66 CHAPTER XIII. THE STEERING OF STEAMERS. § I. TERMINOLOGY. The United States Navy has in recent years adopted a new terminology in connection with the steering of ships, substituting Right Rudder" for "Port Helm" and "Left Rudder" for "Starboard Helm." There is a certain inconvenience resulting from this change, due to the fact that the older terms continue in use in other Navies and in the Merchant Service. The change is, however, logical and reasonable. When the operation of steering was a matter of actually putting the helm (tiller) to port for swinging the ship's head to starboard, it was natural and reasonable that the command should coincide with the act to be performed, even though it was as true then as now that the governing influence in the movement of the ship was the starboard rudder, not the port helm. But in a modern ship, not only is it true that the rudder and the ship's head both go to starboard, it is true that the steering wheel (or lever) also goes to starboard. And it is certain that the helmsman gives no thought to the fact that (possibly) somewhere in the remote recesses of the ship, his action in moving the wheel to starboard has caused a tiller to move to port. In this and succeeding chapters, the terms "Right Rudder" and "Left Rudder" have been adopted, with a footnote indicating the older (and commoner) terminology where this is considered necessary. § II. SINGLE SCREW. The elements which enter into the steering of a screw steamer are so many, and their effects are so varied and conflicting, that any attempt at an exhaustive analysis of them would be altogether out of place in a work on practical seamanship. It is proposed here merely to name and briefly discuss the most important of them and to explain such of their effects as enter prac tically into the handling of a ship. In this discussion it will be assumed, where not otherwise stated, that all conditions are normal and average; that is to say, that the ship is of medium size and draft and loaded to her mean water-line; that she has no unusual features of design; that she draws rather more water aft than forward; that her rudder is of the ordinary type and of medium size, and fitted to a rudder-post immediately abaft the propeller-well. It will be assumed also, except where the contrary is stated, that the weather is calm. For simplicity, the discussion will be confined to a right-handed screw, so that all which is said of the effects of the screw upon the steering must be exactly reversed for a vessel with a left-handed screw. Twin-screws will be treated in a separate section, The factors which we shall consider as entering into the steering of a steamer are the following: I. The ordinary direct influence of the Rudder. 2. The Screw Current,-which we shall find to be made up of two distinct parts. 3. The Force exerted by the revolving blades of the Screw to drive the stern to one side or the other by their direct Sidewise Pressure upon the water. 4. The Wake Current. I. THE RUDDER.-The effect of this is simple and well understood. It calls for no explanation here, except in so far as it is complicated by the screw current, in connection with which it will be fully treated below. 2. THE SCREW CURRENT.-As the screw turns, it draws in a current from one direction and forces it out in the other. In going ahead, the current is drawn in from forward and forced out aft. In backing, it is drawn in from aft and forced out forward. It is found that the in-rushing "suction" current may be considered as flowing parallel to the line of the shaft, but that the current driven off from the screw partakes to an important degree of the rotary motion of the blades and moves diagonally to the shaft and keel. (a) If the screw is turning ahead, the inflowing CHAPTER XIII. THE STEERING OF STEAMERS. § I. TERMINOLOGY. The United States Navy has in recent years adopted a new terminology in connection with the steering of ships, substituting "Right Rudder" for "Port Helm" and "Left Rudder" for "Starboard Helm." There is a certain inconvenience resulting from this change, due to the fact that the older terms continue in use in other Navies and in the Merchant Service. The change is, however, logical and reasonable. When the operation of steering was a matter of actually putting the helm (tiller) to port for swinging the ship's head to starboard, it was natural and reasonable that the command should coincide with the act to be performed, even though it was as true then as now that the governing influence in the movement of the ship was the starboard rudder, not the port helm. But in a modern ship, not only is it true that the rudder and the ship's head both go to starboard, it is true that the steering wheel (or lever) also goes to starboard. And it is certain that the helmsman gives no thought to the fact that (possibly) somewhere in the remote recesses of the ship, his action in moving the wheel to starboard has caused a tiller to move to port. In this and succeeding