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Scientific American Supplement, No. 598, June 18, 1887试读：
THE HAVRE MARITIME EXHIBITION.
The Havre Maritime Exhibition opened on the 7th of May.
Will this exhibition awaken general interest, or will it prove a local affair simply? This is a secret of the weeks that are to follow.
Should nothing chance to discourage the general interest that surrounds Havre, to dampen the enthusiasm of the public, or to act to the prejudice of the exhibitors, whose very evident desire is to show nothing but remarkable products in every line, the International Maritime Exhibition will prove a great success.THE INTERNATIONAL MARINE EXHIBITION AT HAVRE.—THE PRINCIPAL ENTRANCE.
The people of Havre have two points of comparison that more particularly concern themselves: Their Maritime Exhibition of 1868, which, as far as exhibition goes, was a complete success, is the first．The financial results of it were not brilliant, but that was due to certain reasons upon which it is not necessary to dwell．On the contrary, the Rouen Exhibition of 1884 proved profitable.
The Havre Exhibition, under able management, can have only a like good fortune．It must be said that the people of Havre would be deeply humiliated should it prove otherwise.
A very appropriate location was selected for the Exhibition, in the busiest quarter of the center of the city．Its circumference embraces one of the finest docks of the port—the Commerce Dock, thus named because it could not be finished (in 1827) except by the financial co-operation of the shipowners and merchants of the city．For the purposes of the Exhibition, this dock is now temporarily closed to navigation.
In the various structures, wood has been exclusively employed．The main building, which alone has a monumental character, is Arabic in style, and is situated in the center of Gambetta Place, over Paris Street, which here becomes a tunnel．Two facades overlook the ends of this tunnel．A third facade, which is much longer, fronts Commerce Dock.
The edifice is surmounted by a spherical cupola that serves as a base to a semaphore provided with masts and rigging．On each side of the sphere there are two pendent beacons．Wide glazed bays open in the external facades, and allow the eye to wander to the south through Paris Street as far as to the outer port, to the summits of Floride, and to see beyond this point the bay of La Seine, Honfleur, and the coast of Grâce．To the north, the most limited view has for perspective the City Hall, its garden, and the charming coast of Ingonville.
The principal facade, that which fronts Commerce Dock, from which it is separated solely by a garden laid out on Mâture Place, is the most attractive and most ornamented．Here are located the restaurants, the cafes, the music pavilion, and a few other light structures.
Internally, this portion of the Exhibition comprises a vast entertainment hall, brilliantly and artistically decorated with tympans representing the three principal ports of commerce—Havre, Bordeaux, and Marseilles—and with pictures by the best marine painters．It is lighted by an immense stained glass window which fronts Commerce Dock and the garden, and which lets in a flood of soft light.
The galleries to the right and left, over Paris Street, are reserved for the exhibitions of the ministers of state and of the large public departments, and for models, specimens, plans, and drawings of war and merchant vessels, and of pleasure boats, and for plans of port, roadstead, and river works.
Two endless galleries run to the north and south of Commerce Dock, parallel with Orleans Wharf on the one hand and Lamblardie Wharf on the other.
The northern gallery is connected by a foot bridge with the annex of Commerce Place, where is located the colonial exhibition, the center of which is occupied by a Cambodian pavilion, in which are brought together the products of Indo-China and Algeria．For half of their extent, the two galleries are separated from the dock by a promenade provided with seats and covered with a roof．On this promenade, it became necessary to make room for certain belated exhibitors whose products are not affected by the open air.
In Commerce Dock are to be seen, floating, specimens of every ancient and modern naval construction, French and foreign, among which are the state convette Favorite and an English three-master converted into a cafe boat．We find here, too, the giant and prehistoric oak of the Rhine, on board of the Drysphore.
Commerce Dock is divided into two parts by a foot bridge, which allows the visitors to pass from one side to the other without being compelled to tiresomely retrace their steps.
The main entrance to the Exhibition is opposite the portico of the theater, on Gambetta Place．A second entrance is found on Commerce Place in the colonies annex．The others, near the center, are on Orleans Wharf, opposite Edward Larue Street, and on Lamblardie Wharf, opposite Hospital Street and opposite Saint Louis Street.
The garden of the Exhibition and the galleries that surround it are illuminated at night by the electric light.—L'Illustration.
