Maritime craft

Abstract

A maritime craft 1 has a body 5 defining an enclosed volume (11a, Fig 2), at least part of the enclosed volume forming a ballast chamber (65, Fig 3) arranged to be flooded and emptied to vary the buoy

Description

The present disclosure relates to a maritime craft. In particular, but not exclusively, the present disclosure relates to a maritime craft arranged to dock and undock with an underwater vehicle. Boats and other maritime craft are able to move quickly when operating as surface craft. For example, boats may hydroplane or hydrofoil to achieve high speeds. For certain operations, such as covert infiltrations, it is desirable for maritime craft to be submerged. However, boats cannot achieve as high speeds when under water. There is therefore a desire to provide craft that can operate both in high-speed modes (such as planing, hydroplaning or hydrofoiling) to get near a desired location (for example a littoral region), and also submerge for covert operation when it has reached the location. When a craft is operating underwater, foils and other structures cause drag, reducing efficiency of the underwater operation. There is a desire to provide a craft with efficient surface and sub-surface operation. According to a first aspect of the invention, there is provided a maritime craft having: a body defining an enclosed volume, at least part of the enclosed volume forming a ballast chamber arranged to be flooded and emptied to vary the buoyancy of the maritime craft; one or more hydrofoils provided below a lower surface of the body; and a passage extending into the body from an entrance, the passage arranged to dockably receive an underwater vehicle through the entrance, wherein: when the ballast chamber is flooded such that the entrance of the passage is below the water level, an underwater vehicle can dock in and undock from the passage; and when an underwater vehicle is docked in the passage and the ballast chamber is emptied such that the entrance of the passage and the underwater vehicle are above the water level, the maritime craft is arranged to lift into a hydrofoiling mode of operation using the one or more hydrofoils, with sufficient velocity. The maritime craft is able to operate in surface modes of operation, including lifting from displacement mode to hydrofoiling mode, through a number of different intermediate modes. Since the entrance to the passage can be moved below the waterline, a separate under water vehicle docked in the passage may be deployed from the craft and / or may dock with the craft. This ensures that the underwater vehicle can be delivered to a target area at high speed with high efficiency, and then operate underwater without suffering from drag or other negative effects due to the foil structures. The increased efficiency of surface and sub-surface operation enables the use of lower energy density fuel sources, such as battery electric amongst other purposes. The use of electric propulsion above and below water gives greater flexibility in usage and results in a reduction in thermal and acoustic signatures compared to propulsion by combustion engine. The entrance of the passage may be in the stern of the body. The underwater vehicle docks and undocks through the entrance of the passage, by placing the entrance at the stern, the opening and passage do not cause drag when propelled forward. The passage may extend along a length from the entrance to a second end opposite the entrance. The second end may be closed such that the passage is a blind passage. When the ballast chamber is emptied such that the entrance of the passage is above the water level, the maritime craft may be operable in a displacement mode, planing mode or hydrofoiling mode depending on the forward velocity. The craft may be operable in a displacement mode, planing mode or hydrofoiling mode depending on the forward velocity with an underwater vehicle docked or undocked. When the ballast chamber is flooded such that the entrance of the passage is below the water level, and there is no underwater vehicle in the passage, the maritime craft may be operable as a stationary floating platform. When the underwater vehicle is deployed, the craft may remain static as a platform for the underwater vehicle to return to, or it may move to a new location. The craft may be operable as a relay station to relay communications signals between an undocked underwater vehicle operating remotely from the maritime craft and a base station. The craft also enables long range communications to the underwater vehicle by acting as a relay. Communications between the maritime craft and the underwater vehicle may be by one or more of: underwater acoustic communications or plasmonic RF. Communications between the maritime craft and the base station may be by one or more of: satellite communications such as Low Earth orbit SATCOMs or plasmonic RF. The enclosed volume may include at least two chambers, the two chambers including the ballast chamber and a buoyancy chamber arranged to provide buoyancy to the maritime craft. The ballast chamber may be formed in a lower part of the enclosed volume. The maritime craft may include: one or more pumps to