Patent Publication Number: US-10766574-B2

Title: Automatic-opening fairlead and towing device comprising the fairlead

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International patent application PCT/EP2017/075034, filed on Oct. 3, 2017, which claims priority to foreign French patent application No. FR 1601450, filed on Oct. 6, 2016, the disclosures of which are incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a fairlead that is intended to equip a towing device that can be installed on the deck of a ship and makes it possible to tow an object trailed behind the ship. The towing device conventionally comprises a winch, a cable and a fairlead, the cable passing through the fairlead under the action of the winch. This type of device is employed for example in the field of underwater acoustics and more particular for towed active sonars. These sonars generally comprise a transmission antenna integrated into a submersible object or “towfish” and a receiving antenna made up of a linear antenna or “streamer”. During the use of the sonar being towed, the towfish and the streamer are secured to the same cable in order to be towed by the ship. 
     BACKGROUND 
     It is possible to use the sonar in passive mode, i.e. without its transmission antenna, or in active mode with its transmission antenna formed by the towfish and its receiving antenna. In order to ensure these two operating modes, the towfish is fixed and connected removably to the cable. When the towfish is in place on the cable, it is suspended from the cable such that its center of gravity is situated under the axis of the cable. The towfish comprises a body and one or two arms. The free end of each arm is coupled to the cable from above the cable in order to allow the cable to be guided through the fairlead. 
     The cable generally comprises a core formed of electrical and/or optical conductors for transmitting energy and information between the equipment of the sonar that are situated on board the ship and the antennas. The core of the cable is generally covered with a strand of metal wires that ensure the mechanical integrity of the cable. The make-up of the cable imposes a minimum radius of curvature thereon. Below this radius, inadmissible mechanical stresses arise and cause the constituents of the cable to deteriorate. The winch fixed to the deck of the ship has a reel on which the cable can be hauled in when the sonar is inactive and when the antennas are stowed on board the ship. The diameter of the reel makes it possible to ensure that the hauled-in elements are not curved at a radius smaller than the minimum radius of curvature. 
     When the towed elements are in the sea, the cable is guided by the fairlead, which makes it possible to safeguard its effective radius of curvature. The fairlead forms the last element for guiding the cable with respect to the ship before the cable drops into the water. The fairlead comprises a frame fixed to the deck of the ship and a channel in which the cable slides. The channel has an upwardly open section such that the cable is held in the channel by gravity. When the sea is heavy or while the ship is being maneuvered, the cable can escape from the channel, the fairlead then no longer fulfilling its guiding role. In order to prevent the cable from escaping from the channel, it is desirable to close at least a section of the channel. However, the fact of closing the channel prevents the arms of the towfish from passing through the fairlead. 
     The applicant has attempted to internally produce a fairlead having a closed section that an operator can open manually to allow the arms of the towfish through. The position of the fairlead behind the ship, or partially overhanging the transom of the ship makes the opening and closing operation tricky or even dangerous under difficult navigation conditions. It would be conceivable to remote-control the opening and closing of the fairlead, but this would be complicated to implement. Furthermore, the towing device already requires an operator manipulating the winch. If this operator had to move around in order to open the fairlead, this would entail the risk of the fairlead being left open for an excessively long time. A second operator could manipulate the fairlead, but this would generate a higher operating cost for the towing device. 
     The invention provides a solution to this problem by proposing a fairlead with a closed channel that can open automatically during the passage of the towfish. 
     SUMMARY OF THE INVENTION 
     To this end, the subject of the invention is a fairlead that is intended to equip a towing device that can be installed on the deck of a ship and comprises a winch, a cable passing through the fairlead under the action of the winch, the fairlead comprising:
         an open-section channel extending in a main direction for guiding the cable,   a movable bolt closing a section of the channel,   a force sensor that is situated in front of the bolt in one sense of the main direction and is configured to detect an external force, and   a trigger configured to open the bolt when a force exerted on the sensor and oriented along the main axis in the sense exceeds a predetermined force, and to close the bolt when this force ceases.       

     Advantageously, the force sensor is configured to detect an external force in front of the bolt in both senses of the main direction, and the trigger is configured to open the bolt when a force exerted on the sensor and oriented along the main axis in both senses exceeds the predetermined force, and to close the bolt when this force ceases. 
     According to a first embodiment of the invention, the bolt is rotatable with respect to the channel about an axis of rotation substantially perpendicular to the main direction. 
     Advantageously, according to the first embodiment, the force sensor comprises a tab that is rotatable about the axis of rotation. The trigger comprises a pawl that can take up two positions, of which a first position, referred to as the closed position, is effective when there is no force on the tab and keeps the bolt closed, and of which a second position, referred to as the open position, allows the bolt to rotate freely. The pawl is driven by the tab from the closed position to the open position after the predetermined force has been exceeded, the fairlead also comprising a first spring connected between the channel and the tab, the spring stiffness of the spring contributing to the predetermined force and to the realignment of the bolt with the tab. 