OUR COAST DEFENSES.
General H.L．Abbott delivered a lecture before the Academy of Sciences in New York, on the evening of March 21, a summary of which is given by the Herald as follows:
According to General Abbott, the country needs for its coast defenses:
Heavy guns;
Armor-clad casemates;
Disappearing gun carriages in earthworks;
Heavy mortars;
Submarine mines or fixed torpedoes; and
Fish torpedoes.
The lecturer said that this nation may be attacked in four ways: First, by fleet and army combined, as in our revolutionary war; second, by blockading the entrances to all our ports; third, by bombardment of our seaport cities from a long distance; fourth, by a fleet forcing its way into our harbors, and making a direct attack or levying tribute on our people.
The first is not now greatly to be feared．We are too distant from great powers, and too strong on land.
The second should be met by the navy, and is, therefore, outside a discussion of coast defenses.
The third is not probable, though it may be possible．The extreme range of 10 miles for heavy guns cannot be obtained from shipboard, and as an elevation of only 15° or 16° can be given, not over 5 to 6 miles range is attainable.
The fourth is the one which is possible, probable, even certain—if we have war before we have better defenses.
The race between guns and armor began about thirty years ago, and there has been more development in ships and guns in that time than in the two hundred preceding years．The jump has been from the 7 in．rifle as the largest piece to the 110 ton Armstrong; in armor, from 4½ in．of iron to the Inflexible with 22 in．of steel plating．The new Armstrong gun of 110 tons, tried only recently, with 850 pounds of powder and an 1,800 pound shot can pierce all the targets, and so far guns have the victory over armor．This gun developed 57,000 foot tons of energy, and will probably reach 62,000．Imagine the Egyptian needle in Central Park, shod on its apex with hard steel, dropped point downward from the height of Trinity steeple; it weighs 225 tons, and it would strike with just about the effect of one of the 110 ton gun's projectiles．Two of these guns are ready for the ironclad Benbow, and the Italians have several equally powerful of 119 tons from Herr Krupp．The most powerful gun in the United States, the 15 in．or the 12 in．rifle, has a muzzle energy of 3,800 foot tons.
Ships like the Inflexible are the most powerful afloat．A steel water-tight deck extends across the ship, and she has 135 water-tight compartments．Her guns and engines amidships have a protection of 24 in．of armor, and amidships she has a citadel carrying two revolving turrets, each containing two 80 ton guns．Her turret armor is 18 in．thick．She can make 14 knots, and she has cost $3,500,000．But she has a low freeboard, and the guns, therefore, get no plunging fire.
The French ship Meta has her heaviest guns mounted en barbette, high above the water line, giving a splendid plunging fire.
Either of these ships could enter any of our harbors and hold us at her mercy.
The entrance to the harbor of Alexandria, Egypt, is about 5 miles across．At the time of the bombardment the protecting fortifications were situated at the east end, in the center, and at the west end．On the west there were mounted 20 modern guns of great size and power, and there were 7 others at the east end.
Although the Egyptians fought bravely, they did very little harm to the English fleet, while on the second day the defense was silenced altogether．Following the bombardment—as in Paris—came the reign of mob law, doing more harm than the shells had done; and it is a possibility that every such bombardment would be followed by such an overthrow—at least temporary—of all forms of law and order.
The ships that had silenced the Alexandria batteries—which had 27 heavy guns more than we have—could reach our coasts in 10 or 12 days, and we would have nothing to meet them.
Armor-clad casemates are beginning to take the place of masonry．A tremendous thickness of masonry is built up to the very embrasures for the guns in the steel-clad turrets．This (the Gruson) system has been adopted by Belgium, Holland, Germany, Austria, and Italy.
In 1882 England had 434 heavy modern guns behind armored shore batteries; besides these at home, she had 92 in her colonies, of which 13 were in Halifax and 11 in Bermuda—for our express benefit.
What we have are brick and stone casemates and earthworks．A sample granite casemate, with iron-lined embrasure, was built at Fortress Monroe, and 8 shots were fired at it from a 12 in．rifle converted from an old 15 in．smooth bore．This gun develops only 3,800 foot tons of energy—a mere nothing compared with the 62,000 foot tons of the English and German 110 ton guns.