empty the ballast chamber; and one or more closeable apertures to fill the ballast chamber. The one or more closeable apertures may be arranged to be below a waterline of the maritime craft when operating in displacement mode. The maritime craft may have electronics components including one or more of: One or more communications system. One or more systems for controlling autonomous operation of the maritime craft. These systems may include radar, AIS collision avoidance, a controller for operation of the maritime craft. One or more systems of determining a position of the craft. These systems may include a global positioning system, an inertial navigation system, an inertial measurement unit, a Doppler velocity system and a depth sensor. Batteries for providing power to a propulsion system of the maritime craft. At least some of the electronics components may be received in a sealed chamber to prevent ingress of water. The sealed chamber may be received in the enclosed volume. The one or more hydrofoils may be mounted on a distal end of a strut extending from a lower surface of the body of the maritime craft. The maritime craft may include: a first hydrofoil mounted on the strut and a second hydrofoil mounted on the strut behind the first hydrofoil. The first hydrofoil may be arranged to provide a majority of the lift. The second hydrofoil may be arranged to provide trim control. The strut may be removable from the body of the maritime craft. Removing the strut allows for convenient shipping of one or more craft. The one or more hydrofoils may be interchangeably mounted on the strut. Interchangeable foils allows for the setup of the craft 1 to be tailored for different uses. The shape of the body may be arranged to provide a low visual and / or radar profile, especially when the ballast tanks are filled, for example when then craft is operating as a static platform. A shape of the passage may be reconfigurable to enable different underwater vehicles to be received. By configuring the shape of the passage, the craft can be used with existing underwater vehicles. Alternatively, it may be provided with a purpose designed vehicle. The maritime craft may include means to releasably secure the underwater vehicle in the passage. The maritime craft may include means to control docking of the underwater vehicle in the passage. The positioning means comprises one or more of: Means to determine positions of the maritime craft and underwater vehicle in an independent frame of reference. Means to determine positions of the maritime craft and underwater vehicle relative to each other. Acoustic communications means arranged to use time of flight measurements to determine a relative position of the underwater vehicle from the maritime craft. Short range identification beacons or visible markers and corresponding detectors on the maritime craft and underwater vehicle to determine a relative position of the underwater vehicle form the maritime craft by detection of the beacon or marker. The enclosed volume may extend around a circumference of the passage along at least part of the length of the passage. The maritime craft may be powered by electric propulsion means. The maritime craft may have a propulsion means independent of and separate to the propulsion means of the underwater vehicle. According to a second aspect of the invention, there is provided a system including: the maritime craft the first aspect; and an underwater autonomous vehicle, wherein the maritime craft and underwater autonomous vehicle are arranged such that the underwater autonomous vehicle is dockably received in the passage of the maritime craft. It will be appreciated that features discussed in relation to a particular aspect may be applied to any other aspect unless mutually exclusive. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates a perspective view of a maritime craft in accordance with an embodiment of the invention, with a docked underwater vehicle; Figure 2 illustrates the craft of Figure 1 in side on cut-through view; Figure 3 schematically illustrates the craft of Figure 1 in side on cut-through view, showing the water levels of different modes of operation; Figure 4A schematically illustrates the electric system of the craft of Figure 1; Figure 4B schematically illustrates part of the electric system of an underwater vehicle; and Figure 5 schematically shows the operation of an underwater vehicle using the craft of Figure 1. Figures 1 and 2 illustrate an example of a maritime craft 1 with a docked underwater autonomous vehicle (AUV) 3. Figure 1 shows a perspective view whilst Figure 2 shows a sectional view. As will be discussed in more detail below, the AUV 3 can dock and undock from the craft 1. When the AUV 3 is docked, the AUV 3 and craft 1 form a single vessel that moves as an integrated unit. When the AUV 3 is not docked, the craft 1 and AUV 3 may operate independently to each other. In some examples the docking and undocking process may occur only once. For example, the AUV 3 may start docked, and then undock. Once the AUV 3 is undocked, it may not redock (or the AUV 3 may start undocked, and then once docked not undock again). Alternatively, the AUV 3 may