July 13, 1882] 



NATURE 



247 



The main engines drive the turbine, expelling the water 

 with considerable velocity through curved pipes or pas- 

 sages leading to "nozzles" placed on each side at the 

 level of the water-surface. When the vessel is going 

 ahead the jets are delivered stemwards ; if it is desired to 

 move astern the engines are not reversed, but valves are 

 operated in the outlet pipes, and the jets are delivered 

 through the forward ends of the nozzles. These motions 

 of the valves can be made from the deck by an officer in 

 command. If desired, the jet on one side can be delivered 

 ahead, and that on the other side astern, the vessel then 

 turning without headway. This power of control over the 

 movements of the vessel, without reversing the engines, is 

 one of the chief advantages claimed for the system ; and 

 it is undoubtedly of value, especially in war-ships. 

 Another advantage claimed for the jet-propeller is the 

 power of turning it on an emergency, into a powerful 

 pump, by which large quantities of water can be dis- 

 charged from the interior of a ship that has been damaged 

 in action. This latter feature cannot be regarded as of 

 primary importance, however, seeing that modern war- 

 ships are minutely sub-divided into water-tight compart- 

 ments, and must depend for their flotation upon the in- 

 tegrity of the bulkheads and other partitions, if their 

 skins have been broken through by ramming or torpedo- 

 explosions. A further claim on behalf of the jet-propeller 

 for war-ships is based upon the less risk of disablement 

 in action, as compared with screws or paddle-wheels ; and 

 this claim may be admitted. On the other side must be 

 set the fact that all the trials made hitherto in vessels 

 fitted on the Ruthven system have shown a less speed for 

 a given amount of engine-power than would have been 

 obtained with the screw-propeller. It may be urged, of 

 course, that the decrease in speed should be accepted, at 

 least in special cases, in order to secure the undoubted 

 benefit of the hydraulic system. But the general feeling 

 of naval architects and marine engineers is in favour of the 

 use of twin-screws rather than water-jets for war-ships, the 

 duplication of machinery and propellers decreasing the 

 risk of disablement, giving great manoeuvring power, and 

 securing higher speed than could be obtained with the jet 

 propeller. 



Recently further trials have been made with a vessel 

 built in Germany, from the designs of Dr. Fleischer, who 

 claims to have devised a novel and more efficient system 

 of hydraulic propulsion. A brief notice of the invention 

 appeared in Nature, vol. xxvi., p. 18 ; fuller details are 

 to be found in two pamphlets published by the inventor : 

 " Der Hydromotor," and "Die Physik des Hydromotors" 

 (Kiel, 1881). The first of these pamphlets contains a 

 general description of the system, as applied in the 

 Hydromotor (a vessel of 1 10 feet in length, and about 100 

 tons displacement), a summary of her trials, compared 

 with those of earlier vessels enginedon Ruthven's system, 

 and an enumeration of the advantages to be obtained by 

 using jet-propellers instead of screws or paddles. The 

 second pamphlet contains a statement of the experi- 

 mental and mathematical investigations conducted by 

 Dr. Fleischer in working out his system. 



Dr. Fleischer dispenses with a turbine, and allows the 

 steam to act directly upon the water in two large vertical 

 cylinders placed amidships. These two cylinders com- 

 municate with the ejecting nozzles which are situated on 

 either side of the keel. In each cylinder there is a " float" 

 or piston of nearly the same diameter as the cylinder, 

 with a closed spherical top ; when this float is in its ex- 

 treme upper position, the cylinder is full of water. Steam is 

 then admitted into the upper part of the cylinder above the 

 float, the latter is pressed down, and the water is expelled 

 through the nozzle-pipe with great velocity. At a certain 

 portion of the stroke, the admission of steam is shut off 

 automatically, the remainder of the stroke being per- 

 formed during the expansion of the steam, and the 

 velocity of ejection of the water gradually diminishing. 



At the conclusion of the stroke, the exhaust-valve from 

 the steam space to the condenser is opened, the steam 

 rushes out, forming a partial vacuum above the float, and 

 the water enters, pressing the float up. The entry of the 

 water at this ^age is partly through the nozzle, and partly 

 from a separate valve communicating with the water- 

 space of the surface condenser. In order to utilise the 

 vacuum as much as possible, and to increase the 

 effective " head " of water during the down stroke, the 

 cylinders are placed as high as convenient in the vessel. 

 Two cylinders acting alternately were used in the Hydro- 

 motor, for larger or swifter vessels it is proposed to use a 

 greater number of similar cylinders. As in other jet- 

 propelled vessels valves operated from the deck enable 

 the commanding officer to reverse the direction of outflow 

 of either or both jets, making the vessel move ahead or 

 astern, or turn on her centre. The position of the nozzles 

 in the Hydromotor is not so favourable to manoeuvring 

 power as in the Waterivitch, and the difference in 

 behaviour is likely to be appreciable. 



Greater interest attaches to the trials of speed than to 

 those of turning. Unfortunately the records are too 

 meagre to enable a decisive opinion to be formed on the 

 merits of the new system as compared with that of Ruth- 

 ven. Dr. Fleischer claims that the Hydromotor attained 

 a speed of 9 knots with 100 indicated horse-power ; but 

 the conditions under which this speed was attained may 

 have differed considerably from those under which 

 measured-mile speed trials are conducted in this country. 

 Any exact comparison of the performances of two steam- 

 ships with either similar or different systems of propul- 

 sion, demands as its basis the elimination of all varying 

 conditions, the determination of the true mean speed, and 

 the calculation of the engine-power corresponding to that 

 speed. Dr. Fleischer may have done all this, but it does 

 not clearly appear in his publications whether he has or 

 not. He distinctly claims for his system a very high 

 "efficiency" as compared with that of Ruthven, but it 

 will be shown hereafter that the formula which he uses is 

 not absolutely correct ; and what is more important to 

 note is the circumstance that Dr. Fleischer clearly does not 

 possess the experimental data respecting the resistance 

 offered by the water to the motion of the Hydromotor 

 when towed at various speeds, which would enable him to 

 express the true efficiency of the propelling apparatus. 

 On this point a few further remarks may be permittted. 



Supposing a vessel to be towed at any speed, and her 

 resistance to be ascertained by a dynamometer, the horse- 

 power expended in overcoming that resistance can be 

 calculated, and. in the terminology of the late Mr. Froude, 

 is styled the "effective horse-power." Next let it be 

 supposed that the vessel is driven at the same speed 

 by her own machinery, and that the " indicated horse- 

 power" in the cylinders is ascertained. The ratio of 

 the " effective " to the "indicated" horse-power expresses 

 the true efficiency of the propelling apparatus, excluding 

 from the account, of course, the efficiency of the boilers. 

 Now what has been said above respecting Dr. Fleischer's 

 figures simply amounts to this : he does not appear to 

 have ascertained the effective horse-power of the Hydro- 

 motor, and consequently cannot express the true efficiency 

 except as an estimate. 



The excess of the indicated horse-power over the effec- 

 tive in any steam-ship is to be accounted for by the 

 waste-work of the mechanism, the waste-work of the pro- 

 pellers, and the "augment" of the tow-rope resistance 

 produced by the action of the propellers. In good 

 examples of screw-steamers the effective horse-power at 

 full speed has been found to vary from 40 to 60 per cent, 

 of the indicated power. Dr. Fleischer claims for the 

 Hydromotor a corresponding efficiency of about 34 per 

 cent, at full speed ; but not, it would seem, with any cer- 

 tainty. 



Passing by this comparison with screw-propelled ships, 