General Abbott showed most conclusive proof of the worthlessness of masonry forts in pictures showing the effect of the shots．The massive 8 feet thickness of granite was pierced and battered till it looked like a ruin．Not a man inside would have been left alive.
He also showed a "disappearing" gun in an earthwork, the gun recoiling below the level of the parapet and being run up to a firing position by a counterweight．In 1878 Congress stopped all appropriations for defenses, and nothing had been done since.
General Abbott said that we needed submarine mines or fixed torpedoes, which should be thickly interspersed about the channel and be exploded by an electric battery on shore．To prevent these torpedoes from being exploded by the enemy, the surface over them should be covered by plenty of guns．Heavy guns and mortars were needed to resist attacks by heavy iron-clads．Movable torpedoes were valuable, but only as an auxiliary—a very minor auxiliary—compared with submarine mines．We should be cautious not to infer that torpedoes made a satisfactory defense alone, as they must be protected by large and small guns, and they form only a part of the chain of general defenses.
THE STEAMSHIP GREAT EASTERN.
The history of the Great Eastern is full of surprises．It is always that which is most unlikely to happen to her which occurs．Not long since we recorded her sale by auction in Liverpool for £26,000．It was stated that her purchasers were going to fit her out for the Australian trade, and that she would at once be sent from Dublin to Glasgow to be fitted with new engines and boilers, and to undergo thorough renovation．Lord Ravensworth, in his address to the Institution of Naval Architects, spoke recently of the bright future before her in that Australian trade for which she was specially built．Yet at this moment the Great Eastern is lying in her old berth in the Sloyne at Liverpool, and unless something else at present quite unforeseen takes place, she will once more play the undignified part of a floating music hall．It seems that although she was certainly sold, as we have stated, the transaction was not completed．Her owners then cast about for the next highest bidder, who at once took her．He is, we understand, a Manchester cotton spinner, and he paid £25,500 for her．It is no secret that Messrs．Lewis made a considerable sum out of the ship last year, and the knowledge of this fact has no doubt induced her present owner to follow their example．The ship left Dublin on Sunday, April 3, under her own steam and in tow of two Liverpool tugs, the Brilliant Star and the Wrestler, and arrived in the Mersey without accident on Monday, after a passage of only thirteen hours．Mr．Reeves, formerly her chief officer, has been made captain．Mr．Jackson is still chief engineer．We cannot at present explain the fact that she went more than twice as fast as she has done recently, her engines making as many as 36 revolutions a minute, save on the assumption that while lying at Dublin much of the enormous growth of seaweed on her bottom died off, as will sometimes happen as a result of change of water．Her engines and boilers, too, have had a good overhaul by Mr．Jackson, and this may account in part for this improvement．It is much to be regretted that the scheme of using the ship for her legitimate purpose has not been carried out．It is not, however, yet too late．The Great Eastern was not a success in Dublin, for one reason, that a beer and spirit license could not be obtained for her．It is said that notice has been given at the Birkenhead police court that any application for a license of a similar kind will be opposed．Whether the ship will be as popular a resort without as she was with a license, we cannot pretend to say; and we may add that all our predilections are against her degradation to the status of a floating music hall．The greater her failure as such, the greater the chance of her being put to a better use; and it may help to that desirable end if we say here something concerning the way in which she could be rendered a commercial success as a trader.
