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The difference between the Astrcea and the Arrogant, both of the same length, is very marked.

The advance of Arrogant is 350 yards, of Astrcea, 440 yards.

The tactical diameter of Arrogant is 380 yards, or 3'6 lengths. „ „ „ Astrcea is 650 yards, or 61 lengths.

This great difference is due to two causes, viz. the double rudders of Arrogant and the large cut up at the stern of that ship. The ships of the Arrogant class were specially designed as fleet cruisers, and this great turning facility was made a feature of the design.

The following comparison between the turning of the Diadem and Cressy illustrates also the influence of the shape of the stern. The Diadem is 435 ft. X 69 ft. X 25* ft. X 11,000 tons, with a stern like Edgar in Fig. 71. The Cressy is 440 ft. X 69£ ft. X 26| ft. x 12,000 tons, with a stern shaped as shown in Fig. 73. The rudder of the Cressy is rather larger than in Diadem, but the ratio of rudder area to immersed middle line plane is the same in both cases.

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Turning of a Twin Screw Ship.—The above discussion deals with the turning of ships under the action of the rudder alone. A twin screw vessel, however, may be made to turn in a smaller arc by the use of its screws in association with the rudder. The engine on the side to which the rudder is put would be worked ahead, and the other worked astern. This power of turning in the smallest possible circle may be of great value in special circumstances to avoid collision. It is found that the advance with one screw ahead and one astern is about 70 to 80 per cent, of the advance with both screws ahead. The tactical diameter is about 60 to 70 per cent.

Twin screw vessels have a great advantage over single screw ships because of the possibility of steering by the screws alone, by varying the revolutions. Several battle-ships have recently gone long voyages without a rudder at all, the steering being done by the twin screws.

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Turning Trials.—Systematic turning trials are carried out on all H.M. ships, and a record is kept in the ship's book for the information of those officers who have subsequently to navigate the ship. There are two sets of trials; the first those carried out during the official steam trials of the ship when she is in the dockyard reserve, and secondly a series of turning trials at 12 knots and 6 knots, carried out when the ship is in commission.

1. Trials in dockyard reserve.—Most of these are to determine the time of turning, the advance and tactical diameter at the full natural draught power; but some of the trials determine these also for the speed of 10 knots.

2. Trials when in commission}—The trials ordered to be carried out are divided into four sections.

(1) At 12 knots with full helm.

At 12 knots with full helm and one engine at the revolutions for 12 knots astern.

(2) At 12 knots with 25° and 15° of helm.

(3) At 6 knots with full helm.

(4) With helm amidships to determine the time and distance

before the ship loses way.

(a) With engines at 12 knots and then stopped.

(b) With engines at 12 knots and then reversed with

all steam at command. (c) With engines at 6 knots and then reversed with all steam at command.

The object of (b) and (c) of the last section is to ascertain whether the ship can best avoid an object right ahead (as shallow water or another ship) by reversing with all steam at command, or by turning with both screws ahead, or with one screw reversed as in section (1).

It is laid down that each section should be completed in a day, and if possible all the four sections should be undertaken with the ship in similar conditions of trim, in similar weather, and in water over 20 fathoms deep. Full instructions how to proceed with the trials are contained in the form No. S. 347.

In using a range-finder for getting the-distances of the buoy from the ship, notice must be taken of the lower limit of the range-finder, so as not to go too close to the buoy.

1 Previous to 1902 the trials were somewhat different; the principal change has boon in the alteration of speed, then 10 and 5 knots, now 12 and 6 knots.

CHAPTER XXII.

THE RESISTANCE AND PROPULSION OF SHIPS.

Resistance.—The resistance opposed to a ship when moving through water is much more complex than the resistance offered to the motion of a train, say. In first considering the subject, we must leave out of account the disturbance caused by the propelling agent, usually the screw propeller, and imagine that the ship is towed through the water by some other ship. This has actually been done by experimenters on the subject, the most notable series of experiments being those carried out by Mr. W. Froude on H.M.S. Greyhound in 1871. Mr. Froude had the ship towed by H.M.S. Active, as in Fig. 203, to avoid any disturbance due to the wake behind the latter ship. The tow-rope was connected on the

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Fig. 203.

Greyhound to a dynamometer, to register the strain, and it was this strain which was overcoming instant by instant the resistance offered by the water to the onward motion of the Greyhound. The experiments were carried out over a wide range of speed, and as a result Mr. Froude had a record of resistances at various speeds. When such a record as this is obtained, it is convenient to represent it graphically by drawing a base to represent speeds, and erect ordinates to represent the resistances. The spots thus obtained enable a curve to be drawn showing resistance on a speed base. The curve obtained for the Greyhound is shown by AA in Fig. 204, and it is very suggestive. We notice that the resistance does not increase regularly as the speed increases, but the rate of increase is much more rapid at high speeds than at low speeds. Thus to increase the speed from 7 to 8 knots an extra resistance of 1500 lbs. has to be overcome, whereas to increase the speed from 11 to 12 knots an extra resistance of 6000 lbs. has to be overcome, or four times as much for an increment of 1 knot.

This agrees with our experience. We know how much more difficult it is to increase the speed of a ship by a knot, say, near the top speed than at the lower speeds. Fig. 205 shows the curve of I.H.P. on base of speed of H.M.S. Drake, and the following

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shows in tabular form the increase of power necessary for each 2 knots from 10 to 24 knots:—

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I.H.P 1,950 3,200 4,800 7,000 10,000 14,800 21,900 31,000

Additional I.H.P. neces- I

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To increase the speed from 22 to 24 knots requires as much power as is sufficient to drive the ship 17^ knots, and to increase the speed from 20 to 24 knots means more than doubling the horse-power. This great increase of power necessary for high speeds is due to the great increase of resistance.

The rate at which resistance increases as speed increases is therefore a matter of great importance. Mr. Froude found in the Greyhound that up to 8 knots the resistance was varying as the

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square of the speed. That is to say, if Ri is the resistance at speed Vi and Ra is the resistance at speed V2, then—

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or Mr. Froude found, as the curve in Fig. 204 indicates, that the

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