     The first spring may be preloaded, the preload contributing to the predetermined force and to the realignment of the bolt with the tab. 
     Advantageously, according to the first embodiment, the trigger of the fairlead includes a second spring. The second spring being configured such that it tends to close the bolt. Additionally, the second spring being connected in series with the first spring. The bolt is secured at a common point between the first spring and the second spring. 
     The second spring advantageously has a spring stiffness or a spring constant that is less than a spring stiffness or a spring constant of the first spring. 
     The second spring may be preloaded by a value or force that is less than a preloaded value or force of the first spring. 
     According to a second embodiment of the invention, the bolt of the fairlead is movable. In particular, the bolt is movable in translation with respect to the channel along an axis that is substantially perpendicular to the main direction. 
     Advantageously, according to the second embodiment, the force sensor comprises a tab. The tab is configured to be rotatable about an axis of rotation substantially perpendicular to the main direction. Additionally, the force sensor includes a means for converting a rotary movement of the tab into a movement in translation of the bolt. The means for converting can advantageously be configured and structured for irreversible operation. 
     Advantageously, the fairlead of the second embodiment comprises a cam. The cam can be configured and arranged such that it turns with the tab. The fairlead of the second embodiment further comprises a pivoting lever comprising, arranged at a distance from its pivot axis, a pin bearing on the cam and a slot in which the bolt is supported. 
     The fairlead advantageously comprises a return spring that tends to return the cam into a balanced position in which the bolt is closed. 
     A further subject of the invention is a towing device that can be installed on the deck of a ship and comprises a winch, a cable and a fairlead according to the invention, the fairlead and the winch being fixed with respect to one another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be understood better and further advantages will become apparent from reading the detailed description of an embodiment given by way of example, said description being illustrated by the appended drawing, in which: 
         FIG. 1  schematically shows a ship towing an active sonar; 
         FIG. 2  shows more precisely a towing device fixed to the deck of the ship; 
         FIG. 3  shows a fairlead through which a towfish is passing; 
         FIG. 4  shows a perspective view of a first embodiment of a mechanism for automatically opening the fairlead; 
         FIGS. 5 a , 5 b  and 5 c    show the fairlead in profile in different positions of the automatic opening mechanism from  FIG. 4 ; 
         FIG. 6  shows the automatic opening mechanism from  FIG. 4  in more detail; 
         FIGS. 7, 8, 9 and 9   a  show the automatic opening mechanism from  FIG. 4  in cross section; 
         FIG. 10  shows the variation in force on a tab of the mechanism from  FIG. 4  as a function of the travel of the tab; 
         FIG. 11  shows a kinematic diagram of the first embodiment; 
         FIG. 12  shows a perspective view of a second embodiment of a mechanism for automatically opening the fairlead; 
         FIG. 13  shows a side view of the automatic opening mechanism of the second embodiment. 
     
    
    
     For the sake of clarity, the same elements will bear the same references in the different figures. 
     DETAILED DESCRIPTION 
     The invention is described with reference to the towing of a sonar by a surface vessel. It will of course be understood that the invention can be implemented for other towed elements. 
       FIG. 1  shows a ship  10  towing an active sonar  11  comprising an acoustic transmission antenna  12 , often referred to as a towfish, and an acoustic receiving antenna  13 , often referred to as a streamer. The sonar  11  also comprises a cable  14  for towing the two antennas  12  and  13 . The cable  14  also carries signals and power between the ship  10  and the antennas  12  and  13  of the sonar  11 . 
     The antennas  12  and  13  are mechanically anchored and connected electrically and/or optically to the cable  14  in an appropriate manner. Conventionally, the receiving antenna  13  is formed by a tubular linear antenna identical to those found in passive sonars, hence its name of streamer, while the transmission antenna  12  is incorporated into a voluminous structure having a shape similar to that of a fish. The receiving streamer is generally disposed at the rear, at the end of the cable  14 , the towfish being positioned on the part of the cable  14  closest to the ship  10 . During an underwater acoustic mission, the antenna  12  transmits sound waves through the water and the receiving antenna  13  picks up any echoes coming from targets at which the sound waves output by the antenna  12  are reflected. 
     The receiving antenna  13  is generally anchored permanently to the cable  14  while, for its part, the towfish  12  is anchored in a removable manner. To this end, the cable  14  has an anchoring zone  15  for the towfish  12 , in which zone means for mechanically fixing the towfish  12  and for electrically and/or optically connecting it to the cable  14  are installed. 
     The launching and retrieval of the antennas  12  and  13  are realized by means of a winch  16  disposed on a deck  17  of the ship  10 . The winch  16  comprises a reel  18  dimensioned to allow the cable  14  and the receiving antenna  13  to be hauled in. The winch  16  also comprises a stand. The reel  18  turns with respect to the stand in order to haul in the cable. The hauling in of the cable  14  makes it possible to winch the towfish  12  on board the ship  10 , for example onto an aft platform  19  provided for this purpose. 
