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Unformatted text preview: 14 TOWING OF VEHICLES Vehicles which are towed have some similarities to the vehicles that have been discussed so far. For example, towed vehicles are often streamlined, and usually need good directional stability. Some towed vehicles might have active lifting surfaces or thrusters for attitude control. On the other hand, if they are to be supported by a cable, towed vehicles may be quite heavy in water, and do not have to be self-propelled. The cable itself is an important factor in the behavior of the complete towed system, and in this section, we concentrate on cable mechanics more than vehicle characteristics, which can generally be handled with the same tools as other vehicles, i.e., slender-body theory, wing theory, linearization, etc.. Some basic guidelines for vehicle design are given at the end of this section. Modern cables can easily exceed 5000 m in length, even a heavy steel cable with 2 cm diameter. The cables are generally circular in cross section, and may carry power conductors and (Continued on next page) 14.1 Statics 65 Torque s = 0 s = 1 1 0.5 0 0.5 1 1.5 2 / m base Figure 8: Effects of constant Volts/Hz speed control; voltage is constant above electrical frequency base . multiple communication channels (fiber optic). The extreme L/D ratio for these cables obviates any bending stiffness effects. Cable systems come in a variety of configurations, and one main division may be made simply of the density of the cable. Light-tether systems are characterized by neutrally-buoyant (or nearly so) cables, with either a minimal vehicle at the end, as in a towed array, or a vehicle capable of maneuvering itself, such as a remotely-operated vehicle. The towed array is a relatively high-velocity system that nominally streams out horizontally behind the vessel. An ROV, on the other hand, operates at low speed, and must have large propulsors to control the tether if there are currents. Heavy systems, in contrast, employ a heavy cable and possibly a heavy weight; the rationale is that gravity will tend to keep the cable vertical and make the deployment robust against currents and towing speed. The heavy systems will generally transmit surface motions and tensions to the towed vehicle much more easily than light-tether systems. We will not discuss light systems specifically here, but rather look at heavy systems. Most of the analysis can be adapted to either case, however. 14.1 Statics 14.1.1 Force Balance For the purposes of deriving the static configuration of a cable in a ow, we assume for the moment that that it is inextensible. Tension and hydrostatic pressure will elongate a cable, but the effect is usually a small percentage of the total length....
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- Fall '04