ASTREA

moment of resistance offered by a portion of the surface will vary roughly as the cube of its distance from the pivoting point, and as

ARROGANT T. DIA:

the cosine of the angle it makes with the vertical. The flat portions of a ship at the after end are therefore best adapted for offering effective resistance to turning, and on this account the flat portions at the stern of recent ships are cut away in order to improve the turning (see Figs. 72 to 76 for examples). In large cruisers the stern is cut right up, with an underhung balanced rudder. In the fourteen battle-ships of Formidable and Duncan classes the stern is cut away as shown in Fig. 75, being brought down at the sternpost to take the blocks when docking. In King Edward VII. (Fig. 76) this cut away is associated with a partially balanced rudder. The cut up at the bow is of little value in influencing the turning when going ahead, but has some influence when going astern. A ship trimming by the stern more than usual will have a larger tactical diameter in that condition, and the converse will be the case if she trims more by the head. A short ship will turn more readily than a long ship, on account of the less resistance offered to the turning.

5. A ship with heavy weights at the extremities will turn more slowly than a ship of the same size and weight, etc., with the weight concentrated more amidships, and when once turning will be more difficult to get back to the straight again. This is due to the greater moment of inertia of the ship about a vertical axis in the former case.

Suppose two balls each weighing 1 lb. are fastened on a stick 12 in. apart, and two other balls of the same weight are fastened on a stick 60 in. apart. It is readily seen that the latter system is more difficult to start rotating about an axis in the middle perpendicular to the stick than the former, and when once in motion will be more difficult to stop. This is due to the different moments of inertia. In the first case it is roughly 2[1x (1)] (i.e. weight multiplied by square of distance), in the second it is roughly 2[1x()], the ratio being (1): (5), or 1: 25.

The comparison between the turning circles of Orlando, Astrea, and Arrogant (Fig. 202) illustrates the above principles very clearly. The Orlando is 300 ft. long, with a rectangular rudder. The Astrea is 320 ft. long, with a balanced rudder and no cut up at the stern. The Arrogant is also 320 ft. long, but has two balanced rudders with considerable cut up at the stern.

The Orlando, although shorter than the Astroa, does not get into the circular path so soon as the Astræa, on account of the type of rudder, which is not balanced. She turns, however, in a smaller circle than the Astræa on account of the lesser length.

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

The advance of Arrogant is 350 yards, of Astræa, 440 yards. The tactical diameter of Arrogant is 380 yards, or 3.6 lengths. Astrea 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 251 ft. x 11,000 tons, with a stern like Edgar in Fig. 71. The Cressy is 440 ft. x 691 ft. × 261 ft. × 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.

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.

R

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.

(e) 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.

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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 been 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

66

GREYHOUND.

FIG. 203.

-ACTIVE.

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