be able to dock and undock a number of times for repeated usage. The craft 1 has a main body 5 formed by an outer hull 7. The hull 7 is formed by a number of panels 9 defining an envelope 11. At the front 13 of the craft 1, is a nose 15. In the following the forward / backwards direction will be defined relative to the nose 15. The up and down directions will be defined relative to a normal orientation of the craft 1 in use, such that the bottom / base faces into a water surface. From the nose 15, an upper front surface 17 of the craft 1 extends upwards, and outwards as it extends backw ards (increasing the height and width of the cross section of the craft 1) until it reaches a planar top surface 19 of the craft 1. The top surface 19 is substantially rectangular in shape. A lower front surface 21 extends downwards, and outwards from the nose 15 as it extends backwards (again increasing the height and width of the cross section of the craft) until it reaches a planar base surface 23. The extent of the lower front surface along the length of the craft (from the nose 15 to a stern 25) is shorter than the extent of the upper front surface 17 in the same direction, and the angle of incline / radius of the downward curvature is smaller. The planar base surface 23 continues to extend outwards as it extends backwards. A waist 27 is formed where the upper front surface 17 meets the lower front surface 21. Extending backwards from the front 13 of the craft 1, the waist 27 tends upwards from the nose 15. Partway along the length of the craft 1, behind the nose 15, the upper front surface 17 and the lower front surface 21 separate from each other and the waist 27 ends. From this point, side surfaces 29 are formed extending between the upper front surface 17 and the lower front surface 21. The stern 25 of the craft 1 extends over an area defined by the upper front surface 17 and the lower front surface 21 and the side surfaces 29. At least part of the stern 25 of the craft 1 is enclosed by a rear face 31. A passage 33 is formed extending into the body 3 of the craft 1. The passage 33 has an entrance 35 in the rear face 31 of the craft 1. The passage 33 extends from the entrance 35 to an opposite end 37 adjacent the nose 17 of the craft 1. For at least part of the extent along the length of the passage 33, from the entrance 35 to the second end 37, sidewalls are provided around the passage 33. Towards the front of the craft 1, the passage 33 opens out into a wider end chamber 39. At least part of the enclosing surface of the end chamber 39 may be formed by the upper front surface 17. The widening of the passage 33 is a step change in the width. At the step change, internal walls 33a,b,c are formed between the sides of the passage 33 and the sides of the outer hull 7, and along the top and bottom of the passage 33 to the upper front surface 17 and the lower front surface 21. The outer hull 7 and the walls of the passage 33 form an enclosed volume Ila which is watertight. The enclosed volume Ila extends around both sides of the passage 33 and above and below it. The passage 33 extends through the envelope into the end chamber 39. The end chamber 39 and passage 33 are open to water through the entrance 35 of the passage 33. As will be discussed in more detail below, a AUV 3 can be received in the passage 33. In the example discussed below, the AUV 3 is substantially cylindrical in shape, with a drive propeller 41 at the rear. When the AUV 3 is received in the passage 33, the drive propeller 41 projects rearwards of the stern 25 of the craft 1 Any suitable AUV 3, with any desirable function, power source and control means may be used. As shown in Figures 1 and 2, a strut 43 extends downwards from the base surface 23 of the outer hull 7. The strut 43 extends from a proximal end 45 at the base 23 of the outer hull 7 to a distal end 47 opposite the proximal end 45. An upper part 43a of the strut is shaped to reduce drag as the craft 1 moves through the water. A lower part 43b of the strut 43 is shaped and moveable by servo motors 49 to allow it to operate as a rudder. At the distal end 47 of the strut 43, a foil support 51 extends parallel to the length 1 of the craft both ahead of and behind the strut 43. Ahead of the strut 43 a forward wing 53 is provided to cause lift 1 of the craft 1 when hydrofoiling. The forward wing 53 forms a first hydrofoil extending perpendicular to the foil support 51. Behind the strut 43, an electric surface pump-jet propulsor 55 is mounted on the foil support 51, along with a rear wing 57. The rear wing 57 forms a rear hydrofoil extending perpendicular to the foil support 51. The rear wing 57 is moveable by servo motors 59 to provide trim control (pitch, heave). Typically, the front wing / foil 53 provides the majority of lift, while the rear foil / wing is primarily used for trim control. For example, the front wing / foil 53 may provide above 80% or above 90% of the lift. When steering the craft 1, actuation of the rudder causes a hydrodynamic sway (sideways) force on the foils 53, 57. The centre of this force is below the centre of gravity of the craft 1 and results in the foils 53, 57 taking a lateral angle of attack. The