It may be taken as proved that the present value of the ship is about £26,000．Mr．De Mattos gave, we understand, £27,000 for her, and he bought her by auction．The last sale gives nearly the same figures．If we assume that there are 10,000 tons of iron in her, we may also assume that if broken up it would not fetch more than £3 a ton at present rates; but even if we say £4, we have as a total but £40,000．To break the ship up would be a herculean task; we very much doubt if it could be done for the difference between £26,000 and £40,000; her engines would only sell for old iron, being entirely worthless for any other place than the foundry once they were taken out of her; as for her boilers, the less said about them the better．In one word, she would not pay to break up．On the other hand, by a comparatively moderate further outlay, she might be made the finest trading ship afloat．There are two harbors at all events into which she can always get, namely, Milford and Sydney．There are others, of course, but these will do; and the ship could trade between these two ports．By taking out her paddle engines, she would be relieved of a weight of 850 tons．The removal of her paddle engine boilers would further lighten her, and would give in addition an enormous stowage space．By using her both as a cargo and a passenger ship, the whole of the upper portion could be utilized for emigrants, let us say, and the lower decks for cargo, of which she could carry nearly, if not quite, 20,000 tons．She would possess the great advantage that, notwithstanding she was a cargo ship, she would be nearly, if not quite, as fast as any, save a few of the most recent additions to the Australian fleet．There is every reason to believe that she has been driven at 14 knots by about 6,000 horse power．We are inclined to think that the power has been overstated, and we have it on good authority that she has more than once attained a speed of 15 knots．Let us assume, however, that her speed is to be 13 knots, or about fifteen miles an hour．Assuming the power required to vary as the cube of the speed, if 6,000 horsepower gave 14 knots, then about 4,800 would give 13 knots—say 5,000 horse power．Now, good compound engines of this power ought not to burn more than 2 lb．per horse per hour, or say 4.5 tons per hour, or 108 tons a day．Allowing the trip to Australia to take forty days, we have 4,320 tons of coal—say 5,000 tons for the trip．The Etruria burns about this quantity in the run to New York and back．For each ton of coal burned in the Great Eastern about 15,000 tons of cargo and 3,000 passengers could be moved about 3-1/3 miles．There is, we need hardly say, nothing afloat which can compare in economy of fuel with this．Taken on another basis, we may compare her with an ordinary cargo boat．In such a vessel about 3,000 tons of grain can be moved at 9 knots an hour for 600 horse power—that is 5 tons of cargo per horse power．Reducing the speed of the Great Eastern to 9 knots and about 2,000 horse power, we have 9 tons of cargo moved at 9 knots per horse power; so that in the relation of coal burned to cargo moved she would be nearly twice as economical as any other vessel afloat.
The important question is, What would the necessary alterations cost? Much, of course, would depend on what was done．A very large part of the present screw engines could be used．For example, the crank shaft, some 2 feet in diameter, is a splendid job, and no difficulty need be met with in working in nearly the whole of the present framing．If the engines were only to be compound, two of the existing cylinders might be left where they are, two high-pressure cylinders being substituted for the others．If triple expansion were adopted, then new engines would be wanted, but the present crank and screw shafts would answer perfectly．The present screw would have to be removed and one of smaller diameter and less pitch put in its place．All things considered, we believe that for about £75,000 the Great Eastern could be entirely renovated and remodeled inside．Her owners would then have for, say, £100,000 a ship without a rival．Her freights might be cut so low that she would always have cargo enough, and her speed and moderate fares ought to attract plenty of passengers．Sum up the matter how we may, there appears to be a good case for further investigation and inquiry as to the prospects of success for such a ship in the Australian trade, and the opinion of merchants and others in Melbourne and Sydney ought to be obtained．Something would be gained even if the opinions of unprejudiced experts were adverse．We might then rest content to regard the ship as an utter failure, and not object to see her sunk and filled with concrete to play the part of a breakwater．Until, however, such an opinion has been expressed after full discussion, we must continue to regard the ship as fit for something better than a music hall and dancing saloon.—The Engineer.
THE NEW GERMAN CORVETTE GREIF.
Our cut represents the corvette Greif—the latest addition to the German fleet—on its trial trip, March 10．As other naval powers, especially England and France, have lately built corvettes and cruisers which can travel from 17 to 18 knots, while the fastest German boats, Blitz and Pfeil, can make only 16 knots an hour, the chief of the Imperial Admiralty decided to construct a corvette which should be the fastest vessel in the world．The order was given to the ship and engine corporation "Germania," of Berlin and Keil, in April, 1885, the requirements being that the engines should generate 5,400 h.p., and that the vessel, when loaded, should have a speed of 19 knots, a point which has never been reached by any boat of its size．The hull is made of the best German steel of Krupp's manufacture, and measures 318 ft．in length at the water line, with a breadth of beam of 33 ft., the depth from keel to deck being 22 ft．It draws about 11 ft., and has a displacement of 2,000 tons.