     A fairlead  20  guides the cable  14  downstream of the reel  18 . The fairlead  20  forms the last guiding element for the cable  14  before it drops into the water. During towing, the inclination of the cable  14  can vary with respect to the longitudinal axis of the ship  10 . The variations in inclination are caused in particular by changes in the heading and speed of the ship and also by the state of the sea. One of the functions of the fairlead  20  is to ensure that the respective radii of curvature of the cable  14  and of the linear antenna do not exceed a predefined lower limit. The cable  14  comprises for example a core formed of electrical and/or optical conductors for transmitting energy and information between the sonar equipment situated on board the ship  10  and the antennas  12  and  13 . The core of the cable  14  is generally covered with a strand of metal wires that ensure the mechanical integrity of the cable  14  notably the tensile strength thereof. Below the lower limit of curvature, there is a risk of permanent deformations or breakage of constituents of the cable  14 . The same goes for the linear antenna. 
       FIG. 2  shows in more detail a side view (from the starboard side) of the elements of the towing device. The fairlead  20  comprises a frame  21  intended to be fixed to a deck  19  of the ship, on the sea side with respect to the winch  16 . The deck  19  is in this case an aft platform of the ship  10 . In other words, the fairlead  20  is fixed towards the rear of the ship  10  with respect to the winch  16 . In the embodiment in the figures, the fairlead  20  and the winch are not fixed to the same deck but could, alternatively, be disposed on the same deck. A reeling device  22  for correctly stowing the cable  14  on the reel  18  is interposed between the winch  16  and the fairlead  20 . The cable  14  is in this case guided by the reeling device  22  between the fairlead  20  and the winch  16 . Alternatively, the frame  21  is secured to a reeling system  22 . In other words, the frame  21  is fixed to a reeling device intended to effect movements in translation parallel to the axis of rotation of the reel  18  in order to correctly stow the cable  14  on the reel  18 . When the frame  21  is fixed to the reeling system  22 , it is the entire fairlead  20  which effects the movements in translation parallel to the axis of the reel  18  in order to correctly stow the cable  14  on the reel  18 . 
     On the sea side, the cable  14  can oscillate depending on the state of the sea or more simply if the heading of the ship changes. To this end, the fairlead  20  can comprise a plurality of mutually articulated sectors, each for guiding the cable  14 . Such a fairlead is described for example in the patent application WO 2015/014886 A1 filed in the name of the applicant. In that document, the axis of the articulation of the sectors intersects the main axis along which the cable extends. It is possible to dispose the axis of rotation of the articulation of the sectors differently, as described for example in the document WO 2013/068497 A1, likewise filed in the name of the applicant. It is of course possible to implement the invention in a fairlead that comprises only a single sector that is fixed to the frame  21  or is rotatable with respect thereto. 
       FIG. 3  shows the fairlead  20  through which the towfish  12  is passing. The towfish  12  comprises two arms  12   a  and  12   b  for coupling to the cable  14 . 
     The fairlead  20  comprises a first sector  23  that is fixed with respect to the frame  21 , and a second sector  24  referred to as pivoting sector, both of which guide the cable  14 . Each of the sectors  23 ,  24  comprises a channel or groove,  25  for the sector  23 ,  26  for the sector  24 . The cable  14  slides in the channels  25  and  26 , which are substantially in line with one another so as to be able to guide the cable  14  along the entire length of the fairlead  20 . Each of the channels  25  and  26  allows the cable  14  to be curved. The channels  25  and  26  are dimensioned and arranged so as to limit the maximum curvature of the cable  14  to a predetermined curvature. The sectors  23  and  24  are mutually articulated. The sector  24  can pivot about an axis  28  with respect to the sector  23 . The minimum radius of curvature is maintained during the rotary movements of the sector  24  with respect to the sector  23 . 
     The sectors  23  and  24  have sections in the shape of the letter C making it possible to guide the cable in the bottom part of the C and more specifically in the channels  25  and  26 . The opening of the C allows the arms  12   a  and  12   b  of the towfish  12  to pass through. In order to prevent any escape of the cable  14  from the fairlead  20  during unintentional movements of the cable  14 , the open side of the fairlead  20  comprises at least one closed section. According to the invention, this closed section opens and closes automatically during the passage of the arms  12   a  and  12   b.    
       FIG. 4  shows a perspective view of a first embodiment of a mechanism for automatically opening the fairlead  20 . The two channels  23  and  24  extend in a main direction  27  which the cable  14  follows. In the example shown, the direction  27  is curved. Its curvature is defined to limit that of the cable  14 . In the scope of the invention, this direction may also be straight. A section of the fairlead  20  is defined in a plane perpendicular to the direction  27 . 
