§ I.

CHAPTER XVII.

HANDLING A STEAMER ALONGSIDE A DOCK.

PRELIMINARY.

It is clear that the conditions under which work of this kind is to be done may vary almost indefinitely and that the methods. used must be varied correspondingly. It would be hopeless to attempt to illustrate all or even any considerable number of the situations which arise in practice, but it is not so difficult to analyze in a general way the various factors involved, and to show their application to a few special cases.

The factors are: lines, helm, screw, headway or sternway of the ship, current, wind.

We begin by considering the use of lines, first alone, then in combination with other factors.

CASE I.

If the ship is lying dead in the water abreast of a dock, as in Fig. 1, Plate 138, with a bow line leading to the dock, hauling on this line will turn the bow in, of course, but it will also throw the stern out, the ship pivoting about the center of gravity. It should be noted, however, that the stern does not go out quite as much as the bow comes in; for, since the ship is not held rigidly at the pivoting point, the mass as a whole will respond more or less to the force acting on the bow, and the resultant motion will be like that shown in the figure.

If the stern is held by a line to the dock, as in Fig. 2, Plate 138, the pivot is transferred to the stern and the whole length of the ship comes in as shown. This requires much greater effort than to turn the ship about her natural pivoting point as in Fig. 1.

If the bow and stern lines are hauled on at the same time, the ship may be breasted in bodily, but at an even greater expenditure of work than in the preceding case.

If either of the lines described above leads off at an angle from the beam, it constitutes a "spring," which may be defined as a line diagonal to the keel and exerting a force, when power is

1 See also Chapter XXI, "The Handling of Destroyers."

§ I.

CHAPTER XVII.

HANDLING A STEAMER ALONGSIDE A DOCK.

PRELIMINARY.

It is clear that the conditions under which work of this kind is to be done may vary almost indefinitely and that the methods. used must be varied correspondingly. It would be hopeless to attempt to illustrate all or even any considerable number of the situations which arise in practice, but it is not so difficult to analyze in a general way the various factors involved, and to show their application to a few special cases.

The factors are: lines, helm, screw, headway or sternway of the ship, current, wind.

We begin by considering the use of lines, first alone, then in combination with other factors.

CASE I.

If the ship is lying dead in the water abreast of a dock, as in Fig. 1, Plate 138, with a bow line leading to the dock, hauling on this line will turn the bow in, of course, but it will also throw the stern out, the ship pivoting about the center of gravity. It should be noted, however, that the stern does not go out quite as much as the bow comes in; for, since the ship is not held rigidly at the pivoting point, the mass as a whole will respond more or less to the force acting on the bow, and the resultant motion will be like that shown in the figure.

If the stern is held by a line to the dock, as in Fig. 2, Plate 138, the pivot is transferred to the stern and the whole length of the ship comes in as shown. This requires much greater effort than to turn the ship about her natural pivoting point as in Fig. 1.

If the bow and stern lines are hauled on at the same time, the ship may be breasted in bodily, but at an even greater expenditure of work than in the preceding case.

If either of the lines described above leads off at an angle from the beam, it constitutes a "spring," which may be defined as a line diagonal to the keel and exerting a force, when power is

1 See also Chapter XXI, "The Handling of Destroyers."

applied to it, both in the direction of the keel and at right angles to that direction.

A spring is used, therefore, for moving the ship ahead or astern and breasting her in at the same time.

CASE 2.

If the ship has way, either ahead or astern, her momentum enters into the problem of her behavior. We have here another application of a spring, the power involved in this case being furnished by the ship itself.

In Fig. 3, Plate 138, suppose that the steamer shown is moving parallel to the face of the dock with engines stopped and helm amidships, and that the stern spring A K is taut. Disregarding the resistance of the water, which is inconsiderable, the motion of the ship will be that resulting from her momentum along the original course and the tension along A K. The tension on A K may be resolved into two components; one retarding the ship along the line of her original course and thus directly opposing the momentum, the other hauling her in toward the dock and tending at the same time to turn her about the center of gravity, drawing the stern in and throwing the bow out. It is important to note, however, that the momentum, which is concentrated at the center of gravity, forward of the pivot, opposes the turning and tends to keep the ship straight. Thus as a matter of fact the ship does not turn much in cases of this kind, but comes in nearly parallel to the dock (Fig. 4, Plate 138).

CASE 3. (Figs. 1 and 2, Plate 139.)

If the vessel is moving as before, but with a spring from the bow instead of from the stern, the forces acting are similar to those in the preceding case, but with an important difference. In the present case, the momentum acts to increase the turning effect of the spring, instead of opposing it. This will become clear if Fig. 2, Plate 139, and Fig. 4, Plate 138, are compared.

The result of this difference is that the bow of a ship moving ahead on a bow-spring turns sharply in toward the dock, whereas the stern is turned in very little, if at all, by a stern-spring.

If the ship is moving astern, instead of ahead, the conditions are of course reversed, a stern-spring turning the stern in sharply, while a bow-spring has little effect.