utilised for the purpose of keeping a small tank, fitted inside the paddle-box, always full of water, to be used, if necessary, on the bearings of the paddle and intermediate shafts which are above the water-line. The water-service pipes for these journals are also, in general, connected with the delivery-pipe from one of the auxiliary pumping engines of the ship. When the paddle-wheels are overhung, and carried by a single bearing on the ship's side, the journal should be made of larger diameter, and considerably longer than is necessary when an outside bearing is fitted, to resist the increased pressure and strains. There should also be thrust collars on the journal, to prevent end motion when the ship rolls. The bearings for paddle-shafts in the Royal Navy are generally made of gunmetal, though they are sometimes made of lignum-vitæ strips, as in the case of bearings for screw-shafts. When so fitted the shafts should be cased with gunmetal. Stuffing-box on ship's side. The hole in the ship's side through which the paddle-shaft passes is either fitted with an ordinary stuffingbox, or covered with a leather disc to prevent the passage into the ship of water carried round with the paddle-wheel. Disconnecting apparatus.-In paddle-wheel tug-boats, gear is usually fitted to enable the wheels to be disconnected from each other, and each engine worked independently, to facilitate the manoeuvring of the vessel. In many cases an ordinary disconnecting clutch is fitted on the intermediate shaft for this purpose. Another plan consists in fitting a cast-iron disc on the intermediate shaft, in lieu of a crank-arm. This is driven by feathers on the shaft, over which it may be drawn back, clear from the crank-pin, when the engines are required to be worked independently. Engines of this class, of large power, should be fitted with auxiliary steam startingengines and starting-valves to facilitate handling. In the more recent paddle-wheel tug-boats in Her Majesty's service a pair of cylinders, forming a compound engine, has been attached to each crank. The shafts for each wheel may either be connected by a clutch coupling, or left quite independent of each other, for the engines will be entirely under control whether they are coupled or not. སྔོན་༢༨༤༠༨ ུ.. CHAPTER XXV. SCREW-PROPELLERS. EACH blade of a screw-propeller may be regarded as a small portion of the thread of a screw of great pitch, and of considerable depth relatively to the pitch. The generation of the surface of a propeller blade of uniform pitch may be conceived from the following geometrical construction. C B Let A A', Fig. 287, represent the axis of the screw. Suppose a line AB, perpendicular to A A', to move uniformly along A A', and at the same time to revolve uniformly around it. It is clear that the extremity B of the arm A B will travel on the surface of a cylinder, and will trace out a spiral curve B B' on that cylinder. The same will be true of every point in the line AB; the point c, for example, traces out the curve c c', therefore the surface swept out or developed by the line A B will be a spiral or screw surface. Pitch of the screw- -If the line A B made a complete A FIG. 287. E D revolution around a a', the distance of a' from a at the end of the revolution would be the pitch of the screw. It is the distance between two consecutive threads measured parallel to the axis. Length of screw.-An actual screw-blade consists only of a portion of a complete convolution; and the extreme dimension of the blade, measured parallel to the axis, is called the length of the screw. In Fig. 287 this is represented by the line A A'. The aggregate length of all the screw-blades is equal to the length of the screw multiplied by the number of blades. Angle of the screw. The angle B B' D, between the curve and the plane A' D B' perpendicular to the axis, is called the angle of the screw, at the radius A B. It is evident, if the pitch be constant throughout, that the smaller the radius the greater will be the angle of the screw, the angle cc' E, for example, being considerably greater than the angle B B' D. The relations between the pitch, circumference, and angle of the screw may be shown by means of a right-angled triangle, having the pitch as perpendicular and the circumference as base, the tangent of the angle of the screw being equal to the pitch divided by the circumference. In Fig. 288, let AB represent the pitch and BC the circumference of a screw, to any given scale. Then the angle ACB will represent the angle of the screw at the end of the blade. The pitch being assumed the same throughout the blade, the angle at any other part of the blade may be found by determining the circumference BD at the given part, and joining A D; the angle A D B being the required angle. Form of blade. Screw-blades are made of a great variety of forms. The effect of form of blade has not yet been fully ascertained, but the shape suitable for one ship does not always prove equally efficient for a different ship. As a rule it would appear that form has little peculiar value as regards propulsive efficiency, though it may have some influence on the amount of vibration produced; the main points to be considered are, the pitch and surface of the blades in relation to diameter. The work absorbed in friction and the disturbing effect on the stream-line motions must necessarily have some effect in determining the most suitable shape of the blade, but this has not yet been reduced to exact calculation. In consequence of the increased angle of the screw-blade as it approaches the axis, the inner part of the blade, if the boss be small, has very little propulsive efficiency, and only absorbs power in churning the water. This was very noticeable in the earlier forms of screwpropellers, in which the boss was only about twice the diameter of the shaft, so that the inner portions of the blades were nearly in a fore-andaft direction. In these screws also, the length of the screw, or, in other words, the length of the projection of the blades on a fore-andaft plane, was constant throughout the blades, so that the side view of the screw was rectangular. The ends of the blades were, therefore, very broad, and absorbed much power in surface friction, owing to their great velocity. An endless variety of different patents have been originated on the question of the form and arrangement of screw-propeller blades, many of them peculiar and complicated, but they have all failed on trial to give the results anticipated. Hirsch screw.-A variety of screw much favoured at one time was the Hirsch screw, a four-bladed example of which is shown in Fig. 289. In this propeller the blades are curved forward, the axis of the blade being approximately a spiral curve, instead of a straight line as usual, the pitch also increases towards the circumference, and there are other minor peculiarities. The method of obtaining the curve of the centre line of the blade is shown by the dotted construction. It was supposed that this curved form of blade tended to resist the centrifugal motion of the particles of water acted on by the propeller, and that vibration was diminished by the action of the blade being gradual. Griffiths' screw.-Mr. Robert Griffiths was probably the most successful of the early designers as regards propeller proportions. He substituted for the central inefficient portion of the screw a large spherical boss, one-third the diameter of the propeller, which would revolve without agitating the water. This principle is now adopted for most screw-propellers, though the bosses are not usually so large as in the earlier Griffiths' screws. The general diameter of the boss in screw-propellers as now fitted is about one-fifth to one-quarter the diameter of the propeller. In the Griffiths' screw also, the outer ends of the blades, which revolve at the highest velocity, were considerably narrowed, to reduce loss from friction. The widest part of the blade was about four-tenths of the radius from the centre, the blade being somewhat pear-shaped. The tips of the blades were bent forward to the extent of about onetwenty-fourth of the diameter of the propeller. The proportions of Griffiths' two-bladed propeller are :— Width of blade at tip 0.07 pitch Greatest width Width at root Aggregate length of two blades 0.167 0.24 The two-bladed propeller shown in Fig. 290 is an example of a somewhat modified form of Griffiths' screw fitted to a vessel with a lifting screw. FIG. 289. Propeller arrangements for old masted ships. Before describing the modern arrangement of propellers and shafting, some space will be devoted to that of the early single-screw vessels, which were supplied also with sail power. Such vessels were intended at times to proceed under sail alone, and disconnecting couplings were fitted to enable the propeller to revolve freely without moving the engines. This was usually effected by fitting one of the couplings with bolts, that could be readily withdrawn by means of screws. A friction strap was fitted, to hold the propeller during the time the bolts were being withdrawn or replaced, and a thrust-bearing abaft the disconnecting coupling prevented the shaft being drawn back by the propeller when revolving while disconnected. The resistance offered when under sail by the propeller was still great, and to obviate this one of the following plans was adopted: 1. To lift the screw entirely out of the water. |