     The fairlead  20  comprises:
         a movable bolt  30  closing a section of the sector  24 ,   a force sensor  32  that is situated in front of the bolt  30  in one sense  34  of the main direction and is configured to detect an external force, and   a trigger  36  configured to open the bolt  30  when a force exerted on the sensor  32  and oriented along the main axis in the sense  34  exceeds a predetermined force, and to close the bolt  30  when this force ceases.       

     In  FIG. 4 , the sense  34  corresponds to the raising of the towfish  12  toward the winch  16 . The predetermined force corresponds to that exerted by the arms  12   a  and  12   b  when they come into contact with the force sensor  32 . Advantageously, the force sensor  32  can likewise detect a force in the sense opposite to the sense  34  and the trigger likewise opens the bolt  30  when the force detected by the force sensor  32  in the opposite direction exceeds the predetermined force, and closes the bolt  30  when this force ceases. Thus, the bolt  30  opens and closes when the towfish  12 , and more specifically each of the arms of the towfish  12 , passes through the fairlead  20 , both when it is raised toward the winch  16  and when it is lowered into the water. 
     When the fairlead comprises several sectors  23  and  24 , as in the example shown, the fairlead  20  may advantageously comprise its own automatic opening mechanism associated with each sector. The automatic opening mechanisms of each of the sectors  23  and  24  can function simultaneously. The triggering of the opening then takes place with the aid of a force sensor shared by the different mechanisms. Alternatively, the different mechanisms function independently of one another, each having its own force sensor. This independence makes it possible to reduce the opening time of the different bolts as far as possible in order to best safeguard the cable  14  inside the fairlead  20 . 
       FIGS. 5 a , 5 b  and 5 c    show the fairlead  20  in profile in different positions of the automatic opening mechanisms associated with each sector  23  and  24 . In these figures, the bolt  30  and the force sensor  32  of the sector  24  and also a bolt  40  and a force sensor  42  that are associated with the sector  23  can be seen. In  FIG. 5 a   , the bolts  30  and  40  are closed, in  FIG. 5 b   , the bolts  30  and  40  are open so as to allow the towfish  12  to pass through in the direction of the sea, and in  FIG. 5 c   , the bolts  30  and  40  are open so as to allow the towfish  12  to pass through in the direction of the winch  16 . In the variant shown, the bolts are rotatable about an axis,  31  for the bolt  30  and about an axis  41  for the bolt  40 . 
       FIG. 6  shows the sector  24  and the automatic opening mechanism thereof in more detail. The force sensor  32  comprises a tab  33  that is rotatable about the axis of rotation  31 . The force sensor  32  makes it possible to detect a force in front of the bolt  30  in the direction of movement in question for the cable  14 . In other words, when one of the arms  12   a  or  12   b  approaches the automatic opening mechanism, contact is made with the tab  33  which is situated in front of the bolt  30 . Thus, there is no contact with the bolt  30  itself. This is because such contact could hamper the opening thereof and lead to deterioration of the bolt  30  and of the arm  12   a  or  12   b . In  FIG. 6 , the movement of the tab  33  in the two possible senses of movement of the cable  14  can be seen. The movement is for example through an angle of around ten degrees: movement  45  when the towfish  12  passes through the fairlead  20  in the direction of the sea and movement  46  when the towfish  12  passes through the fairlead  20  in the direction of the winch  16 . Other forms of tab  33  are likewise possible and the movement can be defined linearly. The automatic opening mechanism of the sector  23  is realized in a similar manner to the mechanism of the sector  24  with its movements in the two senses of circulation of the cable  14  in the fairlead  20 . 
       FIG. 7  shows the automatic opening mechanism in cross section through the axis  31 . 
     The trigger  36  is situated inside a shell  50  secured to the tab  33 . The trigger  36  mainly comprises a pawl  52  that can take up three positions: a position in which the bolt  30  is closed, as shown in  FIG. 5 a   , and two open positions in which the bolt  30  is open, as shown in  FIGS. 5 b  and 5 c   . The closed position is effective when there is no force on the tab  33 , and the open positions are achieved when a force greater than a predetermined force in one of the two senses of the main axis  27  is exerted on the tab  33 . The pawl  52  may have only one open position if a pressing force on the tab  33  is detected only in one sense. 
     The pawl  52  is likewise visible in the cross sections HH and DD shown in  FIGS. 8 and 9 . The section planes HH and DD are perpendicular to the axis  31  and their position is identified in  FIG. 7 . 
     The automatic opening mechanism has a shaft  54  extending along the axis  31 . The shaft is fixed to the bolt  30  (not shown in  FIG. 7 ). One of the ends  56  of the shaft  54  may be grooved to ensure the positioning of the mechanism with the bolt  30 . The mechanism and the bolt  30  can be held in position by means of a thread  58 . Any other means for positioning and keeping in position is of course possible. The mechanism comprises a frame  60  fixed to the sector  24 . The bolt  30  and the tab  33  are rotatable about the axis  31  with respect to the frame  60  and thus with respect to the sector  24 . 
