Imágenes de páginas
[ocr errors][ocr errors][ocr errors][ocr errors][graphic]

of King Edward VII. class have a portion of the area before the axis (Fig. 76). This renders the steering of the ship easier, because the centre of pressure on the rudder is brought nearer the axis.

For cruisers rudders are now always "balanced," i.e. a portion of the area is before the axis. If we deal with a rectangular plate towed through the water at an angle of 30° to 40°, it is found that the centre of pressure is about one-third the breadth from the


Section At CD.

Fig. 78.—Rudder of battle-ship.

leading edge. So to balance a rudder, i.e. to get the centre of pressure close to the axis, we need to make the area before the axis considerably less than one-half the total (see Figs. 71 and 73). In such a rudder the twisting moment even at high speeds is small, and much smaller power is needed in the steering arrangements than with an unbalanced rudder. This is specially desirable in cruisers, because of their high speed and the limited room available aft to house the gear owing to the fineness of these ships (see Fig. 163). The pressure, per square foot of rudder area, increases as the square of the speed, so that, comparing 24 knots with 19^ knots, the proportion of pressure for equal areas is {ffisf = 1'5» with our fastest battle-ships. If, therefore, we fitted a rudder hinged on the forward edge in a cruiser, the steering gear would need to be very massive, the steering engine would have to be of large power, and steering by hand would be difficult. On these accounts the rudders of cruisers are always balanced, so that the moment of the water pressure about the rudder-head is small even at high speeds.


or an increase of 50 per cent. This represents the increase of pressure to be dealt with in the fastest of our cruisers as compared

The weight of the rudder is taken by the top of the sternpost, as shown in Fig. 77. At the top of the rudder-head a recess is

formed, into which a bearing is placed in two halves. This bearing rests on the rudder cross-head, which has three or four legs. These are connected to a circular bearing ring, which slides in the metal path on the top of the sternpost.

Fig. 79 shows in some detail the construction of the sternpost and rudder of a large cruiser.


In determining the diameter of the rudder-head of an unbalanced rudder, we practically have only the twisting to deal with, but in cruisers the bending is of large amount, while the twisting going ahead is only small. Figs. 80 and 81 show how the two forms of balanced rudder would bend supposing each is held rigidly in the sternpost. In the first case it is like a beam held at one end and simply supported at the other. In the second case it is held at one Fig. 80. Fig. 81. end and supported near the middle,

with the other end free. In either case a considerable force has to be taken by the lower pintle, and a large bending moment at the rudder-head. The condition going astern has to be investigated, as it may happen that this is the worst case. The maximum speed astern is assumed to be about three-fourths the full speed ahead. The centre of pressure then is nearer the after edge, and the twisting moment about the axis is of considerable amount. This twisting moment, combined with the bending moment, will determine the necessary size for the rudder-head, unless the ahead conditions require a larger diameter.

In any rudder, the head being under water, it is necessary that the hole in the sternpost should be made watertight. The hole is lined with gunmetal, and the rudder-head is cased with a gunmetal sleeve, as shown in Figs. 69 and 77. A stuffing gland is fitted at the top to make the hole watertight.

Projections are cast on the rudder on each side to bring up against the sternpost when the rudder is hard over; these, however, are being omitted in some recent ships.

It was formerly the practice to supply each ship with a mould giving the actual shape of the rudder. It is the present practice to supply a sketch of the rudder on a large scale, 1 P.ot.ct.v. complete figured dimensions.

Shaft Brackets.—In twin screw vessels a considerable length of the propeller shafting is outside the ship, and brackets are fitted on either side, just forward of the propellers, to take the weight of the after end of the shafting, etc. For steel ships the brackets are of cast steel, for sheathed ships of phosphor bronze. These brackets do not have to take any Fi0i §2.fore-and-aft thrust (this

being taken in the engine-room by the thrust block), but they have to bear the very considerable weight of the propeller, etc. Because of this the attachment to the structure of the ship has to be very secure.

Fig. 82 shows the arrangement in a recent battle-ship. The arms are flattened out at the top and bottom. The upper palm passes inside the ship, where it is riveted to a thick fore-and-aft plate. The lower palm is shaped with a scarph to fit a corresponding scarph on the other bracket. The two brackets are then

[ocr errors]
« AnteriorContinuar »