foils 53, 57 then give a force perpendicular to their planform, which acts pushing the craft 1 in the direction of turn. This means that the angle of inclination also acts to oppose any side forces from waves or wind. It will be appreciated that the servo motors 49, 59 for controlling the rudder and trim operation may be watertight. The servo motors 49, 59 are also enclosed in hydrodynamically shaped casings 61, 63, to reduce drag. Figure 3 shows a schematic representation of the division envelope I 1 and enclosed volume Ila of the body 3 into different chambers, and the waterline during different modes of operation. The enclosed volume Ila is divided into separate chambers 65, 67, 69 which are separate from each other and watertight from each other. A first chamber 65 is provided in a lower part of the envelope 11. This may be entirely below the passage 33, or may extend partway up the sides of the passage 33, or above the passage 33. In one example, the first chamber 65 extends to a height approximately level with the top of an AUV 3 received in the passage 33. The first chamber 65 is a ballast tank. This has closeable apertures, controlled by valves 66 to allow water to enter the chamber 65 when the apertures are open and below the water level. Pumps 71 are also provided to expel water from the chamber 65 or fill them more rapidly. The water may be expelled through the same closeable aperture or different apertures. By varying the water volume in the first chamber 65, in combination with the forward speed, the position the craft 1 sits in the water can be varied. This will be discussed in more detail below. The second chamber 67 is provided at an upper part of the envelope 11. The second chamber 67 is a buoyancy chamber. This encloses air to provide for buoyancy. As discussed below, at least some of the operational components of the craft 1 may be provided in the buoyancy chamber 67. For example, a radar head 73 of a radar system 75 may be provided in this chamber 67. The third chamber 69 is an electronics bay for holding any electronics components that need to be protected from water ingress. Figure 4A shows the electrical system 77 of the craft 1. Figure 4B shows the electronic components of the AUV 3 that co-operate with the craft 1. It will be appreciated that this is not the full electric system of the AUV 3, merely the components to allow operation in combination with the craft 1. As discussed above, the craft 1 is powered by an electrically driven propulsor 55 at the end of the strut 43. In the current embodiment, batteries 79 are provided in the ballast chamber 69, in watertight and pressure rated vessels 80. It will be appreciated that the batteries 79 may be provided in the electronics bat 69, or any other suitable location. The batteries power the propulsor 55 and other functions of the craft. The AUV 3 may have its own power source, optionally rechargeable from the craft 1 when docked. Recharging may be by cable or wireless power transfer. A controller 81 is provided to provide appropriate control signals to both the propulsor 55 and to the servo motors 49, 59 for operating the rudder 43b and trim control. A power and control connection 89 may be provided along the strut 43 to connect to the rudder and trim servos and the propulsor. In one example, the controller 81 may be arranged to provide autonomous control, or control from a preprogrammed sub-routine to control operation of the craft 1. In other examples, the controller may operate under direct or remote control from an operator. A memory 83 is provided to store any computer programme instructions required to operate the craft 1 and its sub-systems. Various other sensor or control systems may be optionally provided. For example, as discussed above, a radar system 75 may be provided to assist operation of the craft 1. In addition, or instead, an AIS collision avoidance system 85 may be provided, coupled to an antenna 87 on the top surface 19 of the body 3. Positioning systems may also be provided. For example, the craft may have a Global Navigation Satellite Signal (GNSS) location system 103 to provide a location of the craft 1 in known co-ordinate system. Any suitable GNSS system may be used, such as GPS, Galileo, BeiDou or GLONASS. In addition, an inertial measurement unit 88a, inertial navigation system 88b, Doppler velocity sensor and depth sensor may be provided to assist navigation. These may be provided in the craft 1, or at the end of the strut 43, for example in the casing 63 enclosing the servo 59 the rear foil 57 or a skeg of the foils. These systems may be used when GNSS is not available, or to supplement GNSS, or for other purposes. Various communications sub-systems may also be provided. This may include, for example, a satellite communications system 91 with an antenna 93 on the top surface 19 of the body 3, and an underwater acoustic communications system 95 may be provided with a speaker 97a and microphone 97b (or acoustic modem 97) under the waterline of the craft 1. Various other surveillance and payload equipment may also be provided depending on the intended use of the craft 1. The various components of the craft 1 are in communication with each other