II. The effect of the screw, the above being indirect effects of the screw upon the turning. Other effects will be considered at some length further on. In double screw ships the turning effects, such as they are (in view of the greater distance of the screws from the ship's side and rudder), are made to counterbalance each other by causing the two screws to revolve in opposite ways to drive the ship in a given direction, ahead or astern. III. The effect of pitching on the ship's head is indirectly through the effect of draft on screw and rudder, and directly through the heel imparted to the ship. IV. The turning effects of wind and sea are due directly to the pressure they exert on the forward or after body, and indirectly to their influence on the ship's speed and heel. Each factor, then, affects the ship's head, in part directly, and in part indirectly, in connection with one or more of the other causes mentioned. Assuming that there is neither wind nor sea, the features in single screw ships which produce turning effects are the screw and rudder. We shall consider these causes separately, and the effects of the screw in particular. We note first that the screw may be either right or left-handed. A right-handed screw is one which, viewed from aft, turns with the sun to drive the ship ahead. This is the screw in common use on American vessels, and is the one discussed throughout this chapter.* Fig. 1 shows a vessel fitted with STARTING AHEAD Fig.1 RIGHT HANDED B a right-handed screw, an elevation of the screw itself being given below the plan of the ship. *The effects of a left-handed screw are precisely contrary to those of a righthanded screw. DIRECT TURNING EFFECTS OF THE SCREW. The direct turning effects of the screw are due : (a.) To the difference in resistance of the water to the upper and lower blades; (b.) To the pressure of the screw current upon the after body when the engine is reversed; (c.) To the lateral pressure of the screw stream upon the rudder-post and rudder when the vessel is going ahead. (a.) Difference in Resistance to the Upper and Lower Blades. When the vessel starts slowly ahead, the water acted upon by the blade A, Fig. 1, presents a certain resistance to that blade. The water acted upon by the ascending blade D is of gradually decreasing density, while the lower blade C works in the most dense and least disturbed medium, and the descending blade B is gradually meeting an increased resistance. The resistance to the lower blades being greater than that experienced by the upper blades, the centre of shaft being the centre of effort, will incline to move in the direction of least resistance (the direction of the upper blades, shown by the · arrows, Fig. 1), and as the stern of the ship holds this centre of effort, it must tend in the same direction, to the right (to starboard), so that the vessel's bow goes to the left (to port). Moreover, when pushed aside by the lower blade, the denser strata of water experience a speedier inflow than water disturbed by the upper blades; partly owing to the greater density itself and partly on account of the sharper lines of the lower part of the run which permit such quicker inflow. This is an additional reason why the lower blades should experience the greatest resistance, and it therefore increases the tendency of the stern to go to starboard, bow to port. If the ship is backing, contrary effects obtain, the stern going to port, bow to starboard, on account of the differences of pressure above described. In The wake current, occasioned by adhesion and friction when the ship is moving ahead, dams up the upper surface of the screw current, checking its motion to the rear. many vessels, this surface indraught astern is very noticeable when the vessel is going at full speed. Its effect is to increase materially the resistance experienced by the upper blades. The "wake" current, therefore, acts in opposition to the effects due to greater density of the lower water strata. The resultant of the unequal pressures on the upper and lower blades, and hence that part of the direct turning effect of the screw, depends upon the form and sharpness of the run, the draft, the number of revolutions, and the immersion of the screw. When the water is just being set in motion, i. e., when the engine begins to move ahead, the first named cause of turning effect is at its maximum (unequal densities). When the speed increases, the second cause (quicker inflow in the lower strata) attains its maximum, but at the same time the backing up effect of the screw current upon the upper surface of the screw stream increases with great rapidity. Great draft and sharpness in the lower part of the run assist the wake current to equalize the resistance to the upper and lower blades. (b.) Effect of Screw Current on After Body in Backing. When the engine is reversed, the water thrown by the blades moving over to port and downward strikes the lower part of the port side of the run, while the blades which are rising on the starboard side direct their stream against the starboard after body at, or even above the height of the water line. But since at the last named point the screw current, owing to the greater breadth of the ship, strikes at right angles to the vessel, it is therefore of greater effect than the result produced on the other side, where the current from the descending blades impinges, upon the sharp form of the lowest part of the run, and can only exert there a small portion of its strength. Hence, in backing, the screw current tends to push the stern to port, bow to starboard. This increases the effects which we were led to expect under (a) from the difference in densities, and therefore a screw in backing will have a greater effect upon the