As the vessel is to be used principally as a dispatch boat and for reconnoitering, and as—on account of its great speed—it will not be obliged to come into conflict with larger and stronger men-of-war, no great preparations for protection were needed, nor was it necessary that it should be heavily armed, all available room being devoted to the engines, boilers, and the storing of coal; these occupy more than half the length of the vessel, leaving only space enough for the accommodation of the officers and crew at the ends．The armament consists of five Hotchkiss revolving guns on each side, and a 4 in．gun at each end, the latter being so arranged that each one can sweep half the horizon.
The keel was laid in August, 1885, and the ship was launched July 29, 1886, on which occasion it was christened Greif．On the trial trip it was found that the slender shape of the vessel adapted it for the development of a very high rate of speed under favorable conditions, when it can make at least 22 knots an hour, so that the speed of 19 knots an hour guaranteed by the builders can certainly be reached, even when traveling at a disadvantage．In spite of its great length, the Greif can be easily maneuvered．When moving forward at full speed, it can be made to describe a circle by proper manipulation of the rudder, and by turning one screw forward and the other backward, the ship can be turned in a channel of its own length.THE NEW GERMAN WAR STEAMER GREIF.
A large and rapid cruiser, also for the German navy, is being built by the corporation "Germania"．This vessel is of about the same length as the Greif, has more than double its displacement, and will make 18 knots an hour, an unusual rate of speed for a vessel of its class．It will be launched by the last of the summer or early in the fall.
TWIN SCREW TORPEDO BOAT.
We give several illustrations of a sea going twin screw torpedo boat lately built for the Italian government by Messrs．Yarrow & Co., of Poplar．The vessel in question is 140 ft．long by 14 ft．wide, and her displacement approaches close on 100 tons．The engines are of the compound surface condensing type ordinarily fitted by this firm in their torpedo boats, excepting where triple compounds are fitted．The general arrangement is shown by the sectional plan．As will be noticed, there are two boilers, one before and the other aft of the engines, and either boiler is arranged to supply either or both the engines．Yarrow's patent water tight ash pans are fitted to each boiler, to prevent the fire being extinguished by a sudden influx of water into the stokehold．There is an independent centrifugal pumping engine arranged to take its suction from any compartment of the boat．There are also steam ejectors and hand pumps to each compartment．These compartments are very numerous, as the space is much subdivided, both from considerations of strength and safety．Bow and stern rudders are fitted, each having independent steam steering gear, but both rudders can be worked in unison, or they can be immediately changed to hand gear when necessary．The accommodation is very good for a vessel of this class．Officers' and petty officers' cabins are aft, while the crew is berthed forward.TWIN SCREW TORPEDO BOAT FOR THE ITALIAN GOVERNMENT.
The armament consists of two bow tubes built in the boat．There are two turntables, as shown in the illustrations, each fitted with two torpedo tubes．These, it will be noticed, are not arranged parallel to each other, but lie at a small angle, so that if both torpedoes are ejected at once, they will take a somewhat divergent course．Messrs．Yarrow have introduced this plan in order to give a better chance for one of the torpedoes to hit the vessel attacked．There are two quick firing three pounder guns on deck, and there is a powerful search light, the dynamo and engine being placed in the galley compartment.
We believe, says Engineering, this torpedo boat, together with a sister vessel, built also for the Italian government, are the fastest vessels of their class yet tried, and it is certain that the British Navy does not yet possess a craft to equal them．It is an extraordinary and lamentable fact that Great Britain, which claims to be the foremost naval power in the world, has always been behind the times in the matter of torpedo boats.
The official trial of this boat was recently made in the Lower Hope in rough weather．The following is a copy of the official record of the six runs on the measured mile:
SOME RECENT HIGH-SPEED TWIN SCREWS.
By E.A．LINNINGTON.One of the most interesting and valuable features in the development of naval construction in recent years is the great advance which has been made in the speeds of our war ships．This advance has been general, and not confined to any particular vessel or class of vessel．From the first class armored fighting ship of about 10,000 tons displacement down to the comparatively diminutive cruiser of 1,500 tons, the very desirable quality of a high speed has been provided.
