HITCHING A FISH UP TO A TOWED SONAR
The invention relates to an active towed sonar comprising an
emission antenna integrated into a submersible object called a towfish, and
to a, ·for example linear, submersible receive antenna called a streamer.
When the sonar is being used in dependent towing, the towfish and the
5 streamer are connected in succession to one and the same tow cable. Figure
1 illustrates a known configuration of an active sonar in dependent towing.
The deck of the ship 10 is equipped with a towing device comprising a
motorized winch 11 capable, via a fairlead 15, of towing a cable 12 (often
called heavy cable} to which there are connected, on the one hand, a towfish
10 13 and, on the other hand, a streamer 14. A light tow cable 16 may be
interposed between the towfish and the streamer 14.
Launching this assembly into the water is a tricky operation. In
general, the streamer 14 and the light cable 16 are first of all launched into
the water using an ancillary winch (not depicted). Once this initial launch has
15 been performed, the towfish 13 is connected to the light cable 16 and the
main cable 12 is in tum connected to the towfish 13. The connections provide
mechanical and electrical connection of the various subassemblies. Once
these connections have been made, the rest of the launch procedure can be
performed by means of the winch 11 in order to arrive at the configuration
20 depicted in figure 1. The assembly is recovered by reversing the launch
operations.
This procedure exhibits a number of deficiencies.
Mechanical and electrical continuity between the cable 12 and the
streamer 14 is achieved through the towfish. If the sonar is to be operated in
25 passive mode, which means to say with no acoustic emission on the part of
the towfish, then it is still necessary to launch the towfish 13. This is because
it is actually impossible to connect the streamer 14 to the main cable 12,
potentially by way of the light cable 16 without passing via the towfish 13.
The mechanical and electrical connections are made in a hostile
30 environment with a risk of corrosion due to the marine environment and with
the personnel doing the manipulation exposed for a lengthy period to the
swell.
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If certain handling elements break, for example the ancillary winch,
there is a risk of losing the streamer 14, or even the towfish 13.
The invention seeks to alleviate all or some of the problems
5 mentioned above by eliminating the need for an ancillary winch and by
proposing a sonar system in which the mechanical connections are premade
and in which the fitting of the towfish is optional.
To this end, the subject of the invention is an active towed sonar
1 o comprising an emission antenna integrated into a submersible object called a
towfish, a submersible receive antenna called a streamer and a tow cable
intended to tow the towfish and the streamer, characterized in that the tow
cable comprises a termination connected mechanically and electrically to the
streamer, and in that the termination comprises means of removabte
15 mechanical and electrical connection of the towfish to the tow cable,
independently of the connection of the streamer.
The termination comprises several vertebrae articulated to one
another in series, the vertebrae having a substantially cylindrical exterior
surface extending along a main axis X of the tow cable when the tow cable is
20 taut.
A first and a second of the vertebrae may be intended each to
collaborate with a fork of the towfish so as to ensure the mechanical
connection of the towfish to the termination.
A third of the vertebrae may comprise at least one electrical fixed
25 connector allowing the towfish to be connected to the tow cable. The towfish
then comprises an arm terminating in one of the forks. An electrical cable
allowing electrical connection of the towfish extends inside the arm. The
electrical cable comprises a free portion extending outside the arm and
ending in a plug intended to be connected to the fixed connector.
30 The vertebrae referred to as the running vertebrae situated
between the first vertebra and the third vertebra advantageously comprise a
slot oriented along the axis X and produced inside the cylindrical exterior
surface of the running vertebrae concerned. The slot is intended to contain
the free portion of the electrical cable.
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The sonar may comprise a removable electrical extension
arranged between the plug forming the end of the electrical cable and the
fixed connector. The electrical extension is arranged inside the cylindrical
exterior surface of the third vertebra.
The fork advantageously comprises two tines. The first and the
second vertebrae comprise slots, it being possible for the tines each to
become inserted in one of the slots.
Advantageously, the slots pass through the vertebra concerned
along a vertical axis Z perpendicular to the axis X.
The slots may have an entry bevel making it easier to· insert the
fork in the vertebra concerned.
