Underwater vehicle

1. An underwater vehicle comprising:

port and starboard thrusters spaced apart in a port-starboard direction, each thruster being oriented to generate a thrust force in a fore-aft direction perpendicular to the port-starboard direction;

a vertical thruster which is oriented to generate a thrust force substantially perpendicular to the fore-aft and port-starboard directions;

port, starboard and vertical ducts which contain the port, starboard and vertical thrusters, respectively, each duct providing a channel for water to flow through its respective thruster; and

a moving mass which can be moved in the fore-aft direction to control a pitch of the underwater vehicle, wherein the moving mass comprises one or more batteries.

2. The underwater vehicle of claim 1 , wherein the moving mass can be moved relative to the thrusters in the fore-aft direction to control the pitch of the underwater vehicle.

3. The underwater vehicle of claim 1 , wherein movement of the moving mass is configured to determine the pitch of the underwater vehicle.

4. The underwater vehicle of claim 1 , further comprising an actuator for moving the moving mass.

5. The underwater vehicle of claim 1 , wherein the underwater vehicle has a maximum length L in the fore-aft direction and a maximum width W in the port-starboard direction; and wherein 0.8<L/W<1.2.

6. The underwater vehicle of claim 1 , wherein the underwater vehicle has a maximum length L in the fore-aft direction, a maximum width W in the port-starboard direction, and a maximum height H in the height direction; and wherein: W/H>1.5 and L/H>1.5.

7. The underwater vehicle of claim 1 , wherein the underwater vehicle has a maximum length L in the fore-aft direction, a maximum width W in the port-starboard direction, and a maximum height H in the height direction; and wherein: 0.8<L/W<1.2, W/H>1.5, and L/H>1.5.

8. The underwater vehicle of claim 1 , wherein the vertical thruster is shrouded by a vertical shroud having a convex planform external profile when viewed from above in the height direction.

9. The underwater vehicle of claim 1 , wherein the underwater vehicle has a centre of buoyancy and a centre of gravity, and the vertical thruster is positioned so that the thrust force generated by the vertical thruster is offset from the centre of buoyancy and the centre of gravity.

10. The underwater vehicle of claim 1 , further comprising a body which carries the vertical thruster, wherein an orientation of the vertical thruster is fixed relative to the body so that the thrust force of the vertical thruster cannot be re-oriented from the vertical direction.

11. The underwater vehicle of claim 9 , wherein the underwater vehicle has a mid-plane which lies half way between a nose and a tail and passes through the port duct and the starboard duct, and the port and starboard thrusters are reversible so that they can be operated to generate forward thrust to drive the underwater vehicle forwards with the nose leading and operated to generate reverse thrust to drive the underwater vehicle backwards with the tail leading.

12. The underwater vehicle of claim 1 , further comprising a seismic sensor.

13. The underwater vehicle of claim 1 , further comprising a base with a substantially planar downward-facing external surface which can provide a stable platform for the underwater vehicle.

14. The underwater vehicle of claim 1 , further comprising upper and lower shells which meet at respective edges and together provide an external hull of the underwater vehicle.

15. The underwater vehicle of claim 14 wherein the upper shell forms a downward-facing cup and the lower shell forms an upward-facing cup.

16. The underwater vehicle of claim 1 , further comprising a body which carries the port and starboard thrusters, wherein orientations of the port and starboard thrusters are fixed relative to the body so that the thrust forces of the port and starboard thrusters cannot be re-oriented.

17. An underwater vehicle comprising:

a nose and a tail at opposite ends of the underwater vehicle;

port and starboard thrusters spaced apart in a port-starboard direction, each thruster being oriented to generate a thrust force in a fore-aft direction perpendicular to the port-starboard direction;

a vertical thruster which is oriented to generate a thrust force substantially perpendicular to the fore-aft and port-starboard directions; and

a moving mass which can be moved in the fore-aft direction to control a pitch of the underwater vehicle;

wherein the underwater vehicle is configured to land on a seabed and the vertical thruster is arranged to control a pitch of the underwater vehicle when the underwater vehicle is on the seabed.

18. An underwater vehicle comprising:

port and starboard thrusters spaced apart in a port-starboard direction, each of the port and starboard thrusters being oriented to generate a thrust force in a fore-aft direction perpendicular to the port-starboard direction;

a vertical thruster which is oriented to generate a thrust force substantially perpendicular to the fore-aft and port-starboard directions; and

a moving mass which can be moved in the fore-aft direction to control a pitch of the underwater vehicle;

wherein the vertical thruster is operable to control a pitch of the underwater vehicle; and

wherein the underwater vehicle has a centre of buoyancy and a centre of gravity, and the vertical thruster is positioned so that the thrust force generated by the vertical thruster is offset from the centre of buoyancy and the centre of gravity.

19. The underwater vehicle of claim 18 , wherein the underwater vehicle has a nose and a tail at opposite ends of the underwater vehicle in the fore-aft direction, and the vertical thruster is proximate to one of the nose and the tail.