over a communication bus 99 and other wired or wireless connections. Whether the AUV 3 is in the passage 33 or not, the operation of the craft 1 at different velocities is as follows. With the ballast tank 65 configured for surface operation and operating at slow forward velocities or zero forward velocity, the craft 1 sits in the water such that the water line 101 is at a level just below the entrance 35 of the passage 33. The dashed line labelled 101A shows the surface operation low velocity water line. When the ballast tank 65 is configured for surface operation, the tank 65 may be empty or have a relatively low level to achieve the desired water line 101. At low velocity, the craft 1 moves in displacement mode. As the velocity increases from stationary or low velocity, the craft 1 moves through planing mode, then hydroplaning mode, and then at the highest speeds, hydrofoil mode. With increasing speed, the nose 15 of the craft 1 lifts, emptying any water in the passage 33. In the hydrofoil mode, the craft 1 is lifted out of the water so that the water line is within the lower portion 43b of the strut 43. The dashed line labelled 10IB shows the hydrofoiling water line. In the displacement mode of operation, the weight of the craft 1 is supported by the buoyancy of the craft 1 and the smooth surface on the underside of the hull 7, causes flow attachment of the water. As the velocity is increased from the displacement mode, the nose 15 of the outer hull 7 lifts. Water flow is separated from the hull 7, creating lift. By inducing flow separation, a section of the hull 7 runs dry, leading to a reduction in wetted surface area and thus the frictional component of drag. In the planing mode, the hydrodynamic pressure of the water on the hull 7 underside generates a lift component raising the craft 1 higher in the water and thus reducing the wetted surface. The trimming of the craft 1 (positive angle of attack) is a by-product as the force is predominantly forward and thus creates a trimming moment. As the velocity of the craft 1 increases further, the lift further increases, creating further lift until the foiling / hydrofoiling of operation is reached. In the hydrofoiling mode of operation, the wetted surface is significantly reduced, and it is typically only the surface of the lower part 43b of the strut 43 and the foils 53, 57, foil support 51 and propulsor 55 that are under water. It will be appreciated that as the velocity is increased and decreased, the craft 1 may move between the different modes of operation. In some cases, further intermediate modes may be provided, or fewer modes may be provided. It will be appreciated that the operation gradually changes between modes, rather than abrupt changes. At low speeds, the craft 1 can also be configured to allow for docking or undocking of the AUV 3. With the craft 1 in displacement mode, the ballast tank 65 is filled so that the water line 101 is above the entrance 35 of the passage 33. This is shown by the dashed line labelled 101C in Figure 3. In general, the valves 66 for filling the ballast tank 65 are positioned to be below the water level 101A with the tank configured for surface operation and slow velocities, so that the ballast tank can easily filled by opening the valves 66 and moving forward. Alternatively, or in addition, water may be pumped into the ballast tank 65. Further ballast may be provided in the passage 33 and chamber 39 at the end of the passage by reversing the craft when the entrance 35 of the passage 33 is below the waterline. When the craft is configured for docking, a AUV 3 received in the passage 33 can reverse out of the passage 33 and operate in the location it has been delivered to. Alternatively, a AUV 3 can dock in the passage if it is empty, by moving forward into the entrance 35. The process of docking a AUV 3 is described below. In order to complete a docking procedure, the location of the AUV 3 relative to the craft 1 is determined. This may be done in a number of ways. As discussed above, the craft 1 has a GNSS locations system 103 to provide a location of the respective vehicle in a known co-ordinate system. The AUV 3 may also include a similar system, allowing the positions of both vehicles to be known in a common coordinate frame. Where GNSS is not available or accurate, the inertial navigation systems may be used to calculate the positions to calculate the positions of the craft 1 and AUV 3. This may be from a known starting point, such as an accurate GNSS measurement or if the AUV 3 starts operation from the craft 1. In other examples, the craft 1 or AUV 3 may use localising techniques to locate one vehicle relative to the other. For example, one of the craft 1 or AUV 3 may include a beacon and the other a receiver. Various different time of flight techniques or other location techniques may determine a relative spacing and / or angle of the two vehicles 1,3. Alternatively, time of return flight techniques may be used to detect a reflection (for example sonar). As discussed above, the craft 1 includes an acoustic communications system 95 (transceiver) for communications between the craft 1 and AUV 3. The AUV 3 may include a corresponding transceiver / modem 107. The underwater communications system may be used as the beacon for relative location determination. In another example, short range RFID transmitters 109 and receivers 111 may be used to determine relative positioning between the AUV 3 and craft 1. By having the transmitters 109 and receivers 111 at fixed and known locations on the craft 1 and AUV 1, with unique identifiers for each transmitter 109 and receiver 111, the relative positions of the two vehicles 1, 3 can be determined when a particular transmitter 109 detects a particular receiver 111. In yet a further example, QR codes and optical detectors or other visible, or non-visible identifiers and detectors may be used in a similar manner to the RFID transmitters 109 and receivers 111. It will be appreciated that a combination of different locating techniques may be used to determine the relative locations of the craft 1 and AUV 3. Once the AUV 3 is positioned in the passage, it is held in place by a retractable hook 113 that is actuated to engage a holding formation 115 on the AUV 3. The hook 113 can include a wet mate ethernet connector 125 or position the AUV 3 over a wireless pad to allow for data connection to a controller 117 or charging on the AUV 3. This may allow for updated programmes or instructions to be provided to the AUV 3. In order to undock, the hook 113 is actuated to release the AUV 3. When the craft 1 is ready to move off, whether a AUV is docked or not, the ballast tank 65 can be emptied to the surface operation configuration. Figure 5 schematically illustrates operation of the AUV 3 when it is undocked from the craft 1. In one embodiment the AUV 3 operates autonomously. Autonomous operation of AUVs is known and will be understood by the person skilled in the art. The AUV 3 communicates with the craft 1 using acoustic (underwater) communications 127. The craft 1 then communicates with a base / control station 119 via a satellite link 121 using one or more satellites. Therefore, the craft 1 acts as a relay. In one example Low Earth Orbit SATCOMS may be used. When acting as a relay, the craft 1 may remain as a static base platform, with the ballast tanks 65 filled, so that it sits low in the water, with a low radar profile. It may be that without the weight of the AUV 3, further ballast is provided in the tank 65 or passage 33. The craft 1, acting as a relay, may be stationary or may move as described above. The craft 1 may move with the ballast tank 65 filled or emptied (requiring refilling to redock). Communication between the AUV 3 and craft 1 and communication between the craft 1 and the base station 119 may be continuous or burst / intermittent. The communication may be simply to provide status updates / monitoring or may be two way to allow control and modification of the operation of the craft 1 and AUV 3. In general, the craft 1 may be used to quickly and efficiently deliver a AUV 3 to an area where it is needed. Once the AUV 3 is has finished its tasks, it may redock with the craft 1 to remove from the area. The redocking at the same position as the undocking or a different potion. The docking / redocking process may be completed multiple times. In other examples, the AUV 3 may not return to the craft 1, and the craft 1 may return to its base once the AUV is delivered 3. In other examples the craft 1 may be used to collect a AUV 3 that is operating in an area, having been delivered by other means. Various optional features may be incorporated into the craft 1 discussed above. In some examples, the panels 9 forming the outer hull 7 may be formed of carbon fibre or other lightweight materials. At least some of the panels 9 may be made of material transparent to radar, such as e-glass. The radar transparent material may be used in the vicinity of the radar head 75. Furthermore, by placement of carbon fibre panels and radar transparent panels, the radar signature of the craft 1 can be controlled. In at least some embodiments, the strut 43 may be removably connected to the body 3 of the craft 1. A connection point 123 may be provided at the proximal end 45 of the strut 43. Disconnecting the strut 43 allows for easier movement of one or many craft in containers and the like. Furthermore, the foils 53, 57 may be interchangeable to allow for different shape wings to be attached. One or both of the front and rear foils 53, 57 may be interchangeable. The foils 53, 57 may be made by 3D printing or other suitable manufacturing methods and mounted over titanium or other spars to allow them to be removed and replaced. The foils 53, 57 may be chosen based on expected use parameters and the like, for different periods of operation. For example, large wings provide lower top speed but larger range, whilst small wings increase top speed but reduce the range. Furthermore, selection of wing can change the trimming / centre of gravity of the craft. For example, a larger forward wing accommodates a payload with a centre of gravity further forward. In the embodiment discussed above, at least part of the passage 33 is shaped to tightly fit the AUV 3. In one example, the AUV 3 is approximately 80kg in weight, 2.2 metres long and 350mm in diameter. However, it will