Fig. 15. Power consumption (without zero-load power) of caudal fin thruster and screw propeller

3) Measurement results of maneuverability performance

Possible commissions of portable UUV include probe of port and coast, as well as identification and destroy of torpedo [17]. To perform these commissions, AUV often needs to be close to the object in a small distance and at the same time avoids colliding. At that moment low speed maneuverability is particularly important. For example, AUV is often constrained in a narrow space when it is in the state of autonomous navigation. At that moment AUV needs to turn in situ round itself to go back to open sea, which is a maneuverability often used by ROV but difficult for AUV whose advantage lies in its cruising.

The VCUUV achieved 1.2m/ sec and turn rates up to 75°/sec[12]. With flexible caudal hull and four joints caudal fin driven by hydrodynamics power, VCUUV possesses excellent maneuverability. It achieves a turning diameter of two body lengths(BL). Though the hull of SPC-III is completely rigid, and it only has two joints, its special caudal structure enables caudal fin to realize a deflection angle of 0~90°. 90°deflection angle can be used for emergency braking.

Figure 9, 10 shows the circular trajectory of SPC-III and propellers comparison AUV under maneuverability measurements. The trajectory was drawn at GCS300 ground station software using GPS Coordinate data recorded by Autopilot. Note that the calibration of the map scale is 5m. SPC-III adopts a flapping frequency of 2Hz, with a rotation speed of propeller at 7.5r/sec, and correspondent linear speed being about 1.1m. Yet the speed decreases remarkably as the turning radius decreases in turning state. At 45°deflection angle, caudal fin thruster achieves a turning diameter of 2.5BL, while screw propeller which uses rudder achieves a turning diameter of 5BL. Caudal fin thruster achieves minimum

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