These are all twin screw ships, and each of the twins is driven by its own set of engines and line of shafting, so that the propelling machinery of each ship is duplicated throughout．The speeds attained indicate a high efficiency with the twin screws．In all ships, but more especially in high speed ships, success depends largely upon the provision of propellers suited for the work they have to perform, and where a high propulsive efficiency has been secured, there is no doubt the screws are working with a high efficiency．The principal purpose of this paper is to record the particulars of the propellers, and the results of the trials of several of these high speed twin screw ships．The table gives the leading particulars of several classes of ships, the particulars of the screws, and the results obtained on the measured mile trials from a ship of each class, except C．The vessels whose trials are inserted in the table have not been selected as showing the highest speeds for the several classes．Excepting C, they are the ships which have been run on the measured mile at or near the designed load water line．On light draught trials, speeds have been attained from half a knot to a knot higher than those here recorded．No ship of the class C has yet been officially tried on the measured mile, but as several are in a forward state, perhaps the actual data from one of them may shortly be obtained．All these measured mile trials were made under the usual Admiralty conditions, that is to say, the ships' bottoms and the screws were clean, and the force of the wind and state of the sea were not such as to make the trials useless for purposes of comparison．On such trials the i.h.p．is obtained from diagrams taken while the ship is on the mile, and the revolutions are recorded by ruechanical counters for the time occupied in running the mile．Not less than four runs are made during a trial extending over several hours．The i.h.p．in the table is not necessarily the maximum during the trial, for the average while on the mile is sometimes a little below the average for the whole of the trial．The revolutions are the mean for the two sets of engines, and the i.h.p．is the sum of the powers of the two sets．The pitch of the screw is measured．The bolt holes in the blade flanges allow an adjustment of pitch, but in each case the blades were set as nearly as possible at the pitch at which they were cast．The particulars given in the table may be taken to be as reliable and accurate as such things can be obtained, and for each ship there are corresponding data; that is, the powers, speeds, displacements, revolutions, pitches, and other items existed at the same time．There are a few points of detail about these propellers which deserve a passing notice．In Fig.1 is shown a fore and aft section through the boss．It will be observed that the flanges of the blades are sunk into the boss, and that the bolts are sunk into the flanges．The recess for the bolt heads is covered with a thin plate having the curve of the flange, so that the flanges and the boss form a section of a sphere．This method of construction is a little more expensive than exposed flanges and bolts, which, however, render the boss a huge churn．With the high revolutions at which these screws work, a spherical boss is extremely desirable, but, of course, the details need not be exactly as shown in the illustration．The conical tail is fitted to prevent loss with eddies behind the flat end of the boss, and is particularly valuable with the screws of high speed ships．The light hood shown on the stern bracket is for the purpose of preventing eddies behind the boss of the stern bracket, and to save the resistance of the flat face of the screw boss．The edges of the blades are cast sharp, instead of being rounded at the back, with a small radius, as in the usual practice—the object of the sharp edge being the diminution of the edge resistance．The driving key extends the whole length of the boss, and the tapered shaft fits throughout its length.FIG.1.
These points of detail have been features of all Admiralty screws for some years.
The frictional resistance of screw propellers is always a fruitful source of inefficiency．With a given screw, the loss due to friction may be taken to vary approximately as the square of the speed．This is not to say that the frictional resistance is greater in proportion to the thrust at high than at low speeds．The blades of screws for any speed should be as smooth and clean as possible, but for high speed screws the absolute saving of friction may be considerable with an improvement of the surface．There is no permanent advantage in polishing the blades．No doubt there is some advantage for a little time, and, probably, better results may thereby be secured on trial, but the blades soon become rough, and shell fish and weed appear to grow as rapidly on recently polished blades as on an ordinary surface．These screws are of gun metal．They were fitted to the ships in the condition in which they left the foundry．It appears that within certain limits mere shape of blade does not affect the efficiency of the screw, but, with a given number of blades and a given disk, the possible variations in the form or distribution of a given area are such that different results may be realized．The shapes of the blades of these propellers are shown in Figs.2, 3, and 4．It will be seen the shapes are not exactly the same for all the screws, but the differences do not call for much remark.FIG.2., FIG.3．& FIG.4.