be appreciated that the craft 1 may be used with different types of AUV 3. In some examples, the passage 33 may include removably connectable panels (not shown) to modify the size and shape of the passage 33 for different AUVs 3. The removably connectable panels may be secured by bolts, clips or other releasably connectable means. The shape of the outer hull 7 discussed above and shown in the Figures is given by way of example only. The outer hull may have any suitable shape, and size. In the example discussed above, the upper front surface 17 is sloped to ensure a low radar signature, both with the ballast tank 65 filled and with it empty. This is optional, and other shapes or configurations may be used to provide a low radar signature. In the example discussed above, the craft I and AUV 3 are operated autonomously. It will be appreciated that this is by way of example only. One or both of the craft I and AUV 3 may be operated by a user. This may be remotely or within the vehicle. For example, the AUV 3 may be operated from a user within the AUV 3 or remotely from a user on the craft 1. Alternatively, the craft 1 and / or AUV 3 may be operated by a user remotely from the base station 119 or some other location connected via the base station. The autonomous operation may be any suitable style. For example, in one case, the craft 1 and / or AUV 3 may simply follow preprogrammed paths are routes. In other examples, various levels of autonomation may be implemented. For example, the craft may be reactive to the detected environment and condition around the craft 1 and / or AUV 3.39 In the example discussed above, the craft 1 communicates either continuously or in bursts with a base station 119 over a satellite link 121. The base station 119 may be a launching vessel, a ground based base station, a control aircraft or any other suitable base station. Alternatively, the craft 1 may operate without any communications to the base station 119. Similarly, the craft 1 may not necessarily communication with the AUV 3 for part of or all of the time the AUV 3 is operating independently to the craft 1. In the example discussed above, the AUV 3 is held in the passage 3 by a hook 113. It will be appreciated that this is by way of example only. Any type of holding or clamping mechanism may be used, mounted on either the passage 33 or AUV 3. Furthermore, any suitable data connection may be used to provide a physical or wireless connection between the craft and AUV 3. This may include USB, Bluetooth, Wi-Fi and the like. Any physical connector may be part of the holding or clamping mechanism, or separate. It will be appreciated that the use of acoustic and satellite communications is by way of example only. Any suitable communications system can be used between the AUV 3 and craft I, and the craft 1 and a wider base station 119 / network. For example, plasmonic RF communications may be used in place of satellite or acoustic communications. In the example discussed above, three separate chambers 65,67,69 are provided. This is by way of example only. In some cases, one or more of the chambers may be divided into separate sub-chambers. Furthermore, in some examples the buoyancy chamber 67 may function as the electronics bay such that no separate electronics chamber 69 is provided. In some cases, various electronic components which are able to operate in underwater environments, such as pumps, or that are fitted inside pressure vessels, may be provided in the ballast chamber 65, or in the passage 33 (or enlarged end chamber 39 of the passage). It will be appreciated that any suitable locating methods can be used to locate the AUV 3 relative to the craft 1 when positioning the two relative to each other for a docking operation. This may include means for separately determining the location of the two vehicles in a common frame of reference (such as GNSS) or means for locating one of the craft 1 and AUV 3 relative to the other. In the example discussed above, the foils are mounted on a single strut 43. This provides reduced drag. However, it will be appreciated that any number of struts and foils may be used. Furthermore, any type and arrangement of foils may be used. In the example above, the strut 43 incorporates a rudder for steering of the craft 1 by causing a hydrodynamic sway (sideways) force on the foils 53, 57. This is by way of example only. The person skilled in the art will appreciate a variety of different ways for steering the craft. The example of a pump jet propulsor 55 is given by way of example only. Any suitable propulsion means may be used to propel the craft 1. In one example, the use of a single strut, with active control surfaces (for trimming operation) can result in a range of over 100 nautical miles travelling at over 20 knots, with an 80kg AUV 3 docked. The top speed may reach 25 knots, or even faster, for example 45 knots. The range and speeds are exemplary only and depending on the load, trim, foils and the like, the range and speed can be varied. In the above description, the craft 1 and AUV 3 are operated as a system. However, it will be appreciated that in some examples, the craft may operate on its own, without an associated AUV 3.

