172 



NATURE 



\yiine 19, 1879 



is increased, the curve of total resistance ascends more or 

 less above the curve of frictional resistance, and in some 

 cases departs very largely from it. The rate of departure 

 as the speed increases differs largely in different ships, 

 and in vessels of the same section, but of different 

 lengths. Mr. Froude conducted a series of experiments 

 upon models of the same cross section and form of ends, 

 but with the length successively increased by lengths of 

 parallel middle body. These models corresponded to ships 

 of lengths varying from 160 feet to 480 feet in total length. 

 At the lower speeds, up to about eleven knots, the resist- 

 ance increased by about equal increments with equal in- 

 crements of length of the ship, but at higher speeds this 

 harmony disappeared. At thirteen knots the 200-feet ship 

 makes considerably more resistance than the 240-feet ship, 

 and at 14^ knots the 200-feet ship makes almost as much 

 resistance as a 360-feet ship of 2,275 tons more displace- 

 ment. Similar anomalies appear in the comparison 

 between other ships. These were shown by Mr. Froude 

 to be due to the influence of wave-making resistance, 

 and to depend upon the positions occupied with reference 

 to the stern of the ship by the waves generated by the 

 bow. The practical point involved is that a ship may 

 sometimes be lengthened considerably without any loss 

 of speed for the same application of power ; whereas if 

 the conditions favourable to this are not complied with, 

 she may, on the other hand, require a disproportionate 

 increase of power to keep up her speed. Mr. Froude has 

 shown how the most favourable conditions can be realised 

 in this respect. 



These investigations into the resistance experienced by 

 ships at different speeds have thrown great light upon 

 that long-disputed problem in naval architecture, "the 

 form of least resistance," and has gone far to enable ship- 

 builders to arrive at the best form and proportions for 

 the speed required, which is compatible with other re- 

 quirements. Economically such a discovery is of great 

 value in enabling more work to be done in steamship 

 propulsion for a given engine-power and expenditure of 

 fuel. 



Mr. Froude' s dynamometric experiments upon the 

 resistance of the Greyhound, and some results of the 

 steam trial performances of other ships, showed him that 

 the actual resistance of a ship was much less than had 

 been generally supposed. At eight knots spged the pull 

 upon the tow-rope of the Greyhound — a ship of 1,150 

 tons displacement — was only i\ tons, and at ten knots 

 4J tons. This was so very much below the thrust the 

 screw was supposed to exert when driving the ship at 

 those speeds, that Mr. Froude set to work to investi- 

 gate the relation between the indicated horse-power of 

 marine engines as represented by the work done by 

 the steam in the cylinders, and tile power that is use- 

 fully employed in propelling the ship. He found, as the 

 result of many experiments, that in ordinary ships at 

 full speed the former is 27 times the latter, or that the 

 effective horse-power, as given out by the thrust of the 

 screw, is only 37^ per cent, of the power indicated in the 

 cylinders. 



Mr. Froude decomposed the indicated horse-power of 

 the engine into its constituent parts, and approximately 

 quantified each element as follows : — 



1. The useful thrust or ship's true resistance = e.h.p. 

 or the effective horse-power. 



2. The augmentation of the ship's resistance by the 

 induced negative pressure under the stern consequent on 

 the tlirust of the screw. Mr. Froude often called atten- 

 tion to this cause of resistance, and showed that it might 

 be greatly reduced by placing the screw a short distance 

 abaft its usual position. He ascertained by experiment 

 that with ships of ordinary form the resistance is in- 

 creased on account of the action of the screw by about 40 

 or 50 per cent, of her nett resistance. The power required 

 for this therefore = o'4 E.H.P. 



3. The friction of the screw blades in passing through 

 the water, which was found to be = o-i e.h.p. 



4. The constant friction due to dead weight and the 

 tightness of the moving parts. This is at all speeds about 

 one-seventh of the total load of the engines when working 

 at full speed and pressure. It is therefore = 0T43 I.H.P., 

 I.H.P. being the total indicated horse-power. 



5. Friction due to working load of engine. This is also 

 = 0-143 IH.P. 



6. Air-pump resistance. This is approximately equal 

 to 0-075 i-H.P. 



Summing up these several elements it will be found 

 that the effective horse-power at full speed is little more 

 than 37J per cent, of the indicated horse-power. 



This analysis of the manner in which the power of the 

 engines is employed is very valuable in indicating the 

 manner in which loss of power may be treated in detail, 

 and also in furnishing a reliable means of comparison 

 between the efficiency of different engines. It will take 

 time to come into general use, but cannot fail to do so as 

 it becomes understood and appreciated. Already some 

 of our most intelligent shipbuilders, such as Mr. Denny 

 of Dumbarton, and Mr. Inglis of Glasgow, have applied 

 Mr. Froude's theories to practice, and are working upon 

 the improved methods he has laid down both for increas- 

 ing the effect in the propulsive power of the engines, and 

 in diminishing a ship's nett resistance. 



The general adoption of these theories will be a great 

 boon to science, as well as a practical benefit, in point of 

 economy to the ship-builder and ship-owner. Science 

 will benefit by having the performances of ships recorded 

 in such a manner as will be available for correcting the 

 present theories, and throwing light upon such laws of 

 hydrodynamics as are yet undiscovered or but imperfectly 

 understood. 



One great difficulty in making any scientific use of 

 steamship performances has arisen from the absence of 

 any method of determining the power delivered to the 

 screw. All that was ever ascertained was the power indi- 

 cated in the cylinders, and, as we have seen, this was 

 found to be enormously in excess of the effective power 

 employed in propulsion. As the problems of resistance 

 and propulsion can only be accurately treated by dealing 

 with the thrust exerted by the screw, it became desirable 

 to have some means of measuring this in actual ships. 

 Mr. Froude, who had frequently pointed this out, was 

 asked by the Admiralty to devise a dynamometer that 

 would measure the power delivered at the end of the screw 

 shaft in large marine engines. The problem was a most 

 difficult one, but Mr. Froude solved it in a most complete 

 and admirable manner. He described the instrument 

 he had invented in a paper read before the Institution 

 of Mechanical Engineers in July, 1877, and a description 

 of it is given in Nature, August 2, 1877. It fulfils all the 

 conditions of giving a true indication of the power, being 

 simple, compact, and easy of application. A machine for 

 dealing with an engine of 2,000 l.H.P. is all included in 

 the circumference of a circle of three feet radius, and as 

 its power increases as the fifth power of its linear dimen- 

 sions, it can be applied to any size of engine without 

 becoming unduly large. 



We have glanced hastily over some of Mr. Froude's 

 achievements in science, but it is impossible within the 

 Umits at our di^osal to do more than glance at them, or 

 even to refer to many. He was a scientific worker of the 

 best and rarest type, and was constantly employed in per- 

 fecting the details of his theories or striking out new lines of 

 thought. He was a master of the delicate art of experi- 

 mental and theoretical investigation, and a study of his 

 work would show many perfect examples of the manner 

 in which, by induction, a knowledge of the causes of 

 phenomena may be arrived at. Being an excellent 

 mechanic, and a most conscientious and ingenious 

 experimentalist, Mr. Froude put all his theories and 



