INTRODUCTION

Based on the success of the torpedo drifter (Cresswell at al. 1978; Fig 1) developed and used by Cresswell of CSIRO in the Indian Ocean, the Tasman Sea, and in eddys off the east Coast of Australia, (Cresswell and Vaudrey, 1977; Cresswell and Wood, 1977) and the Ferranti ORE version (Smith et al 1984; Fig 2) used by Luyten in the Agulhas current off Africa (Luyten and Smith, 1985), the ORE torpedo drifter was planned for use in the Yellow Sea Experiment, a joint USA-PRC coastal processes study to be conducted in 1986. However, a test performed at the Scripps Institution of Oceanography (p.p. Nuler, personal communication) indicated that despite the advantages of the torpedo shape in reducing wind and wave drag force errors might ocean current areas, in regions having low velocity current regimes these errors might still be significant. Therefore, an intercomparison experiment was conducted using the ORE torpedo drifter, small spherical drifters, and the Scripps spherical drifter-drogue system. Various drogues were used with the torpedo and the small spherical drifters. While wind and sea conditions at the time of the experiment were not extreme enough to conclusively answer the question raised by the Scripps observations, it is believed an important aspect of the use of the use of drogue drifters was revealed.

The Drifter Buoys

Of the four drifter buoys used in this experiment, three were essentially identical foam-filled fiberglass spheres 45cm in diameter, each having a heavy duty steel pad in the lower pole for the drogue attachment point. These spheres displace 113 pounds in seawater. The ORE spherical buoy (Fig 3) weighing 28 pounds contains within the sealed watertight shell a 12 V alkaline battery pack providing in excess of six months? operation, the ARGOS satellite transmitter (PTT Model 2101A), a new disc-shaped 401.650 mHz antenna developed by Chua Associates. The antenna provides a full horizon-to-horizon radiation pattern ideal for a spherical buoy. The SIO spherical buoy was not instrumented for these tests since visual tracking was employed. The ORE torpedo drifter consists of a crosslinked polyethelence moulded hull filled with high-density polyurethane foam containing a PVC cylinder housing the ARGOS transmitter and an eighteen month operating-life alkaline battery pack. The ARGOS antenna and associated ground plane are sealed under a moulded blister on the upper surface. An internal steel keel supports the drogue attachment fitting and internal structural bulkheads. The sea surface temperature sensor is a waterproof probe located inside the skeg. The flat upper surface provides for battery-charging solar cells whereil long operating life in excess of eighteen months is required. Operation for over two years can be expected using these and lead-acid gell cell. This drifter weighs 90 lb and displaces 275 lb in seawater.

Fig 1 CSIRO torpedo drifter buoy; overall length ca. 1.5m. (from Cresswell et al, 1978) (available in full paper)

Fig 2 Low-cost torpedo-shaped drifter, Ferranti ORE Model 830; overall length 1.24m (available in full paper)

Fig.3 Low-cost spherical drifter, Ferranti ORE Model 831, diameter 45cm

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