to deduct i'rom the eft"ective 

 pressure on the piston first 1 lb. per square inch for the friction with- 

 out load, and then one-eighth of the remainder for the friction due to 

 the load, we find only 240-285. The two methods would, however, 

 give precisely the same result if the pressure in the boiler were 5-38 

 lbs. above the atmosphere ; but it is probable there are some inaccu- 

 racies in the data of both calculations, and the discrepancy is not very 

 great, the ratio of the two numbers obtained being very nearly 9 : 10. 

 However, to give the Americans the advantage of every doubt, we 

 will assume the pressure in the boilers to have been 5 lbs. on the square, 

 inch. (It would be unreasonable to allow more). In this case the 

 gross power would be 331 horse power, and the disponible power by 

 M. de Pambour's method, 202-3)5 horse power. The ratio of the 

 latter to the gross power 331, or k, is thus equal to -79249. 



The gross power being assumed to be 331, instead of 305-81 horse 

 power, makes P', the gross power required to propel the larger vessel 

 of the same form as the Ruby at the rate of 16-55 miles an hour 892-46 

 horse power, which multiplied by k, gives 707-27 for the disponible 

 power required to be applied to tlie paddle wheels. 



The amount of jjower absorbed by friction and other losses in the 

 engines is thus, on the principle of the Ruby's engines 185-19 h. p., 

 on that of the Rochester's 397-03 h. p. 



So that the London engineers are not only capable of constructing 

 engines which would propel vessels at the rate of 16-55 miles an hour 

 (which has only been claimed for the Americans in one solitary in- 



