CHAPTER VII.

STEMS, STERNPOSTS, RUDDERS, AND SHAFT BRACKETS.

Stems. The simplest form of stem is that formed by a flat bar, to which the plating at the forward end is secured. This form of stem is adopted in merchant vessels and in the smaller classes of ships in the Royal Navy, as third class cruisers and destroyers. For larger war vessels, however, a stronger form of stem is necessary, because it is desirable that such vessels should be able to effectively ram an enemy's ship, without at the same time sustaining serious damage herself. The most effective form of stem

for this purpose is one having a ram below water, projecting well forward, so that it shall damage the slight under-water portions of the structure well in from the side before being brought up by the strong structure of the armour or protective deck of the other vessel (see Fig. 65).

Stems of steel vessels are now made of cast steel, a material possessing good strength and ductility (see Chapter II.), and capable of being cast into most efficient forms for the special purpose required. These castings are a great advance on the iron forgings

formerly in use, and a much more efficient ram has been by this means rendered possible.

When a vessel is sheathed with wood and copper we cannot use cast steel for the stem because of the galvanic action that would in all probability ensue between the copper and the steel. In such ships, therefore, the copper alloy, phosphor bronze, is used, but on account of the low strength of this material, the

casting has to be much more massive than a corresponding one of cast steel.

In the earlier ships with cast steel stems (Royal Sovereign to Canopus), the stem casting was carried well down into the body of the ship (Fig. 66), and on this account it had to be made in two pieces, because of the difficulties attending the manufacture and transport of such a large and intricate casting. The two pieces were connected together, as shown, by a scarph (Fig. 66), a tapered key being driven in to draw the parts together, and the whole well secured by screw bolts. The scarph was necessarily

a place of weakness, and this has been avoided in more recent ships by making the casting all in one piece. In order to do this the lower portion is made much shorter (see Fig. 67). This sketch shows the general shape of the casting, the ship being swelled out way of the ram. Inside the ram projection two webs are cast,

in

one vertical and one horizontal.

It is most important to support the stem effectively by the

UPPER DECK.

FIG. 67.-Stem of battle-ship.

adjacent structure. The following arrangements are made with this object in view, viz.

(i.) The outer bottom plating is doubled in thickness and recessed into the casting.

(ii.) The bow protection, in this case 2 in., is recessed into the casting for one half its thickness.

(iii.) The lower deck, which is a thick deck, is well connected to a large projection on the stem casting.

(iv.) At the level of the platform a 2-in. plate is worked, well connected to the horizontal web inside the ram. This 2-in. plate extends back to within 3 ft. of the collision bulkhead.

(v.) The vertical keel runs up to the stem, and is well connected to the vertical web inside the ram.

It is thus seen that every precaution is taken to support the stem to make it efficient for ramming purposes, and (iii.) and (iv.)

above not only provide a direct support, but they would resist the side bending action that would ensue when the ships swung together after the blow was struck.

The sketches each show a "cofferdam" bulkhead 3 ft. abaft

the collision bulkhead. It would be advisable to pack the space between these bulkheads, like an ordinary cofferdam, before ramming, to limit the flow of water aft, in case the collision bulkhead was damaged; access is obtained through the various decks for this purpose.

The phosphor bronze stem of a sheathed second class cruiser is shown in Fig. 68. In this case the stem is cast in two pieces, and the plating and planking have both to be recessed into the casting. The wood keel also has to be recessed as shown.

In the stems of recent ships the casting has been stopped at the main deck, the strength to the upper deck or forecastle being provided for by a bent plate.

Sternposts. The remarks already made as to the necessity of phosphor bronze stems in sheathed ships apply in this case also.

The sternpost of a single screw ship has to be formed to receive the propeller as well as to form a support on which to hang the rudder. Very nearly all the vessels now in the Royal Navy are, however, twin screw, so that the main function of the sternpost is to receive the rudder.

The shape of the sterns of ships varies considerably in different classes, but in all war-ships (except the smallest) an essential