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Where the rudder stock enters the vessel, watertightness must be ensured by fitting a trunk having a stuffing box and gland at its top. This latter, however, may be dispensed with where a carrier is arranged for, this being an additional element in favor of the adoption of these supports. Before fixing on the counter dimension of the rudder trunk, care should be taken that ample clearness is given to ship and unship the rudder. It will be seen, therefore, that the hole through the counter is much in excess of the diameter of stock, and if not filled in would be unsightly, besides allowing a considerable voluine of water continually at play inside. It is covered in with a tail plate fitted in halves and secured with hexagon head taps to the counter plating, so as to be easily removable to permit of unshipping the rudder.
Good proportions for such details as pintles, gudgeons, braces, couplings, etc., to meet most requirements are shown in Fig. 147.
These brackets for supporting the outer end of tail shaft are generally of pear-shaped section as being the form of least resistance. It is usual to cast them in steel, although they are also sometimes
In selecting a suitable area of arm shipbuilders are guided mostly by experience, hence the divergent results seen in practice. The author has therefore devised the formula given on p. 109, in which he has attempted to secure a uniform relationship between the size of these struts and the power transmitted through them.
Where possible the centre of the propeller bracket should be placed on a frame to obtain the maximum of stiffness, and the palms of upper and lower arms may be cast on or connected with angle clips. A web spur is sometimes cast or worked on keel length of stern post to take the palm of lower arm instead of flanging the latter and riveting it through the keel to it, securing independent connection for each strut.
In wake of the upper palm additional stiffening must be worked by fitting a short local doubling on shell and a stringer inside. The number and diameter of palm fastenings should be developed according to the sectional area of the arm, these being in most cases overdone.
The sectional area of arms must not be tapered towards the boss, as, although theoretically considered as a cantilever, this would be rational, it must not be lost sight of that the greatest stresses are borne by the ends of the arms adjoining the boss, and are, besides, alternating ones inducing fatigue.
The engineer will determine the length of boss barrel suitable for bearing and also the finished diameter of the hole, but ample
allowance should be made for boring out to this dimension and also adjusting to centre line of shaft; this is most important when dealing with cast steel, as it provides the opportunity to detect hidden blow holes. A inass of metal should be avoided where the arm swells to meet the boss either by reducing the fillet to a minimum or coring out the metal inside the boss, as otherwise internal stresses will be set up in cooling or dangerous blow holes developed.
In high speed vessels it is important to make the pattern "wind" conforming to the run of the water line, thus obviating the arms being dragged across the stream lines and creating eddies. It is surprising the amount of power absorbed by this resistance when brackets are badly set or not set at all.
Spectacle frames have nearly superseded the open A brackets for large merchant vessels. They are enveloped in the hull of the ship, the plating being webbed out and bossed around the shaft for this purpose, as fully explained in the chapter on Design, which see.
Where the plating ends on the arms of these frames a good riveted connection must be made, usually double and increased to treble tap rivets around the boss. Local strengthening must also be fitted in wake of spectacle frames by increasing the deep floors in thickness and doubling the ship's frames in their vicinity.
CASTING AT STERN TUBE.
The outboard end of stern tube in vessels fitted with A brackets is supported by and connected to a steel casting or forging. Its function is similar to the boss on body post of a single screw steamer.