80 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[IVIaroh, 



the best of their way to their port without reference to the saving of 

 fuel, time, in fact, being frequently of much greater value than a few 

 tons of coals. We state this as a practical fact, for the saving of fuel 

 by the expansion of steam is now well known to those who have 

 devoted the slightest attention to the subject. 



Nominal horse power is certainly but a conventional expression, de- 

 signed by the late Mr. Watt, as an approximative measure of the com- 

 mercial value of his engines, it was very applicable to the circumstances 

 of his times, and perhaps, until something letter is produced, as much 

 so to ours. It is not, as it has been called, " empirical," because it is 

 founded on rational grounds, and if me could gel all engineers to agree 

 to one standard of computation, it is doubtful if any better system could 

 be introduced — it is a mechanical test from which no engineer can 

 escape, and we are enabled, with the aid of the indicator, to apportion 

 to its work just as much merit as it possesses ; by its use we imme- 

 diately discover the principles on which the engine is constructed, the 

 rate of expansion, the state of the vacuum, the proportions and setting 

 of the sliding valves, and other detail absolutely necessary to the good 

 performance of an engine — in fact, we can distinguish between a good 

 and a bad engine, between science and pretension — and in conjunction 

 with data relative to the consumption of fuel can precisely compute 

 the value of each man's work. 



We say it will be difficult to produce anything better than this. 

 The commercial world is satisfied, because they may be supposed to 

 know more of a two foot rule, and its applicability to measure the 

 diameter and length of a cylinder, than they can possibly be of the 

 ratio, or of the rarefaction of water when converted into steam, or of 

 the more intricate theory of expansion. No, the commercial man will 

 continue to buy his steam engines by the capacity of their cylinders, 

 and with the aid of his coal merchant's account, and the amount of 

 nork done, will come to a pretty fair conclusion, as to which engineer is 

 most deserving of his confidence. 



These observations have particular reference to marine engines, in 

 which it is well known that space occupied and weight is of paramount 

 importance, and it has been the aim of much consideration and reflec- 

 tion to obtain these, with the exertion of the greatest power at a mini- 

 mum expence. We call this the ne plus ultra of marine engineer- 

 ing. 



We may, therefore, doubt the judgement of the engineers of the 

 Great Britain, who are using cylinders equal to 1200 horses, and by 

 expansion, reducing the same to 700 horses, occupying both the 

 weight and space of the former power ; but perhaps we are again in 

 fault, now reasoning on the assumption of Mr. Enys, as in the former 

 paper on that of Mr. Hill. 



Mr. Enys observes. " that nominal horse power is a most inaccurate 



basis for calculations of this nature," that is, the consumption of coal 

 and speed of the Great Britain. 



With reference to the first proposition, the consumption of fuel, we 

 most freely assent. The value and the theory of expansion has been so 

 fully explained by many writers, perhaps especially so by Farey, that 

 it is beyond dispute — it is no new light which has burst upon us. A 

 few years since, some attention was drawn to the published logs of 

 certain steamers, in which assertions were made, that the consumption 

 per horse was (as now reproduced by Mr. Enys,) as low as bi lb. 

 nominal h. p. per hour, and which was satisfactorily shewn to have 

 been produced by expansion, during fine weather and favourable winds ; 

 but that, when the full power was exerted, the consumption of fuel 

 reached from nine to 10 lb., the general result of the best machinery, 

 so will it be with the Great Britain. If, as we had supposed, from 

 the lucubrations of Mr. Hill, that her engines are designed, when neces- 

 sary, to work to their full power, and by full power we mean expan- 

 sion by the sliding valve alone, and exciting an average pressure on 

 the piston of at least 14 lb. per square inch, exclusive of 4 the friction, 

 then will our computation of consumption at 10 lb. per horse be found 

 correct ; that is, supposing the engines and boilers, scientifically and 

 practically constructed ; but if any credit is to be attached to the deli- 

 neations of Mr. Hill, we must be allowed to doubt if even this point 

 of economy will be attained. 