The fork may be articulated with respect to the towfish in such a
way as to allow the fork to rotate about a horizontal axis Y perpendicula~ to
the axis X. · · ·
15 The fork may have a shape that curves perpendicular to the axis
Y.
The shape of the fork is advantageously curved such that a
concave part of the curved shape comes to bear against a first flank of the
corresponding slot and such that a free end of each of the tines comes to
20 bear against a second flank of the corresponding slot, opposite to the first
flank.
25
30
The first and second vertebrae and the forks are advantageously
pierced in such a way as to allow the passage of a rod allowing the fork to
be locked in position on the corresponding vertebra.
The sonar may comprise a pin making it possible to prevent the
disengagement of the rod that locks the fork.
The mechanical connection may ensure the positioning of the
towfish with respect to the termination, keeps it in position and reacts the
forces applied by the towfish to the termination.
The invention will be better understood and further advantages will
become apparent from reading the detailed description of one embodiment
given by way of example, which description is illustrated by the attached
drawing in which:
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figure 1, already introduced, illustrates the overall structure of an
active towed sonar;
figure 2 depicts a first embodiment of how a towfish is attached to
a tow cable;
figure 3 depicts in greater detail the connection between the
towfish and a termination that forms one end of the tow cable;
figure 4 depicts the termination alone without the towfish;
figures 5a and 5b depict two half-vertebrae of the termination,
these being equipped with means of retaining an electrical cable coming from
10 the towfish;
figures 6a, 6b, 6c and 6d depict, in the case of the first
embodiment, an example of the mechanical retention of the towfish on the
termination;
figure 7 depicts a second embodiment of,the attachment of the
15 towfish to the tow cable;
figure 8 and 9 depict in greater detail part of the termination of the
second embodiment;
figures 10a to 10j depict, in the case of the second embodiment,
an example of the mechanical retention of the towfish on the termination.
20 For the sake of clarity, the same elements will bear the same
references in the various figures.
Figure 2 depicts a termination 18 of a tow cable 12 from which
there is suspended a towfish 13 comprising a submersible body 21, intended
25 to receive within its structure an emission antenna of an active sonar. The
streamer 14 may be connected to the free end 19 of the termination directly
or via the light tow cable 16.
The towfish 13 is suspended from the tow cable 12 by two
asymmetric connecting arms 22a and 22b allowing the connecting arms to
30 pass laterally through the fairlead 15.
The tow cable 12 is intended to tow the towfish 13. The cable 12
extends along a towing axis X that is substantially horizontal when the
towfish 13 is being towed. The towfish 13 is suspended from the cable 12
under the effect of its weight. An axis Z is defined perpendicular to the axis of
35 the cable 12. A final axis Y is defined that is perpendicular to the other axes
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X and Z. The axes X, Y and Z are defined with respect to the cable 12. By
convention it will be assumed hereinafter that the axes X and Y are horizontal
and that the axis Z is vertical. The horizontality of the axis X and the
verticality of the axis Z come into effect only when the towfish 13 is
5 suspended under gravity and is stable. The axis X is the towing axis. In
practice, the axis X may adopt an inclined position which may be as much as
40° to 60° on launch or recovery of the towfish 13. The axis X may also veer
to starboard or to port in turns. The axis X may be called the roll axis, the axis
Y the pitch axis and the axis Z the yaw axis.
10
Figure 3 depicts in greater detail the connection between the
towfish 13 and the termination 18. More specifically, it is possible to
distinguish the mechanical connection at the arm 22b. The arm extends
along the axis Z from the towfish 13. A finger 23b is fixed at an upper'end
15 24b of the arm 22b. The finger 23b extends along the axis Y. The finger 23b
covers the termination 18 to be attached thereto. The mechanical fixing will
be detailed later on. The arm 22a is mechanically identical to the arm 22b.