     The pawl  52  comprises two fingers  61  and  62  that are rotatable with respect to the frame  60  about an axis  64 . The fingers each have a hook:  65  for the finger  61  and  66  for the finger  62 . When the pawl  52  is in the closed position as shown in  FIG. 8 , the hooks  65  and  66  come into abutment against the shaft  54 . Thus, the bolt  30  is immobilized with respect to the frame  60  and cannot move with respect to the sector  24 . The pawl  52  comprises a spring  68  that keeps the two fingers  61  and  62  in abutment with the shaft  54 . The hooks  65  and  66  can come into direct abutment with the shaft  54  or advantageously with a cam  67  joined to the shaft by screws  69  forming mechanical weak links. In normal operation, the cam  67  and the shaft  54  are secured to one another. If the automatic opening mechanism fails, the screws  69  can break and release the cam  67 , which can then turn with respect to the shaft  54 . Failure may be for example due to the fingers  65  and  66  seizing against the cam  67 , preventing the bolt  30  from opening, even if the force on the force sensor exceeds the predetermined threshold for opening. 
     During a rotary movement of the tab  33 , a pin  70 , secured to the tab  33 , makes it possible to open the pawl  52  by moving away one of the fingers  61  and  62 . In practice, the pin  70  is fixed to the shell  50 , which is itself fixed to the tab  33 . 
     Moreover, a first spring  72  opposes the rotation of the tab  33  with respect to the bolt  30 , which is fixed in the closed position of the pawl  52 . Moreover, in addition to the spring  72 , the internal shape of the fingers  61  and  62  against which the pin  70  presses and the shape of the hooks  65  and  66  are configured to define the force above which the pawl  52  opens in order to release the bolt  30 .  FIG. 9 a    is an enlarged part of  FIG. 9  in which it is possible to see the shape of the fingers  61  and  62  in the vicinity of the point of equilibrium in which no force is exerted on the tab  33 . When pressure is applied to the tab  33 , the pin  70  moves, pushing for example the finger  62 . At the start of travel, the internal shape of the finger is substantially flat so as not to bring about any movement of the finger  62 . This flat zone bears the reference  76 . Next, as its travel continues, the pin  70  reaches an inclined shoulder  78 , forcing the finger  62  to move away from the shaft  54 . The hook  66  is released from its abutment. It is during the passage of this shoulder that the bolt  30  is released. While continuing its travel, the pin  70  reaches a substantially circular zone  80  about the shaft  54 . In this zone, the hook  66  is kept at a distance from its abutment. The internal shapes of the other finger  61  are for example symmetric. Asymmetric forms are possible, in particular to offset the movement in one sense with respect to the other or to obtain different forces to be applied by one of the arms  12   a  or  12   b  of the towfish  12  in one sense and in the other. A difference in force can be useful since, when lowering toward the sea, only the drag force of the streamer  30  drives the towfish, whereas, while the towfish is being raised, the winch  16  can exert a greater force. Moreover, while the towfish  12  is being raised, the fairlead  20  and thus the tab  33  are likely to ship seawater. Consequently, it is useful to differentiate the predetermined force values to be exerted on the tab  33  to open the bolt  30  on raising the towfish  12 , corresponding to the sense  34 , and on lowering the towfish, corresponding to the opposite sense. The predetermined force value for the sense  34  is thus advantageously greater than the predetermined force value for the opposite sense. 
     It is likewise possible to differentiate the force necessary for opening the pawl  52  in the two senses of rotation by doubling the spring  72 , one acting in one sense and the other acting in the other sense. For each of the two springs, it is possible to choose different spring stiffnesses and different preloads. 
     Once the pawl  52  is open, the shapes thereof no longer prevent the rotation of the bolt  30 . The spring  72  then applies a return action on the bolt  30  in order to realign the bolt  30  with the tab  33  and thus to prevent contact between the arm  12   a  or  12   b  and the bolt  30 . 
     Advantageously, the mechanism comprises a second spring  74 , which is connected between the frame  60  and the first spring  72  and tends to close the bolt  30 , which is secured at the common point between the two springs  72  and  74 . By choosing a spring stiffness of the second spring  74  that is less than that of the first spring  72 , it is possible to limit the force necessary to completely open the mechanism and to maintain a high triggering force of the pawl  52  and thus to maintain the minimum force to be exceeded in order to trigger the opening of the bolt  30 . Disposing the two springs  72  and  74  in series between the frame  60  and the tab  33  with the bolt  30  fixed at the common point of the two springs  72  and  74  makes it possible to maintain an angular offset between the tab  33  and the bolt  30  and thus to avoid any contact between the arm  12   a  or  12   b  and the bolt  30 . 
