JAMSTEC has developed a new underwater towed vehicle for oceanographic measurements, the Underwater Sliding Vehicle (USV). Unlike other conventional towed vehicles, the USV is not linked to the end of the tow cable but slides along it, while the tow cable catenary is stabilized by a hydrodynamic depressor. This method allows the USV to have good manoeuvrability although the vehicle is small (1 m long) and of light weight (about 10 kg in air, with sensors). The vehicle is equipped with conductivity, salinity and depth sensors. The vehicle motion is controlled from a deck control unit. The USV scans the vertical plane in which the tow cable advances horizontally, and measures the distributions of those parameters in this plane with high spatial resolution. The measured data are digitized, multiplexed and Frequency Shift Keying (FSK) modulated, and are transmitted up to the deck unit using an inductive coupling data communication system which utilizes an electric current loop formed by a tow cable and the sea water a theoretical analysis of the vehicle motion was also performed before the vehicle was designed, which indicated that no restriction would be imposed on the undulation amplitude of the USV. Field tests were carried out in the Sagami Bay. The USV was towed from a tugboat by a 220 m hydrodynamically faired cable at towing speeds from 4 to 10 knots, and it measured the distributions of water temperature and conductivity down to a depth of 175 m. Data are monitored on a CRT as vertical colour tomograms of the ocean space with respect to one selected parameter (conductivity, temperature or salinity), in which differences in colour correspond to differences of the parameter values.
Conventional towed vehicles used for oceanographic data collection are usually linked to the end of the tow cable. The vehicle varies its depth or undulates in the ocean space to measure the distributions of oceanographic parameters such as temperature and salinity. Towed vehicles of this type must overcome the hydrodynamic forces which are accompanied by the changes of the tow cable catenary, as shown in Fig 1. As the magnitude of the hydrodynamic forces necessary for the undulation increases with undulation amplitude, a large vehicle is needed for a large undulation amplitude. Hydrodynamic drag of the tow cable also resists the fast undulating motion through the sea water. Figure 2 shows the configuration of the USV system. The undulating motion of the USV is schematically shown in Fig 3. As the USV need not change the catenary of the tow cable during the undulating motions, a large undulation force is not necessary for the USV. Thus, the USV can be small and of light weight without any affect on the vehicle performance. The rolling motion of the vehicle which, in many cases, cause serious stability problem in the operation.
Fig. 1 Undulating motion of a conventional towed vehicle (available in full paper)
Fig. 2 The USV system (available in full paper)
Fig. 3 Undulating motion of the USV (available in full paper)