The concept of path keepingis strongly related to the concept of coursestability or stability of direction. A body is said to be stable in any particular state of equilibrium in rest or motion if, when momentarily disturbed by an external force or moment, it tends to turn, after release from the disturbing force, to the state of equilibrium existing before the body was disturbed. In the case of path keeping, the most obvious external disturbing force would be a wave or a gust of wind. For optimum path keeping, it would be desirable for the ship to resume its original path after passage of the disturbance, with no intervention by the helmsman. Whether this will happen depends on the kind of motion stability possessed.
The various kinds of motion stability associated with ships are classified by the attributes of their initial state of equilibrium that are retained in the original path of their centers of gravity. For example, in each of the cases in the next figures, a ship is initially assumed to be travelling at a constant speed along a straight path.
Figure 4.9 shows the straight-line or dynamic stability; the signal path after release from a disturbance retains the straight-line attribute of the initial state of equilibrium, but not its direction.
Figure 4.10 shows the directional or course stability; the original path after release from a disturbance retains not only the straight-line attribute of the initial path, but also its direction. In some cases the ship does not oscillate after the disturbance, but passes smoothly to the signal path.
Figure 4.11 shows the path or positional motion stability; the ship returns to the original path, i.e. the original path not only has the same direction as the original path, but also its same transverse position relative to the surface of the earth.
The foregoing kinds of stability form an ascending hierarchy. Achieving straight-line stability is the designer’s usual goal for most ships when steered by hand. The other cases require various degrees of automatic control.
All these kinds of stability have meaning with control surfaces (rudders) …xed at zero, with control surfaces free to swing, or with controls either manually or automatically operated. The first two cases involve only the last two elements of the control loop of figure 4.8, whereas the last case involves all of the elements of the control loop. In normal marine usage, the term stability implies controls-…xed stability; however, the term can also have meaning the controls working.
The following examples indicate distinctions:
² A surface ship sailing a calm sea possesses positional motion stability in the vertical plane (and therefore directional and straight line stability in this plane) with controls …xed. This is an example of the kind of stability shown in figure 4.11. In this case, hydrostatic forces and moments introduce a unique kind of stability which in the absence of these forces could only be introduced either by very sophisticated automatic controls or by manual control. The fact that the ship’s operator and designer can take for granted is that this remarkable kind of stability does not detract from its intrinsic importance.