     The two springs  72  and  74  are preloaded so as to allow the return toward the closed position when the pressure on the tab  33  stops. It is possible to regulate the preload and the spring stiffness of the spring  74  to a value lower than that of the spring  72  in order to further reduce the force necessary to reach the open position of the bolt  30 . 
     Alternatively, it is possible to use only one spring that applies a return force to the bolt  30  with respect to the frame  60  and a return force to the tab  33  with respect to the frame  60 . However, the use of one spring (per sense) has the drawback of leaving the tab  30  free during the opening of the pawl  52  and it is one of the arms of the towfish that pushes against the bolt  30  after the pawl  52  has been unlocked. Moreover, this variant, for one and the same predetermined force for triggering the opening of the bolt  30 , results in a force necessary for complete opening, shown in  FIG. 5 b    or  5   c , that is greater than the triggering force in the variant with two springs (per sense), and a larger size of the spring in order to accept the opening amplitude. 
     The spring  74  is preloaded between two flanges  82  and  84  that are free to rotate with respect to the frame  60 , in each case in an angular sector giving the possible angular travel for the bolt  30  in one of the senses of rotation. The balanced position is visible in  FIG. 9 , where the flange  82  is in abutment against a key  86  fixed to the frame  60 . The flange  82  comprises a free angular sector  88  allowing it to turn with respect to the frame  60  during the rotation of the bolt  30  in one of the senses of rotation. In the example shown, the maximum rotation of the bolt  30  is 110°. A maximum rotation value of around 90° or slightly greater allows the bolt  30  to be retracted sufficiently during the passage of the arms of the towfish  12 . The flange  84  comprises a similar angular sector allowing the rotation of the bolt  30  in the other sense of rotation. The free angular sectors of the flanges  82  and  84  may be different depending on the maximum travels desired for the bolt  30  in the two senses of rotation. 
     Just like the spring  72 , it is possible to double the spring  74  in order to differentiate the spring stiffness and the preload in the two senses of circulation of the cable  14  in the fairlead  20 . 
       FIG. 10  shows, in the form of a curve, the force applied to the tab  33  as a function of the movement thereof in one of the senses of the main direction  27 . In practice, the springs  72  and  74  are torsion springs in the variant shown, and the force is given in the form of a torque denoted C. In addition, with the tab  33  moving in rotation, the movement thereof is expressed as an angle denoted a. A functional clearance α 1  of for example around 1° is provided between the cam  67  and the pawl  52 , more specifically between the fingers  65  and  66  and the cam  67 . This clearance makes it possible to ensure that the pawl  52  returns into the closed position and thus that the bolt  30  returns into the closed position. A torque C 1  represents the preload of the spring  74 . At the start of the movement of the tab  33  on account of a pressure in one of the two senses of the main direction  26 , the functional clearance α 1  is taken up by a tension of the spring  74 . Once this clearance has been taken up, the pawl  52  bears against the cam  67  and the torque necessary for rotation of the tab  33  is the preload torque C 2  of the spring  72 , which is greater than the torque C 1 , hence the vertical part of the curve between the torques C 1  and C 2  for the angular position α 1 . Beyond the position α 1 , the pin  70  travels through the flat zone  76  and the spring  72  is tensioned from a preload C 2  until reaching a position α 2  of for example around 2.5°. In this position, the pin  70  comes into contact with the shoulder  78 . The gradient of the curve between the positions α 1  and α 2  is substantially given by the spring stiffness of the spring  72 . Next, the pin  70  moves over the shoulder  78  and the curve becomes substantially vertical so as to achieve the predetermined triggering force C 5  to be exceeded in order to release the rotation of the bolt  30  and thus allow it to open. The force C 5  is achieved for example for an angular position α 3  of 3°, which is less than the movement of the tab  33  with respect to the bolt  30 . This movement is depicted in  FIG. 10  by an angular position α 4  of for example around 10°. Thus, the bolt  30  opens before the object (in this case the towfish) that has triggered its opening reaches it. 
     Between the balanced position where α=0° and the position α 3 , the tab  33  moves angularly without the bolt  30  turning. When the bolt  30  is released, the latter is realigned with the tab  33 . In other words, beyond the position α 3 , the tab  33  returns to the advanced position that it had on the bolt  30  in the rest position for α=0° in order to prevent any contact between the arm  12   a  or  12   b  and the bolt  30 . The spring stiffness of the spring  72  contributes to the realignment of the bolt  30  and the tab  33 . 
     Following the opening of the bolt  30 , the curve in  FIG. 10  returns to a lower value and follows a moderate gradient given by the spring stiffness of the second spring  74 . The descent of the curve is due to the transition between the zones  78  and  80  of the finger  62  and to the releasing of the hook  66  which was rubbing against its abutment with the shaft  54 . The preload C 1  of the second spring  74  is, in the example shown, less than the preload C 2  of the first spring  72 . Alternatively, it is possible for the first spring  72  not to be preloaded provided that its spring stiffness is high enough for its return torque to exceed the preload torque C 1  of the second spring  74  for the angular position α 2 . 
