referring now to fig1 a string of salter ducks 11 are shown on a common spine 10 in the sea and aligned broadside to the waves . each salter duck 11 as shown in fig2 comprises a pivotable member 12 shaped to define an enlarged hollow beak portion 13 which is pivotable by means of bearings 14 about a portion 15 of the spine 10 and is sealed from the marine environment . the rear portion 19 of the salter duck 11 is circular so as to minimise energy transfer between the rear portion 19 and the waves . four gyroscope assemblies arranged in two pairs of gyroscope assemblies 20a , 20b , are mounted in the beak portion 13 , each assembly 20a , 20b being pivotable about an axis substantially perpendicular to the direction of the spine portion 15 , and a hydraulic motor 3 - phase electric alternator assembly 21 is mounted midway between the pairs of gyroscope assemblies 20a , 20b . as shown in more detail in fig3 each gyroscope assembly 20a and 20b , comprises a disc - like rotor 25 drivably connected at each side thereof to an auto - variable swashplate - type hydraulic motor 26 . the rotor 25 is rotatably held in a mounting 27 which is pivotably mounted in the body of the salter duck 11 at 28 , 29 along an axis perpendicular to the spin axis of the rotor 25 . cam shaped discs 32 of respective ring cam pumps 33a , 33b are rigidly supported one above the other from the body of the salter duck 11 and engage on each side thereof respective rollers 39 each mounted as shown in fig4 on a link 42 pivoted at 44 and pivotally connected to a rod 45 extending from a piston 46 moving in a cylinder 47 which discharges into ring - shaped manifolds 48a , 48b in the mounting 27 . as shown in fig4 the manifolds 48a , 48b , of one of the gyroscope assemblies 20a or 20b discharge into high pressure mains 37 , 38 respectively connected in parallel to a swashplate type hydraulic motor 40 of the hydraulic motor / alternator assembly 21 , the motor 40 being drivably connected to one side of a 3 - phase electric alternator 41 of the assembly 21 . the other manifolds 48a , 48b , of the other pair of gyroscope assemblies 20b or 20a are similarly connected to high pressure mains ( not shown ) connected in parallel to a swashplate - type hydraulic motor 43 of the assembly 21 and drivably connected to the other side of the alternator 41 . the hydraulic motors 26 are connected in parallel across the high pressure mains 37 , 38 , and low pressure mains 49 , 50 form the return path of the system of fig4 . in operation , the rotors 25 are set spinning with the rotors 25 of a pair of gyroscope assemblies 20a , 20b spinning in opposite directions , and when the beak portion 13 of the salter duck 11 pivots about the spine portion 15 in response to waves , the gyroscope assemblies 20a , 20b precess in opposite directions about the pivotal mountings 28 , 29 , and cause the pistons 46 of the ring cam pumps 33a , 33b , to be operated by the lobes of the cam - shaped discs 32 engaging the rollers 39 . the high pressure mains 37 , 38 are thus energised by the ring cam pumps 33a , 33b and drive the hydraulic motors 40 , 43 , which thus drive the alternator 41 . if the pressure in the high pressure mains 37 , 38 rises above a mean value as a result of a burst of wave energy , the angular deflection of the swash plates on the hydraulic motors 26 increases so as to allow the extra energy to speed up the rotors 25 . if the pressure in the high pressure mains 37 , 38 is at a mean value , the swash plates in the hydraulic motors 26 move to their zero displacement angle and all the oil in the high pressure mains 37 , 38 flows to the hydraulic motors 40 . if during a lull in the wave pattern on the surface there is less oil from the pumps 33a , 33b than is necessary to maintain a required pressure in the high pressure mains 37 , 38 , the swash plates in the hydraulic motors 26 open and draw the energy deficit from the rotors 25 . as the gyroscope assemblies 20a , 20b of a pair precess in opposite directions , the output torques thereof are in opposite directions and are absorbed in the beak portion 13 without having any significant effect on the spine portion 15 of the salter duck 11 . the lobes on the cam - shaped discs 32 are shown as being on the sides normal to the respective pump 33a , 33b axis , but the lobes may be defined along circumferential surfaces of the cam - shaped discs 32 . ring cam pumps may be obtained from messrs mactaggart - scott , poclain , and from haggelund . each cylinder 47 may have an electronically controlled poppet valve ( not shown ) so that the decision about which poppet valves are to open may be made by a computer to which information is fed such as the duck &# 39 ; s pivotal angle , angular velocity , pressure field , recent history of power levels , rotor speed and precession angle of the gyroscope assembly , condition of the power take - off system , and the deflections of the spine of the string of ducks . the poppet valves may also be used to isolate a cylinder having a defective piston or roller which has been detected by an acoustic transducer . although the invention has been described in relation to a salter duck , it may be incorporated in other apparatus , for example of the cockerell raft , and the shrivenham triplate . referring to fig5 and 6 , a representation of a cockerell raft 55 is shown having a leading pontoon 56 , and a rear pontoon 57 hingedly connected at 58 . two arrangements of gyroscope assemblies 59 and a power take - off system similar to those described in relation to fig1 to 4 are installed in the leading pontoon 55 so that when the leading pontoon 55 pivots about the hinge connection 58 , electric power is produced in a similar manner to that produced by the angular pivotal motion of the beak portion 13 of the salter duck 11 of fig1 to 4 . in the representation of the shrivenham triplate of fig7 a front plate 60 pivoted at 61 is positioned in front of two fixed plates 62 , 63 respectively which are braced by diagonal ties 64 . the plates 60 , 62 , 63 are mounted on a buoyant member 65 which is arranged to present the plates 60 , 62 , 63 just above the mean wave height and parallel to each other normal to the incident wave direction . the fixed plates 62 , 63 are spaced half a wavelength at the optimum conditions for which the triplate is designed , and the front plate 60 and the fixed plate 62 are spaced apart by a quarter of this wavelength . an arrangement of gyroscope assemblies 66 and a power take - off system similar to those described in relation to fig1 to 4 are mounted on the front plate 60 . in operation the resonant wave between the fixed plates 62 , 63 , forms a reference frame , and the incoming wave is fully reflected by the fixed plate 62 so that the front plate 60 is at the point of maximum horizontal motion . thus the front plate 60 pivots about 61 and electric power is produced from this angular motion in a similar manner to that produced from the angular pivotal motion of the beak portion 13 of the salter duck 11 of fig1 to 4 . it will be understood that the invention may be incorporated in other apparatus for extracting energy from waves from the angular motion of a member in response to the waves . it will also be appreciated that although a ring cam pump is preferred , other suitable hydraulic pumps might be used , or alternative work performing means arranged to be energised from the precession of the gyroscopes . if desired , a single or an uneven number of gyroscopes might be used although the balanced output of an even number of gyroscopes is preferred . in order to minimise losses by windage it is desirable for the rotors of the gyroscope assemblies to rotate in a reduced pressure , partial vacuum environment .