ship's direction than when the engine is working ahead. (c.) Pressure of Screw Current on Rudder-post and Rudder. When the engine is working ahead, the blade moving to starboard and downward directs its stream against the lower starboard side of the rudder-post and rudder; the blades moving to port and upward, send their stream against the upper port side of the rudder and rudder post. As the rudder is usually broader at the bottom than at the top, and as the stream from the upward moving blades meets with the least resistance and distributes itself with the least effect, it follows that the current from the blades moving downward has greater influence than the stream from the upward moving blades. The effect will be greater or less, according as the rudder happens to be turned toward the blade moving downward and inward, or toward the blade moving upward and outward. With the helm amidships, the effect of the screw current on the rudder-post and rudder, ship moving ahead, is to turn the stern to port, bow to starboard. This effect is therefore opposed to the results due to the moving of the screw blades in media of different density, while it unites with and increases the effects due to the wake current. The greater the width of the lower half of the rudder in proportion to the upper half, and the more the after portions of the screw blades incline to the rear, the greater will be the turning effect above noted. The final resultant of the direct turning effects of the screw will therefore depend in different ships upon the relative importance of the elements above described. II. INDIRECT TURNING EFFECTS OF THE SCREW. These effects are due to the influence of the screw upon the steering powers of the rudder: (a.) By causing the speed of the ship and consequent way current with its pressure on the rudder; (b.) By causing the pressure of the screw current upon the rudder when the ship is moving ahead; (c.) By suspending the rudder effect when the ship is moving ahead with the engine working astern, the way current being thrust aside by the screw current. Of the cause and effect in the first case (a), it need only be said that the ship's speed itself is affected in turn by the rudder, speed decreasing as rudder angle increases. There is therefore here, within certain limits, a reciprocal action. Under (b) may be noted that the screw current increases the effect of the way current on the rudder when the ship is moving ahead. Both screw and way current are strengthened by increase in the number of revolutions. The effect of the number of revolutions on the turning power of the rudder. as expressed by the time and diameter of turning in a circle, has been investigated with the German corvette "Hertha," with the following results: In regard to the time of turning.-Change in the number of revolutions with different rudder-angles, had great influence on the time of turning : Change in the number of revolutions when the engine is moving slowly is of greater proportionate influence on the time than an equal increase in number of revolutions when moving at great speed. In regard to the diameter of the circle.-Change in the number of revolutions has but slight effect on the diameter. The ratio of revolutions to diameters as observed in the "Hertha" at a mean rudder-angle of 20° was as 66: 62: 46: 30 : 18 = 1.21 : 1.17: 1.63: 0.97 : 1. Hence the time of turning varies inversely as the speed, and the diameter varies directly as the speed. The greater the number of revolutions the less the time and the greater the diameter of the circle. Under (c) it may be said that in vessels moving ahead the suspension of the regular rudder effect due to a reversal of the engines will be more or less complete according to the relative value of the opposing forces. The screw stream being thrown forward, tends to push aside and away from the rudder the way current coming from forward, due to the ship's onward motion. The regular steering effect of the rudder decreases, while the turning effects of the screw become, in most cases, the controlling force. Apart from influences due to wind, sea, and pitching; the greater the rudder surface and angle, the less the diameter of the screw, the smaller the number of revolutions, and the sharper the upper immersed part of the run-the greater will be the steering effect of the rudder. Under reverse conditions, the greater will be the turning effect of the screw. To summarize the results due to the screw alone, we may say 1st. That the screw has its greatest effect upon the ship's head in backing. 2d. That the screw has its least effect upon the ship's direction when going ahead, and that effect decreases as the vessel gathers headway. See also note, p. 544. 3rd. That these effects are greatest when the ship's draft is light, the screw being, however, immersed. Racing. What is said throughout this chapter of the screw effect presupposes that the screw is properly immersed. If this is not the case the effects may be precisely contrary to those described. No data obtained for a given ship at her normal draft can be relied upon when the vessel is badly out of trim or very light. Chief-Engineer Isherwood, U. S. Navy, observes that inasmuch as the screw current is due to the slip, its strength and effects will depend entirely upon the amount of said slip. The same authority points out the increase in the screw current, and its consequent effect on the rudder when the vessel is in very shoal water. One can scarcely fail to notice the different effect of the screw motion on the wake when in shoal water, as compared with the appearance of the water astern when off soundings. It is to be noted also that the effect of the screw upon the rudder depends very much upon the distance of the |