586 



NA TURE 



{April 22, i! 



up to the highest speed reached by the latter, althou.i^h 

 she was 380 feet long, 58 feet broad, and of 8S50 tons only. 

 The Warrior reached 145 knots only ; the Houie attained 

 17 knots. Improvements in marine engineering made 

 this tremendous speed possible in the Hcnve. In her each 

 ton weight of propelling apparatus corresponded to 10 

 indicated horse-power ; in the Warrior 6 indicated horse- 

 power required i ton. This economy of weight in the 

 propelling apparatus was shown to be due to several 

 causes, including a higher steam-pressure, quicker- 

 running engines, the use of forced draught in the stoke- 

 holds, and the introduction of wrought iron, steel, and 

 gun-metal instead of cast iron. 



Two papers dealt with the interesting subject of " forced 

 draught " from different points of view. Mr. Sennett 

 described at some length the Admiralty system of " closed 

 stoke-holds," by means of which air is delivered into the 

 boiler-rooms by powerful fans, and at a sensible pressure. 

 The stokeholds being thus in plenum, the air can escape 

 only through the furnaces, and combustion is quickened 

 greatly. With the best natural draught, about 10 indi- 

 cated horse-power per square foot of furnace (or grate 

 area) is considered a good performance : with forced 

 draught and closed stoke-holds, this may be increased 

 from 60 to 80 per cent. It will be seen therefore that for 

 war-ships, which only require to steam occasionally and 

 for comparatively short periods at full speed, the system 

 is admirably well adapted. And it has been proved to be 

 not nearly as wasteful of fuel as might have been sup- 

 posed ; while it certainly makes the stoke-holds cooler 

 and more comfortable to work in. For the mercantile 

 marine the conditions are different : ships have to steam 

 ordinarily at practically their full speed ; the restrictions 

 of weight and space are not so great as in war-ships ; 

 and economy in coal consumption is of primary import- 

 ance. Still even here forced draught promises to sup- 

 plant iratural draught, and to enable large economies to 

 be made in weight and size of boilers concurrently with 

 savings in coal. Mr. Howden described his system of 

 forced combustion, which has been tried at sea over a 

 long period, and promises to be successful. He does not 

 close in the stoke-holds, but delivers air under pressure 

 from fans direct into the furnaces and ash-pits, this air 

 having been heated by passing through a special apparatus 

 placed in the up-takes. Great economy is claimed for 

 this system, and it was well spoken of by competent 

 authorities in the discussion which followed. Competing 

 plans are also being tried, so that more will certainly be 

 heard of forced draught in the mercantile marine. 

 Hitherto economy has been sought in higher pressures 

 and in the iist' of steam in the engines : now engineers 

 are turning attention to the boiler, and the means of 

 generating steam with a minimum e.-cpense. 



Hard times in the mercantile marine have led to a 

 wholesale conversion of compound engines into engines 

 of the triple or quadruple expansion type. Mr. Cole read 

 a thoughtful and well-considered paper on this subject, 

 which is of general interest to shipowners just now. It 

 may prcrve a very desirable thing to reduce the coal-bill 

 by 20 per cent., even at the cost of converting the 

 machinery to the more highly expansive type. 



It is a natural transition from the propelling machinery 

 to the propellers of steamships. Mr. R E. Froude, who 

 has succeeded his father in the superintendence of the 

 Admiralty model-experimental works, contributed one of 

 the most valuable and scientific papers read at the meet- 

 ings, on " The Determination of the most Suitable 

 Dimensions for Screw Propellers." He attempts from 

 experiments with models of ships and screws to ascertain 

 the resistance experienced by a ship moving at a given 

 speed, and the "augment" of that resistance produced 

 by the action of the propeller behind her. By means of 

 a lengthy series of experiments with model screws he 

 further attempts to fix the best diameter and pitch for a 



given number of revolutions of the engines. And finally, 

 the results are thrown into a form adapted for practical 

 use. The paper is in all respects admirable, but we are 

 bound to say that it can be regarded only as another step 

 forward on a very difficult road, and may be treated as 

 provisional rather than conclusive. Some of the infer- 

 ences do not accord, either, with the results of general 

 experience. It is to be welcomed, however, for as yet 

 the theory of the screw propeller is not in a satisfactory 

 condition ; and it is well known that very remarkable 

 economies are frequently realised by changes in pro- 

 pellers. In the course of the discussion Mr White men- 

 tioned a case of recent occurrence, where, by a change of 

 screw only, the speed of a ship was raised from 12 to 13J 

 knots per hour. 



M. Marchal, of the French G^nie Maritime, con- 

 tributed an interesting paper, in which the results of a 

 number of experiments, made by order of the Govern- 

 ment, were described. It was desired to obtain data for 

 guidance in deciding on the relative advantages of two 

 or three screws as applied to an ironclad of 10,000 tons. 

 A model steamer of 10 tons was built, and tried at 

 " corresponding speeds," with two screws and with three. 

 The publication of this paper marks a distinct change of 

 policy in France, and it places before English designers 

 a mass of valuable facts, which may prove very useful 

 hereafter as the speeds of ships are increased. 



Mr. Hall read a paper on " Flexible Shafting for Screw 

 Steamers," describing a plan by which he hopes to 

 reduce the number of breakages or serious accidents to 

 the screw shafts of ocean-going steamers. His conten- 

 tion was that in not a few cases there is a want either of 

 accuracy in the line of shafting and shaft -bearings, or of 

 rigidity in the hulls of steamships ; so that, by special 

 joints between the various lengths of shafting, a certain 

 amount of llexibility might advantageously be secured. 

 Experience will prove whether he is correct or not in the 

 anticipation that his plan will reduce accidents or break- 

 ages — serious matters in single-screw ships carrying large 

 numbers of passengers and having very small sail- 

 power. 



Another important group of papers are those dealing 

 with the use of rolled and cast steel for shipbuilding. It 

 is well known that steel is rapidly gaining upon iron, and 

 Mr. Martell (of Lloyd's) stated some very interesting 

 facts as to the extension of its employment in the mercan- 

 tile marine. War-ships are now all steel-built. Seven 

 years ago only 4470 tons of steel ships were built as 

 against 518,000 tons of iron ships. In 1885 over 165,000 

 tons of steel ships were built as against 290,000 tons of 

 iron. Confidence in steel was expressed by Mr. Martell 

 in his paper, echoed by Mr. Ward in another excellent 

 paper recording eight years' experience in building steel 

 ships, and indorsed by all who took part in the discussion. 

 Incidentally the question arose of the introduction of 

 steel made by the "basic" process for shipbuilding pur- 

 poses ; as yet this " make " of steel has not found much 

 favour, but the Admiralty authorities are now about to 

 undertake a series of experiments from which much may 

 be learnt. Everyone agrees that thorough and systematic 

 testing has done much to secure the excellent quaUties of 

 steel now made by both the Bessemer and the Siemens 

 processes ; and even the manufacturers are in favour of 

 maintaining the full severity of the tests in order to pre- 

 vent any deterioration in quality. Of more recent date 

 than the use of " mild-steel " plates and bars is the intro- 

 duction of mild-steel castings in lieu of iron forgings. 

 Mr. Warren, who had been chairman of a Committee 

 appointed by the Admiralty to look into this question, 

 gave to the meeting an excellent summary of the results 

 of their inquiries. There can be no question but that 

 heavy iron forgings are doomed to give place to steel 

 castings, which can be produced rapidly and cheaply, of 

 sound and ductile quality, and in finished forms, avoiding 



