## 3 Architecture of the Robot  
The proposed device is designed to carry a minimum payload of 75 kg. The victim is approached from underwater and then helped into the net when the robot emerges on the surface. Then the mission can stop, wait for a support ship or a helicopter, or can continue, bringing the victim to a safer place. Since the water could be dirty and opaque, the robot would be equipped with a set of sensors (350 lumens search LED light, infrared and temperature sensor etc.) to facilitate the localization procedure. This issue is beyond the aim of this paper. The maximum speed of the robot is 10 km (5.15 m/s). The minimum duration of the mission is 15 min and the operations may be repeated many times, accounting for the possibility of missing the victim at first attempt. The device has to be able to stop and maintain a fixed relative position with respect to the body. Furthermore, the person should acquire a horizontal position once saved, as it is medically demonstrated that, in this way, the risk of further injuries is minimized. Owing to these requirements, six different solutions are proposed in this section. Every configuration is composed of a stretcher supported by two lateral hulls. Two propellers (1 in Figure 3) are positioned under the hulls to avoid contact with the victim. Since the robot is remotely controlled, it is sufficient to drive the propellers with different velocities to make the system yaw in the desired direction. This solution is more compact from the construction point of view and effective also in the case of low speed maneuvering if compared with a configuration based on a single propulsion system combined with a rudder. The six architectures are illustrated in Figure 3 where (a), (b), (c), and (d) are designed for the static diving strategy while (e) and (f) exploit the normal and full dynamic diving approach respectively. The final selected configuration is (f). Solution (a): the vertical motion is provided by filling and emptying four diving chambers (3) (two per each hull) with two reversible electro-pumps (2). The amount of air needed to fill or empty  
Figure 2: Hydroplanes tilting sequence for normal dynamic diving (**a**) and full dynamic diving (**b**). The diving path is illustrated by the dashed lines. the chambers is taken over the surface of water by means of two snorkels (11). Motors supplying propellers, electro-pumps, batteries, and electronic devices are contained in the paraboloid heads of the hulls, separated by the diving chambers through watertight plates. The rescue method is as follows: the robot localizes and reaches the victim using the horizontal propellers (1), immerses by filling the diving chambers with water, moves right under the person by switching on the propellers again, and then emerges vertically by emptying the diving chambers. Solution (b): the concept is based on the same idea and the same rescue method of (a) but aims at reducing the weight and mass moments of inertia. The cylindrical hulls here do not take up the entire length of the robot. One diving chamber per hull (3) is present and thus the control of pitch movement has to be performed with additional rudders (not illustrated in the Figure). Solution (c): the robot can immerse by changing its volume with the deformable hulls (4). The device can move both underwater and at the surface level during the outward phase and only at the surface level when the victim is onboard. The hull is conceived to be an ellipse during the outward phase (c.1), with an eccentricity that will depend on the decided trim (underwater or displacing) and a cylinder in the most loaded condition (c.2), i.e., when the victim is onboard. The solution presents problems of dimensions, repeatability, and fatigue of the hulls. Solution (d): the device is equipped with cylindrical vessels storing compressed air used to inflate two dinghies responsible for the device trim.