Fig.2 shows the blades for the A screw．C and D have the same form．Fig.3 shows in full lines the blades of the B screw, and, though very narrow at the tips, they, like A, are after the Griffith pattern．The blades of E and F are of a similar shape, as shown in Fig.4, and approach an oval form rather than the Griffith pattern．The particulars of these propellers would be considered incomplete without some reference to their positions with respect to the hulls．When deciding the positions of twin screws, there is room for variation, vertically, longitudinally, and transversely．For these screws, the immersions inserted in the table give the vertical positions．The immersion in A is 9 ft., showing what may be done in a deep draught ship with a small screw．Whatever the value of deep immersion may be in smooth water, there can be no question that it is much enhanced in a seaway．The longitudinal positions are such that the center of the screw is about one-fifth of the diameter forward of the aft side of the rudder post．The positions may, perhaps, differ somewhat from this rule without appreciably affecting the performance, but, if any alteration be made, it would probably be better to put the screws a little farther aft rather than forward．The forward edges of the blades are from 2 ft．to 3 ft．clear of the legs of the bracket which carries the after bearing．The transverse positions are decided, to some extent, by the distance between the center lines of the engines．As regards propulsive efficiency, it would appear that the nearer the screws are to the middle line, the less is the resistance due to the shaft tubes and brackets, and the greater is the gain from the wake in the screw efficiency, but, on the other hand, the greater is the augment of the ship's resistance, due to the action of the screws．Further, the nearer the screws are to the hull, the less are they exposed．But experience is not wanting to show that the vibration may be troublesome when the blades come within a few inches of the hull．The average of the clearances between the tips of the blades and the respective hulls is about one-eighth of the diameter of the screw.
An interesting and noteworthy fact in connection with these propellers is the wide differences in the pitches and revolutions, though the products of the two do not greatly vary．Such differences are extremely rare in the mercantile marine for similar speeds, but in war ships they are inseparable from the conditions of the engine design．As a general rule, with (revolutions × pitch) a constant, an increase of revolutions and the consequent decrease of pitch allow a diminution of disk and of blade area—other modifying conditions, such as the thrust, slip, number, and pattern of blades, being the same．The screws for E and F are interesting, because, with practically the same speeds and slips, there is a considerable difference in the revolutions．It will be observed that F is a vessel of finer form and a little less displacement than E, and, therefore, has less resistance．Although E has the greater resistance and the screw the smaller pitch/diameter, the higher revolutions permit the use of a smaller screw．But from this example the influence of the high revolutions in diminishing the size of screw does not appear so great as some empirical rules would indicate．The screws for A and B are also worthy of attention．Although the ship A has a much greater resistance than B, the screw of the former is much the smaller, both in the blade area and the disk．A's screws, however, in addition to 22 per cent．more revolutions than B, have a much larger slip, and the blades have rather a fuller form at the tips．Compared with the practice in the mercantile marine, the revolutions of these screws are very high, and from the foregoing remarks it may appear that much larger screws would be required for a merchant ship than for a war ship of the same displacement and speed．There would, however, be several items favorable to the use of small screws．For a given displacement the resistance would be less in the mercantile ship, and with the lower revolutions the proportion of blade area to the disk could be increased without impairing the efficiency．Thus in passing from the war vessel to a merchant ship of the same displacement, there are the lower revolutions favorable to a larger screw, but, on the other hand, the smaller resistance, larger proportion of blade area, and the coarser pitch, are favorable to a diminution of the screw．The ship B has a very large screw at 88 revolutions, but the tips are very narrow．If the blade were as dotted for a diameter of 16 ft., the same work could be done with the same revolutions, but with a little coarser pitch and a little more slip.
There is something to be said for large screws with a small proportion of blade area to disk．For instance, two bladed screws have frequently given better results than four bladed screws of smaller diameter, neglecting, of course, the question of vibrations．Twin screws, however, should, as a rule, be made as small as possible in diameter without loss of efficiency．The advantages of small twin screws are the shorter shaft tubes and stern brackets, deeper immersion, and less exposure as compared with large screws．The exposure of the screws is usually considered an objection, but, perhaps, too much has been made of it, for those well qualified to speak on the subject consider that careful handling of the ship would, in most cases, prevent damage to the screws, and that where the exposure is unusually great, effectual protection by portable protectors presents no insuperable difficulty.
The slips of these screws vary from 10 to 17½ per cent., which is certainly not an extensive range, considering the widely different working conditions．Slip, as an indication of the efficiency of the screw, is not only an interesting subject, but it is often one of importance．In these ships, however, there is nothing about the slips which would give rise to any doubts as to the fitness of the screws for their work.
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