     Beyond the position α 3 , the rotation of the tab  33  continues as far as the position α 5 , for example around 110°, in which position the return torque C 3  is substantially a function of the spring stiffness of the second spring  74 . 
     The variant with one spring (per sense) is also depicted by dashed lines in  FIG. 10 . Starting from a preload C 4 , the single spring is tensioned until it reaches a torque C 6  for the position α 5 . The torque C 6  arises from the spring stiffness of the single spring and from the minimum torque C 5  desired for the torque upon the opening of the mechanism at the position α 3 . The variant with one spring results in a value C 6  that is much greater than the value C 3  if the spring stiffness of the spring is high. It is possible to choose, for this single spring, a lower spring stiffness (gradient less pronounced for the dashed curve), but this requires a very large increase in size. 
     The curve is substantially symmetric with respect to the y-axis give or take the adaptations described above, the maximum torque value C 5  and angular amplitude which can be adjusted differently in the two senses of rotation. Thus, the bolt  30  tends to return to its closed balanced position regardless of its sense of rotation. 
     Returning to the variant with two springs  72  and  74 , when the force on the spring  33  ceases, the tab  33  and the bolt  30  close, following a direct curve from the point on the curve (α 5 , C 3 ) to the point (0, C 1 ) and then (0,0). The preload C 1  of the second spring  74  ensures the closure of the bolt  30  and the return of the pin  72  to its balanced position. 
     The return of the tab  33  with respect to the shaft  54  takes place in a similar manner to that of the bolt  30  with respect to the frame  60 . The spring  72  is preloaded between two flanges  90  and  92  that are rotatable with respect to the shaft  54 . The flange  90  is coupled to the shaft  54  via a key and the flange  92  is coupled to the shell  50  and thus to the tab  33  via a pin. The angular travel of the flange  90  is around 10° with respect to the shaft  54 , and corresponds to the movement  45  and  46  of the tab  33  with respect to the bolt  30 ; it can be ensured, as above, by means of a key fixed to the shaft  54  and a free angular sector realized in the flange  90 . 
       FIG. 11  shows a kinematic diagram of the first embodiment. This diagram shows several variants with respect to the depictions in cross section in  FIGS. 7 to 9 . More specifically, in  FIGS. 7 and 8 , a spring  68  that tends to return the two fingers  61  and  62  into abutment against the shaft  54  via hooks  65  and  66  can be seen. In the kinematic diagram in  FIG. 11 , the spring  68  has been replaced by two springs  68 . 1  and  68 . 2 . The spring  68 . 1  is disposed between the finger  61  and the frame  60 . The spring  68 . 1  tends to return the finger  61  into abutment with the shaft  54 . Similarly, the spring  68 . 2  is disposed between the finger  62  and the frame  60 . The spring  68 . 2  tends to return the finger  62  into abutment with the shaft  54 . This doubling of the spring  68  makes it possible to differentiate the force necessary for opening the pawl  32  in the two senses. 
     The pin  70  secured to the shell  50  and the tab  33  appears in the diagram in  FIG. 11 . The contact that the pin  70  can exert on one of the fingers  61  or  62  is shown in the form of a rectilinear link. A punctiform link is likewise conceivable. It is clear that the pin  70  exerts only one contact at a time, either on the finger  61  or on the finger  62 . Consequently, only one of the rectilinear links is effective at a time, the other being absent. 
     In the kinematic diagram in  FIG. 11 , the springs  72  and  74  have likewise been doubled as mentioned above. For one of the senses, the function ensured by the spring  72  is ensured by the spring  72 . 1  held between the two flanges  90 . 1  and  92 . 1 . For the other sense, the function ensured by the spring  72  is ensured by the spring  72 . 2  held between the two flanges  90 . 2  and  92 . 2 . 
     Similarly, for one of the senses, the function ensured by the spring  74  is ensured by the spring  74 . 1  held between the two flanges  82 . 1  and  84 . 1 . For the other sense, the function ensured by the spring  74  is ensured by the spring  74 . 2  held between the two flanges  82 . 2  and  84 . 2 . The key  86  secured to the frame  60  is likewise doubled and shown in  FIG. 11 . The flange  82 . 1  bears against the key  86 . 1 . The flange  82 . 2  bears against the key  86 . 2 . These bearings are shown schematically in the form of rectilinear links that it is possible to lose when the corresponding flange turns with respect to the shaft  54 , as for example in the free angular sector  88  for the flange  82 , as visible in  FIG. 9 . Simple punctiform links can likewise replace the different rectilinear links. 
       FIG. 12  shows a perspective view of a second embodiment of a mechanism for automatically opening the fairlead  20 . The two sectors  23  and  24  are apparent. It is clear that this second embodiment can be implemented in a fairlead with one sector. 
