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65. In practice, a good margin of coal should be shown as left over by the calculations, for the reason that the actual coal consumption at the reduced speed will, as a general rule, be in excess of the calculated consumption by reason of the decrease in economy of the engine, induced by reducing the developed horsepower.
TURNING EFFECTS OF THE SCREW AND RUDDER
66. The following are the results obtained from experiments made with different types of vessels, ranging from a first-class battle ship down to a steam launch. They apply to single-screw vessels.
Ship and Screw Going Ahead. – A single-screw vessel, with the screw either right- or left-handed, does not show any decided or invariable deflection of the bow when starting from rest with the helm amidships. In the majority of cases, however, the bow of a vessel with a right-handed screw has a slight tendency to turn to port, while that of a ship with a left-handed screw turns to starboard; but this tendency gradually disappears with the increase of speed.
Again, when steaming at high speed there seems to be a tendency of a left-handed screw to deflect the bow to port, while a right-handed screw turns it to starboard. Generally, however, as soon as the vessel begins to move ahead from a standstill, the rudder attains its steering power and promptly overcomes the turning tendency of the screw.
When starting from rest, with the helm hard over, ships with a left-handed screw turn quickest with a port helm, while those with a right-handed screw answer the starboard helm quickest.
When moving ahead at a fair speed, with the helm hard over, a ship with a left-handed screw turns quickest with a starboard helm, while a ship with a right-handed screw answers the port helm quickest; the most rapid turning
motion is obtained when the rudder is turned toward the descending blade of the screw.
The wind has its least effect on the ship when starting; when moving ahead at speed the ship has a tendency to come to with a beam wind, this tendency sometimes overcoming the screw-turning effect and often requiring considerable weather helm to counteract it, if the breeze is fresh.
68. Ship and Screw Backing. - In backing from a standstill the bow of vessels with a left-handed screw turns to port, and if the screw is right-handed the bow turns to starboard. When moving astern in calm and smooth water this deflection of the bow generally continues in the same direction. The rudder has little effect when starting to back from rest. It will only effect the ship when some sternway has been gathered, and even then to a much less degree than if the ship were moving ahead at an equal speed. Often, it only suffices to keep the ship on a straight course astern. Ships with a left-handed screw will answer the port helm quickest, those with a right-handed screw the starboard helm.
The wind has a less pronounced effect on a vessel starting astern. When going astern, its influence, if the breeze is sufficiently strong, will become the controlling one; the ship will throw her stern to wind against both screw and rudder.
69. Ship Going Ahead, Screw Backing. – With the helm amidships, the bow of a vessel with a left-handed screw turns to port; with a right-handed screw the bow turns to starboard. With the helm hard over, vessels with a lefthanded screw answer quickest to the port helm; vessels with a right-handed screw obey the starboard helm quickest. The helm must not be put over too soon, especially if the vessel is still going ahead at high speed, otherwise it may act as for headway. The helm is best laid after the screw commences to back, when it will act as if the ship had sternway.
Vessels fitted with steam steering engines can insure the intended maneuver by short period of helm laid for headway before the screw begins to back, when the helm must be shifted; in other words, a vessel with a right-handed screw steered by steam and going ahead at full speed, wishing to stop and turn her head to starboard, will first port the helm and shift it to hard astarboard by the time the screw begins to back. In this maneuver the wind, if moderate, has no great effect.
70. Ship Going Astern, Screw Working Ahead. If the helm is amidships, ships with a left-handed screw turn first to starboard, those with a right-handed screw to port, but the bow subsequently may deviate in the same direction as if the ship were moving ahead. The helm may be put over as soon as the screw is reversed and working ahead, and it will affect the ship as if she were starting ahead. If a vessel has a left-handed screw, she will answer her port helm quickest, and if a right-handed screw her starboard helm quickest.
The wind has no special influence on the ship in this maneuver; in fact, as far as noted, the principal effect of a breeze is on vessels in motion, either with headway or sternway; a beam wind in this case will cause them to come to when going ahead and to fall off when going astern. Immediately upon starting in either direction the influence of a moderate breeze is small.
71. A ship fitted with twin screws has always one screw right-handed and the other left-handed; and as general rule, the former is placed to starboard and the latter to the port side of the ship. By this arrangement, therefore, the stern of a ship may be thrown either way simply by stopping one screw and backing the other. Thus, if it be required to throw the stern to starboard, we have only to stop the starboard screw and back the port one.
Ship and Both Screws Going Ahead. – With the helm amidships, and with both screws working ahead at equal rate, the ship should not deviate either way, except when influenced by wind and sea. With only one screw working ahead, the ship should be conveniently steered by a small amount of helm toward the side of the revolving
73. Ship Going Ahead and Both Screws Backing. In this case the steering effect of the rudder is considerably reduced but not changed; in other words, as long as the ship has headway the bow will turn slowly according to the helm given. When headway ceases and the ship begins to gather sternway, she is usually under the control of the rudder and should answer the helm readily, although not with the same ease and sensitiveness as when going ahead.
74. One Screw Working Ahead and the Other Backing. – When going ahead at a fair speed and it is desired to make a rapid turn to either side, the helm should be used in the ordinary way, with the screw on the side toward which it is desired to turn backing; the ship will then gradually lose her headway and turn sharply, throwing the stern toward the opposite side. In order to attain the greatest possible rapidity in this maneuver, the helm should be laid over before the screw is reversed, so as to impart to the bow an additional turning tendency in the desired direction.
75. When starting from rest with one screw working ahead and the other backing, the helm being amidships, the bow will, as a rule, turn rapidly toward the side of the backing screw. This tendency, however, does not apply to all vessels, and in cases where it does not exist it may sometimes be necessary to give the ship some headway and use the helm before the turning can be effected. 76. Concluding Remarks. – The preceding
brief remarks on the turning effect of the screw and rudder are general, and may be aptly termed the "average characteristics' under normal conditions. In other words, they apply to ships of medium size, of usual design, and normal draft, such as are represented by the combined freight-and-passengercarrying steamer. A great variety of elements, the effect of which is varied and perplexing, will necessarily make their appearance when conditions are changed, and it therefore becomes a matter of great importance that each officer in charge should familiarize himself at the outset of his service with the behavior and peculiarities of his ship under various conditions. This can be attained only by a close and attentive study, and the most important points then to be observed and noted are as follows:
The time and distance required, when running at full and at half speed, to bring the ship to a standstill by backing with
The effect of the rudder in different positions when backing.
The most effective way to apply the helm and engines in order to turn the ship quickly in the smallest space.
The effects of wind and sea under different speed and draft.
ENGINE-ROOM SIGNALS 77. The following code of signals was adopted by the Board of Supervising Inspectors of Steam Vessels, and approved by the Secretary of the Treasury:
1 whistle or 1 bell means Go ahead.
2 whistles or 2 bells when the engine is working ahead will always be a signal to stop or back strong.