Claims

1. A maritime craft having:a body defining an enclosed volume, at least part of the enclosed volume forming a ballast chamber arranged to be flooded and emptied to vary the buoyancy of the maritime craft;one or more hydrofoils provided below a lower surface of the body; and a passage extending into the body from an entrance, the passage arranged to dockably receive an underwater vehicle through the entrance, wherein:when the ballast chamber is flooded such that the entrance of the passage is below the water level, an underwater vehicle can dock in and undock from the passage; andwhen an underwater vehicle is docked in the passage and the ballast chamber is emptied such that the entrance of the passage and the underwater vehicle are above the water level, the maritime craft is arranged to lift into a hydrofoiling mode of operation using the one or more hydrofoils, with sufficient velocity.

2. The maritime craft of claim 1, wherein the entrance of the passage is in the stern of the body.

3. The maritime craft of claim 1 or claim 2, wherein the passage extends along a length from the entrance to a second end opposite the entrance, wherein the second end is closed such that the passage is a blind passage.

4. The maritime craft of any preceding claim, wherein, when the ballast chamber is emptied such that the entrance of the passage is above the water level, the maritime craft is operable in a displacement mode, planing mode or hydrofoiling mode depending on the forward velocity.

5. The maritime craft of claim 4, w;herein the craft is operable in a displacement mode, planing mode or hydrofoiling mode depending on the forward velocity with an underwater vehicle docked or undocked.

6. The maritime craft of any preceding claim, wherein when the ballast chamber is flooded such that the entrance of the passage is below the water level, and there is no underwater vehicle in the passage, the maritime craft is operable as a stationary floating platform.

7. The maritime craft of any preceding claim, the maritime craft is operable as arelay station to relay communications signals between an undocked underwater vehicle operating remotely from the maritime craft and a base station.

8. The maritime craft of claim 7, wherein:communications between the maritime craft and the underwater vehicle are by one or more of: underwater acoustic communications or plasmonic RF; andcommunications between the maritime craft and the base station are by one or more of: satellite communications such as Low Earth orbit SATCOMs or plasmonic RF.

9. The maritime craft of any preceding claim, wherein the enclosed volume includes at least two chambers, the two chambers including the ballast chamber and a buoyancy chamber arranged to provide buoyancy to the maritime craft.

10. The maritime craft of claim 9, wherein the ballast chamber is formed in a lower part of the enclosed volume.

11. The maritime craft of any preceding claim, including:one or more pumps to empty the ballast chamber; andone or more closeable apertures to fill the ballast chamber, the one or more closeable apertures being arranged to be below a waterline of the maritime craft when operating in displacement mode.

12. The maritime craft of any preceding claim having electronic components including one or more of:one or more communications system;one or more systems for controlling autonomous operation of the maritime craft, optionally including radar, AIS collision avoidance, a controller for operation of the maritime craft;one or more systems of determining a position of the craft, optionally including a global positioning system, an inertial navigation system, an inertial measurement unit, a Doppler velocity system and a depth sensor; andbatteries for providing power to a propulsion system of the maritime craft.

13. The maritime craft of claim 12, wherein at least some of the electronics components are received in a sealed chamber to prevent ingress of water, the sealed chamber received in the enclosed volume.

14. The maritime craft of any preceding claim, wherein the one or more hydrofoils are mounted on a distal end of a strut extending from a lower surface of the body of the maritime craft.

15. The maritime craft of claim 14, including:a first hydrofoil mounted on the strut arranged to provide a majority of the lift; anda second hydrofoil mounted on the strut behind the first hydrofoil, the second hydrofoil arranged to provide trim control.

16. The maritime craft of claim 14 or claim 15, wherein the strut is removable from the body of the maritime craft.

17. The maritime craft of any of claims 14 to 16, wherein the one or more hydrofoils are interchangeably mounted on the strut.

18. The maritime craft of any preceding claim, wherein a shape of the passage is reconfigurable to enable different underwater vehicles to be received.

19. The maritime craft of any preceding claim, including means to releasably secure the underwater vehicle in the passage.

20. The maritime craft of any preceding claim, including positioning means to control docking of the underwater vehicle in the passage.

21. The maritime craft of claim 20, wherein the positioning means comprises one or more of:means to determine positions of the maritime craft and underwater vehicle in an independent frame of reference;means to determine positions of the maritime craft and underwater vehicle relative to each other;acoustic communications means arranged to use time of flight measurements to determine a relative position of the underwater vehicle from the maritime craft; andshort range identification beacons or visible markers and corresponding detectors on the maritime craft and underwater vehicle to determine a relative position of the underwater vehicle form the maritime craft by detection of the beacon or marker.

22. The maritime craft of any preceding claim, wherein the enclosed volume extends around a circumference of the passage along at least part of the length of the passage.

23. The maritime craft of any preceding claim, wherein the maritime craft is powered by electric propulsion means.

24. The maritime craft of any preceding claim, wherein the maritime craft has a propulsion means independent of and separate to the propulsion means of the underwater vehicle.

25. A system including:the maritime craft of any preceding claim; andan underwater autonomous vehicle,wherein the maritime craft and underwater autonomous vehicle are arranged such that the underwater autonomous vehicle is dockably received in the passage of the maritime craft.

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