In order that the vessel may pass through the Suez Canal, the extreme draught should not exceed 25 ft. 7 in., which may be obtained in the following

manner:

1. Remove all water from the hydraulic tanks, the after fresh-water tanks, and all except 20 tons from fresh-water tanks forward.

2. Remove all water from boilers in the after boiler-rooms; the water in the other boilers should be to working height.

3. Empty the reserve feed-tanks, with the exception of about 10 tons of water.

4. Remove all coal except 200 tons in bunkers abreast the four forward boilers.

5. Remove all provisions, bread, and spirits, except 20 tons; the provisions, etc., removed should be principally from the after store-rooms.

6. Remove all officers' stores and slops except 30 tons, removing principally from the after store-rooms.

7. Remove 55 tons of shell from forward 12-in. shell-room, and an equal quantity from the after 12-in. shell-room; also 40 tons of shell from forward 6-in. shell-room, and an equal quantity from the after 6-in. shell-room.

Generally.

1. When the ship is floating at or near a mean draught of 27 ft., a weight of 675 tons added to or removed from the ship will increase or decrease the mean draught by 1 ft.

2. A longitudinal moment of about 15,800 foot tons will alter trim by 1 ft., i.e. if w tons is weight moved, and d feet the distance the weight is moved (w × d) longitudinally, then 15,800

is the change of trim in feet, or the increase of

draught aft plus the decrease of draught forward, or vice versâ.

3. A weight placed on board or removed from about station 93, which is about the middle of the after boiler-room, will not affect the trim.

4. To ascertain the combined effect on draught and trim of removing a weight or placing a weight on board, the effect on draught only is first obtained by rule (1) above, supposing the weight put on board at station 93; the effect on the trim is then obtained by rule (2) above, the distance moved through being the distance in feet between station 93 and the actual position of the weight.

Note.-The removal of any weight before station 68, which is 12 ft. abaft the forward bulkhead of forward boiler-room, will not diminish the draught aft. Similarly, any weight removed from abaft station 116, which is abreast the mainmast, will not decrease the draught forward.

It should be stated that more recent battle-ships and cruisers have been designed to float at a rather less draught with legend coal, etc., than the ships of Majestic class, and this, together with the increase of draught now allowable, renders the operation of lightening these ships to pass through the canal considerably simpler than the above.

P

Stability of a Submarine Boat.-In a vessel totally submerged the shape of the displacement does not alter for any

=

inclination (Fig. 181), and therefore the upward force of the buoyancy must always act through the same point, viz. the centre of buoyancy. The stability at any angle is W x BG x sin 0, varying directly as sin 0. It will be a maximum at 90°, where sin 0 is a maximum, and the angle of vanishing stability1

will be 180°, where sin 0. In order to give good stability, therefore, BG must be as large as possible. This is done by so arranging the weights and ballast that G is below B.

For longitudinal inclinations the distance between B and G is the same as for transverse inclinations, so that the case is very different from an ordinary ship, in which the stability for fore-andaft inclinations is very great. In a submarine, therefore, the stability is the same for all directions of inclination, and such a boat is exceedingly sensitive to anything tending to disturb the fore-and-aft position.

Change of Trim after Bilging.-When a vessel is bilged near either end both bodily sinkage and change of trim occur.

This is well illustrated by the case of the Victoria in Chapter XXIV. In that case the change of trim was so considerable as to bring the upper deck forward under water, and consequently water gained

1 See next chapter.

access to the ship through hatchways, etc., and the movement down by the head was greatly accelerated. Fig. 182 has been drawn for a box-shaped vessel 175 ft. long, 30 ft. broad, 15 ft. deep, 8 ft. draught, before damage. If an empty compartment between bulkheads 25 ft. and 55 ft. from the bow is laid open to the sea the vessel will float at a draught of 13 ft. 5 in. forward and 6 ft. 8 in. aft. It is seen that the stem head is quite close to the water, and although the loss of buoyancy is not very considerable, yet this, with the change of trim, causes a dangerous condition. It is thus seen to be most important to carry watertight transverse bulkheads well above water. Figs. 52 and 54, which show the watertight subdivision of a large and small cruiser respectively, show that most of these bulkheads are carried to the upper deck.

CHAPTER XIX.

STABILITY AT LARGE ANGLES OF INCLINATION.

WE have seen that the stability of a ship at any angle is the effort she makes to return to the upright when put over to that angle. For small angles of inclination, up to 10 to 15°, this depends directly on the metacentric height. Thus at 10° the Royal Sovereign, with 3 ft. GM and 14,150 tons displacement, will have a righting moment of

14,150 x 3.5 × sin 10° = 8,600 foot tons.

It is possible, however, for a vessel to have sufficient metacentric height but insufficient stability at large angles. This was specially brought out in the investigations which followed the loss of H.M.S. Captain. Metacentric height alone, apart from other

W

considerations, principally freeboard, will not ensure a vessel having sufficient stability, and special calculations are necessary to determine the righting moment at large angles of inclination.

Curve of Stability.-Take a vessel inclined to a large angle 0, as Fig. 183. The upward force of the buoyancy acts through B', the new centre of buoyancy, and the couple tending to right the ship is Wx GZ, GZ being the righting lever. The length of this righting lever will depend on how far the centre of buoyancy shifts out, and this length will vary for different angles. Thus for H.M.S. Captain the following values of GZ were calculated, viz. 7°, 4 in.; 14°, 8 in.; 21°, 103 in.; 28°, 10 in.; 35°, 73 in.; 42°,

FIG. 183.

N

5 in.; 54, zero. A convenient way of representing these results is to draw a base line to represent angles of inclination and set up as ordinates the lengths of GZ as found. A curve drawn through the spots thus obtained is a curve of statical stability. The curve for the Captain is in Fig. 184.

Fig. 185 shows a curve of stability constructed as above. The angle at which GZ obtains its maximum value is termed the angle of maximum stability (in this case 47°). The angle at which the

curve crosses the base line (in this case 77°) is termed the angle of vanishing stability, or the range of stability. Up to this angle the vessel possesses a righting lever which will take her back to the upright. At 77° the ship is in equilibrium, the C.G. and C.B. being in the same vertical, but this equilibrium is unstable, and a small inclination either side of 77° will take her away from that angle; if to 75°, say, she will go back to the upright; if to 79°, say, she will capsize.