If it be intended to work these engines at a permanent rate vf expan- 

 sion beyond that of the ordinary sliding valve, a corresponding saving 

 will accrue in the exact proportion of the length of the stroke worked 

 by the dense steam ; or, in other words, the quantity of steam used or 

 admitted into the cylinder, before the communicatio.i with the boiler 

 is cut off. If the stroke be six feet, and the steam shut off at three 

 feet, the consumption of fuel will be reduced A (we speak prac- 

 tically), if at i of the stroke, it will be f , and so on. The reduction of 

 the power will be in a much less ratio ; and from all we have gathered 

 from a considerable experience, the velocities of the ship will be, as 

 the cube roots of the reduced powers. These, then, are the advantages 

 of expansion. 



In order to bring this question more fully before our readers, we 

 have made the following calculation relative to the engines and 

 boilers of the Great Britain, and which will at a glance show what 

 will be the consumption of fuel per horse per hour — the power 

 exerted — the cubic feet of steam per horse per minute — the cubic feet 

 of water to be evaporated per horse per hour — and, though last the 

 most important, what we conceive will be the velocity of the ship, 

 with full steam pressure, and with expansion from ^ to jj of the stroke, 

 giving the results for every six inches — from 12 inches to 04 inches, 

 or # of the stroke. 



Engines SS inch cylinder; CO stroke; 223 feet per minute: 294 horse power each. 



Expansion in inches of the stroke 



Horse Power with dense steam 



Horse Power during expansion 



Total power, whole stroke 



Total power with 4 cylinders, as Great Britain 



Contents of cylinder at this expansion 



Cubic feet of steam 2i lb. above the atmosphere per horse per minute 



Cubic feet of steam at the atmospheric pressure per horse per minute 



Water, per horse per hour, to be evaporated 



Coals per horse per hour, at 8 lb. per horse 



Tons per 24 hours 



Speed of vessel with paddle wheels 



Speed of vessel with screw 



12 

 245 



44-59 



289-59 



1153-3 



211-15 



27-6 



32-5 



1 14 



90 



111-5 



13-9 



11-C 



IS 

 220-5 



61-96 

 2S2-4U 

 1129-8 

 100- 

 25-5 

 30-1 

 1-09 

 8-51 

 103 

 13-S 

 11-50 



24 

 196-0 

 79-38 

 275-38 

 1101-5 

 167- 

 23 5 

 27-75 

 •9S 

 7-74 

 92 

 13-7 

 11-40 



30 

 171-44 

 91-00 

 262-44 

 10497 

 147-S 

 21-5 

 25-4 

 •912 

 7-12 

 SO 

 13-5 

 11-25 



36 

 147- 

 101-87 

 248-87 

 995-5 

 126-7 



19-5 



230 

 •840 

 6*55 



70 



13-25 



111 



42 

 122- 

 117-18 

 229-68 

 918-72 

 105-6 

 17-5 

 20-6 

 •72G 

 5-87 

 5S 

 12-9 

 10-75 



48 



98-0 



107-7 



205-7 



882-8 



84-5 



15-8 



18-7 



•60 



5'15 



45-5 



12-4 



103 



54 



735 



101-87 



175-37 



701-48 



63-4 



13-7 



16-2 



•571 

 4-5 

 34- 

 11-7 

 9-S5 



! This table corroborates Mr. Eny's general reasoning, but we differ in 

 detail arising from different estimations of the full power, and we 

 would suggest to Mr. E., that he is obviously incorrect in assuming 

 the " evaporation of a cubic foot of water to be equal to one-horse 

 power," because that depends entirely on the rate of expansion used 

 in any particular engine. 



We may remark that we have constructed the foregoing table on 

 the plan laid down by Mr Farey, the truth of which we have prac- 



tically proved by indicator experiments — presuming that low pressure 

 steam at 3A to 4 lb. is to be used, we have supposed its density in the 

 cylinder to be about 24 lb. on the square inch, and have reduced this 

 to atmospheric density for the calculations relating to the consump- 

 tion of fuel. 



We have been more liberal than Mr. Enys, and have allowed that 

 good boilers can evaporate S lb. of salt water with 1 lb. of coal ; that is, 

 if they are kept in a fair state, by blowing off, or by the use of the 