     In the second embodiment, the tab  33  for detecting a force is apparent. A bolt  100  which, unlike the first embodiment, opens and closes in a movement in translation along an axis  102 , is apparent. The bolt is guided in translation with respect to the sector  24  along the axis  102 . 
       FIG. 13  shows a side view of the automatic opening mechanism of the second embodiment. The tab  33 , as before, is rotatable about the axis  31  with respect to the sector  24 . As before, the force sensor  32  detects a force in front of the bolt  30  in the sense of movement in question for the cable  14 . This movement is clear in  FIG. 12 , where the tab  33  protrudes from the bolt  100  in at least one of the senses of the main direction  27  followed by the cable  14  in the sector  24 . In the example shown, the tab  33  protrudes from the bolt  100  in both senses. The external shape of the tab  33  against which the arms of the towfish  12  are intended to press can define the movement with respect to the bolt  100 . 
     A pinion  104  is secured to the tab  33 . The pinion  104  turns about the axis  31 . A second pinion  106  is rotatable with respect to the sector  24 . The axis of rotation  108  of the pinion  106  is different from the axis of rotation  31  of the pinion  104 . The pinion  106  is driven by the pinion  104  via a belt  110 . The tab  33 , the pinions  104  and  106  and the belt fulfill the function of the force sensor  32 . 
     A cam  112  is secured to the pinion  106 . An arm  114  can pivot at one of its ends  116  with respect to the sector  24  about an axis  118  different from the axes of rotation  31  and  108  of the two pinions  104  and  106 . The arm  114  comprises a roller  120  that forms a cam follower and presses on the cam  112 . The bolt  100  comprises a pin  122  that can slide in a slot  124  made in the arm  114  at its second end  126 . The cam  112 , the arm  114  and the roller  120  fulfill the function of the trigger  36 . 
     The arm  114  forms a lever for moving the bolt  100  in translation along its axis  102 . The shape of the cam  112  is defined to coordinate the movement in translation of the bolt  100  depending on the angular movement of the tab  33 . The distance ratio between the pin  122  and the axis of rotation  118 , for the one part, and the roller  120  and the axis of rotation  118 , for the other part, makes it possible to amplify the movement in translation of the bolt  100  with respect to the rotation of the tab  33 . This amplification can be modified by the ratio of the diameters of the pinions  104  and  106 . In the example in question, the pinions  104  and  106  and the arm  114  amplify the movement in translation of the bolt  100 . A reduction is likewise conceivable. 
     Any other means for converting  200  the rotary movement of the tab  33  into a movement in translation of the bolt  100  is possible within the scope of the invention, for example a system of the rod-crank type. 
     In order to avoid a situation in which the roller  120  loses contact with the cam  112 , the latter advantageously comprises a groove  130  in which the roller  120  moves. The roller  120  thus remains in contact with the two flanks of the groove  130 . 
     The profile of the cam  112  against which the roller  120  bears is advantageously defined such that the mechanism is irreversible, i.e. a force on the bolt  100  cannot open it. This makes it possible to prevent friction of the cable on the bolt  100  being able to raise it. Thus, only a force on the tab  33  that tends to pivot it about its axis  31  makes it possible to open the bolt  100 . 
     The profile of the cam  112  is symmetric with respect to the point of equilibrium shown in  FIG. 13 . This point of equilibrium corresponds to the bottom position of the bolt  100 , in which it closes the sector  24 . The symmetric shape of the cam  112  allows identical movements of the bolt  100  depending on the rotation of the tab  33  in the two senses of the direction  27 . It is possible to provide different shapes for each of the two senses depending on the desired movements for the bolt  100 . 
     The mechanism comprises a return spring  132  that tends to keep the bolt  100  in the closed position. A preload of the spring  132  makes it possible to define the minimum force to be exerted on the tab  33  in order to open the bolt  100 . The spring  132  can be directly fixed between the sector  24  and the bolt  100 . This disposition of the spring  132  only functions if the mechanism is reversible. In the case of an irreversible mechanism, the spring  132  can be directly fixed between the sector  24  and the cam  112  in order to exert a torque on the cam  112 , this torque tending to keep the roller  120  at the balanced position. In the example shown, in order to accentuate the effect of the spring  132 , the mechanism comprises a crown wheel  134  that is rotatable with respect to the sector  24  and a pinion  136  secured to the cam  112 . The crown wheel  134  and the pinion  136  roll without sliding on one another. To this end, the crown wheel  134  and the pinion  136  comprise for example cooperating gear teeth. With respect to the plane of  FIG. 13 , the pinion  136  is situated behind the cam  112 , while the pinion  106  is situated in front of the cam  112 . The spring  132  is fixed between the sector  24  and the crown wheel  134 . The diameter ratio between the pinion  136  and the crown wheel  134  amplifies the return force of the spring  132 .