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956
[STEERING
SHIPBUILDING


on rudder, the tuming moment on the rudder head, and the power required for working the rudder are also less. A type of balanced rudder devised by Professor Biles and adopted in some large Atlantic liners is shown in fig. 66.

Broader and shallower rudders are adopted in warships owing to the necessity of keeping the whole of the steering gear (fig. 74), which had, in addition to the usual rudder at the stern, a double-balanced rudder in the bow, which could be drawn up into recesses in the hull; the two rudders were about 3 ft. apart and when in use worked together.

The results of the turning trials of some of the Experimental principal classes of warships are given in the following results. below the water-line for protection. table: Area ofd

Displacement Length Longitudinal gpeed m Advance Tflctlcal Efctlcal Ship or Class. in in Plane C 'lots at in D 1a.meter '?"Peter Tons. Feet. divided by Ommencement ' Yards. m dwlded Area of of Turn. Yards. by Length

Rudder.,

Dreadnought 17,900 490 37-5 19 490 440 2-7 Lord Nelson 16,500 410 40-5 17 400 370 2-7 King Edward VII. ' . 16,350 425 44-8 16% 450 440 3-1 F formidable . . 15,000 400 45-2 I4§ 440. 500 3-7 Majestic . 14,900 390 47-8 16 450 500 3-9 Minotaur 14,600 49° 43'4 19 480 600 3-7 Monmouth - 9.800 440 44-4 23% 590 790 5~4 Drake 14,100 500 46-8 23% 700 810 4-9 Diadem . 1 1,000 435 44-5 20% 650 920 6-3 Powerful 14,200 500 50-3 22 800 I-120 6-7 Minerva 5,600 350 48-3 18 540 770 - 6-6 A-'f°Kf'mt' - 5,750 320 33-5 17 350 380 3-6 Helm angle about 35° in all cases.

The unbalanced type was mainly used in British battleships up to H.M.S. “ Formidable " (1901) and “ Duncan ” (1903) (fig. 67). In the “ King Edward II.” class

(1905) (fig. 68) the rudder was

balanced, about one-fourth of its

area being placed before the axis;

balanced rudders supported at

about mid-de th were fitted in

P

- the “ Yashima " (1897) and the

FIG. 69.-H.M.S." Lord Nelson.”

"Yashima" and H M Ss “Swif

t-

sure, " “ Warrior” and “ Minotaur”

similar.

behind the inner propellers (fig. 7 “ Lord Nelson " class (1905)

(Hg. 69). In H.M.S. “ Dreadnought

" (1906) and recent

battleships, twin-balanced

rudders are fitted immediately

0), to obtain additional steering

effect from the propeller race, and to enable the ship to be steered from rest in getting under way. Owing to the higher speeds of first class cruisers, balanced rudders were used; those fitted in “ Diadem ” bf -

Section at A.P. FIG. 70.-H.M.S. “ Dreadnought." and “ Powerful " classes (1897-1900) are shown in fig. 71, and for “ Ciessy, " “ Monmouth " and “ Devonshire " classes (1901-1905) ~ ~ in fig. 72. In “Warrior” and

" M1notaur" classes (1907-1908)

the rudders are as shown

in fig. 69. The older second class

cruisers had rudders and

sterns of the type shown for

H.M.S. “ Powerfulf' in fig.

71, with the exception of the

Arrogant " class (1898), in

which two rudders were fitted

in conjunction with a considerable cut-up at the stern in order to obtain increased manccuvring capacity (fig. 73). Recent second class cruisers have rudders of the type shown in fig. 69. H ° "" T

FIG. 71.—H.M.S. “ Powerful.”

H.M.S. “ Diadem ” similar.

” Al

FIG. 72.—H.M.S. “ Devonshire." FIG. 73.-H.M.S. I'I.%.Ss.“Cressy"and“ Monmouth” “ Arrogant.” sim ar.

Auxiliary rudders have been fitted in H.M. ships in a few instances. An interesting example was that of H.M.S. " Polyphemus " ~ ~ as

FIG. 74.-H.M.S. “ Polyphemus.”

In the last column the tactical diameter is expressed in terms of the length of the ship; this ratio enables a rough comparison between the steering capacities of different ships to be expressed. The improvement in turning in modern warships has been due largely to the increase of rudder area in relation to the area of the immersed middle-line plane, which has been made possible by the adoption of balanced rudders. Considerable improvement has also been effected by cutting away the after deadwood; this will be seen on comparing the performances of H.M.Ss. “ Monmouth" and “ Diadem, " and “ Drake” and “ Powerful "; the former ship of each pair has her after deadwood partially cut away and hasa smaller tactical diameter. In the “ Yashima 7' the whole of the deadwood is removed and a very large rudder fitted; her tactical diameter is twice her length. The rudder area is relatively much less in merchant vessels, where the necessity, for a small tactical diameter does not arise. Experiments have been made to ascertain separate effects of angle of helm, time of putting helm over, and draught and trim of ship.

The effect of variation of helm angle is shown in table below: Z Tactical Diameter in Yards at about 12 knots speed. Y First- l Second- Torpedo-Ship.

Battleship. Class Class Boat

Cruiser. Cruiser. ' Destroyer.

10° helm 750 1400 1600 700

20° helm 550 1000 1000 500

35° helm 450 750 800 300

In ships having unbalanced rudders and fitted with hand-steering gear considerable time is required to put the helm hard over at full speed; and consequently the tactical diameter and the advance are greater at high speeds than at low speeds. When steam-steering gear is provided the helm can usually be put hard over in from IO to 20 seconds at any s eed; and in modern warships the speed is found to have little infihence on the path described when turning. In the case of torpedo-boat destroyers marked increases in the tactical diameter and in the advance occur at high speeds, the cause of which is not fully known. In such vessels of length 270 ft. and displacement 900 tons, the tactical diameter is about 550 yds. at 30 nots and 300 yds.. at 15 knots.

A moderate variation in the mean draught-has little effect on the course, but additional trim by the stern results in a greater space being required for turning., -

By working one propeller ahead and the other astern the space required for turning may be shortened, but the time of turning is frequently increased. The character of the path described depends on the relation between the revolutions of the screws. In a single-screw ship, with the propeller well immersed, the upper blades experience greater resistance to rotation than the lower blades, since the forward velocity of the frictional wake is greatest at the surface; hence a right-handed screw tends to turn the ship's head to starboard, and requires starboard helm. The reverse is occasionally experienced when the upper portion of the screw is incompletely immersed.

When a ship is going astern manoeuvring is performed with some

uncertainty, as the rudder is near the pivoting point.