chapters, the terms "Right Rudder" and "Left Rudder" have been adopted, with a footnote indicating the older (and commoner) terminology where this is considered necessary. § II. SINGLE SCREW. The elements which enter into the steering of a screw steamer are so many, and their effects are so varied and conflicting, that any attempt at an exhaustive analysis of them would be altogether out of place in a work on practical seamanship. It is proposed here merely to name and briefly discuss the most important of them and to explain such of their effects as enter prac tically into the handling of a ship. In this discussion it will be assumed, where not otherwise stated, that all conditions are normal and average; that is to say, that the ship is of medium size and draft and loaded to her mean water-line; that she has no unusual features of design; that she draws rather more water aft than forward; that her rudder is of the ordinary type and of medium size, and fitted to a rudder-post immediately abaft the propeller-well. It will be assumed also, except where the contrary is stated, that the weather is calm. For simplicity, the discussion will be confined to a right-handed screw, so that all which is said of the effects of the screw upon the steering must be exactly reversed for a vessel with a left-handed screw. Twin-screws will be treated in a separate section, The factors which we shall consider as entering into the steering of a steamer are the following: 1. The ordinary direct influence of the Rudder. 2. The Screw Current,-which we shall find to be made up of two distinct parts. 3. The Force exerted by the revolving blades of the Screw to drive the stern to one side or the other by their direct Sidewise Pressure upon the water. 4. The Wake Current. 1. The RuddeR.—The effect of this is simple and well understood. It calls for no explanation here, except in so far as it is complicated by the screw current, in connection with which it will be fully treated below. 2. THE SCREW Current.—As the screw turns, it draws in a current from one direction and forces it out in the other. In going ahead, the current is drawn in from forward and forced out aft. In backing, it is drawn in from aft and forced out forward. It is found that the in-rushing "suction" current may be considered as flowing parallel to the line of the shaft, but that the current driven off from the screw partakes to an important degree of the rotary motion of the blades and moves diagonally to the shaft and keel. (a) If the screw is turning ahead, the inflowing suction current from forward moves along the lines of the run toward the screw and has no appreciable effect upon the steering. It is different with the outflowing current thrown off and driven aft by the screw. This current, by reason of the rotary motion which has been referred to, strikes diagonally upon the rudder-post and the rudder (which we will for the moment suppose to be amidships) and exerts a distinct force tending to throw the stern to one side. We must recognize here, however, two factors opposing each other. The upper blades of our righthanded screw move over from port to starboard and drive their current against the port upper side of the rudder-post and rudder. At the same time, the lower blades drive in their current upon the starboard lower side. As the lower part of the rudder is commonly larger than the upper part, it is natural that the current from the lower blades, tending to drive the stern off to port, should more than overcome that from the upper blades, and that the resultant of these two forces should tend, upon the whole, to move the stern to port (in going ahead, with helm amidships). The conclusions of theory in this matter are confirmed by an interesting experiment in which the rudder was divided into two parts by cutting it horizontally. With the two parts left free to move and the screw turning ahead (the ship being at rest), the upper part took up a position at an inclination of nearly 10° on one side of the keel, while the lower part stood at an equal angle on the other side. In other experiments, where the full rudder was left free to move under the influence of the screw current alone, it took a position at a small but distinct angle with the keel on the side toward which the current from the lower blades tended to deflect it. If the rudder, instead of being kept amidships, is put over to one side, the sternward velocity of the current will have its effect, as well as the rotary velocity. With the ship and screw both going ahead, this part of the current will add its effect to the ordinary steering effect of the rudder, and simply produce a greater turning force than would otherwise exist. It may happen, however, that the screw is moving ahead and sending its current aft against the forward side of a hard-over rudder, while the ship is moving astern and developing its ordinary effect upon the after side. This is the case when a ship, while moving astern with some velocity, suddenly |