It is now possible to simulate, almost in real time, the dynamic response of a towed body to oscillations of the cable top produced by the towing ship's reaction to the waves. It is equally important that one may vary the magnitudes of the physical and hydrodynamic properties of the system to discover which have a dominant influence on the towed body (the "fish").
This technique has been systematically applied to the problem of reducing the angular rotation of passive towed fishes and has shown how far, by careful design, it is possible to reduce the magnitude of ship-induced perturbations in pitch, roll and yaw.
The need for such reductions is well recognized for fishes transmitting or receiving sonar signals, e.eg for seabed surveying.
It may be possible to correct sonar data for changes of body orientation (if the latter is recorded during towing), or to mount equipment on gimbals to isolate it from such changes. However, in both cases it becomes difficult, or expensive, to cope with the horizontal accelerations that may also be present in the body motion. Under these circumstances the precise measurement of attitude requires gyroscopes and accelerometers. In other words, there is no substitute for a steady towed platform experiencing acceptably small angle changes.
The purpose of this chapter is to describe the forms of motion imposed on a single towed fish by the ship, and to emphasize how the choice of fin areas and towstaff length govern the magnitude of the resulting attitude variations.
The properties of the towed fish required to describe its two-dimensional motion in the plane of symmetry may be divided into two classes, the physical and the hydrodynamic. In the former are the mass, the submerged weight, and the location of the centres of mass and of weight-plus-buoyancy forces; in addition the location of the tow point and the radius of gyration in pitch about it, in the latter class are the drag coefficient, the "lift curve slope" (the rate of change of lift with angle of incidence) and the location of the "aerodynamic centre" (the point at which acts the lift produced by a small change of angle of body incidence). In general terms the magnitude of the lift curve slope increases with the size of the horizontal fins, and the aerodynamic centre location depends upon their size and location on the body. Information is also needed about the damping moment, about the tow point, due to a constant angular velocity in pitch. Some of these properties are illustrated in Fig.1 for the fish configuration used for these studies.
The theoretical work on which this chapter is based is contained in chapman (1982, 1984a, b), which provide much more detail than can be included in this review.
It is useful, initially, to consider the ship-induced motion of a fish in two dimensions when the fish has the (hypothetical) form of a heavy sphere, i.e. non-lifting, requiring no fins, and subjected to only one hydrodynamic force, its drag.
Fig.1 The fish configuration (available in full paper)