The towfish 13 is electrically connected to the tow cable 12 by
means of an electrical fixed connector 25 belonging to the termination 18 and
20 of an electrical plug 26 arranged at the end of an electrical cable 27 emerging
from the towfish 13. The cable 27 extends along inside the arm 22b and the
finger 23b. The electrical cable 27 comprises a free portion extending outside
the finger 23b and terminating in the plug 26 intended to be connected to the
fixed connector 25. A cap 28 advantageously covers the plug 26 and the
25 fixed connector 25 in order to afford them mechanical protection.
Figure 4 depicts the termination 18 alone without the towfish 13.
The termination 18 is formed of several vertebrae articulated together in
series. One running vertebra 30 is articulated to the previous one by means
30 of a pivot connection 31 allowing freedom of rotation about the axis Z. The
same vertebra 30 is articulated to the next one by means of a pivot
connection 32 allowing freedom of rotation about the axis Y.
When the tow cable 12 is taut, the various vertebrae of the
termination 18 are aligned along the axis X. In this configuration, the exterior
35 surface of the running vertebrae 30 forms a substantially cylindrical surface
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of axis X so as to minimize the drag in the water. The pivot connections 31
and 32 are contained within the cylindrical surface.
The termination 18 comprises three special vertebrae. One
vertebra 35 comprises the fixed connector 25 that allows the towfish 13 to be
5 electrically connected to the cable 12. More specifically, the cable 12 is
formed of a core along which the electrical or possibly optical conductors run,
making it possible to carry signals and power between the winch 11 and the
towfish 13 and between the winch 11 and the streamer 14. The cable 12 is
covered by an armature that provides its mechanical· integrity. ·The
1 o conductors also run along inside the vertebrae and those intended for
connecting the towfish 13 emerge from the termination 18 via the fixed
connector 25. The. vertebra 35 comprises a cylindrical exterior surface of axis
X. Once the electrical connection of the towfish 13 has been made, the fixed
·• , . connector 25, the plug 26 and the cap 28 do not protrude from the ·cylindrical
15 exterior surface of the vertebra 35. In this alternative forrn, in order to allow
the fitting of the fixed connector 25, the outside diameter of the exterior
surface of the vertebra 35 is greater than the diameter of the exterior
cylindrical surface of the running vertebrae 30.
Two other special vertebrae 36 and 37, advantageously identical,
20 allow mechanical connection of the towfish 13 to the termination 18. The
mechanical connection ensures the positioning of the towfish 13 with respect
to the termination 18, keeps it in position and reacts the forces applied by the
towfish 13 to the termination 18. Included among the forces applied by the
towfish 13 to the termination 18 are mainly the weight of the towfish and the
25 hydrodynamic forces applied by the water to the towfish 13 when the latter is
being towed. It is possible to have, in succession, a vertebra 36, several
running vertebrae 30, the vertebra 35,. several running vertebrae 30, the
vertebra 37 and, once again, several running vertebrae 30 to form the
termination 18.
30 The running vertebrae 30 situated between the vertebra 36 to
which the arm 22b is fixed and the vertebra 35 may comprise lugs 38 and 39
for holding the electrical cable 27 in position along the exterior surface of the
vertebrae 30. It is also possible to provide lugs on the vertebrae 30 situated
between the vertebra 35 and the vertebra 37 in order to accommodate an
35 electrical cable coming from the arm 22a. The vertebra 35 may be symmetric
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so that a cable coming from the arm 22a or from the arm 22b can be
connected to it. It is possible to foresee a towfish 13 equipped with two
electrical cables, one passing through each of the arms 22a and 22b. For
that reason, the vertebra 35 comprises two fixed connectors 25, each
5 intended to accept one of the cables.
In order to assemble the vertebrae 30 and notably to assemble the
pivot connections 31 and 32, the running vertebrae 30 are formed of two halfvertebrae
30a and 30b each. Figures 5a and 5b depict two half-vertebrae
30a and 30b each equipped with a lug 38 and 39 respectively.
10 Figures 6a, 6b, 6c and 6d depict an example of mechanical
attachment of a finger to the vertebra intended to receive same, in this
instance the finger 23b on the vertebra 36. The finger 23a is advantageously
fixed to the vertebra 37 in the same way.
· ·•. Tile vertebra 36 is formed of two .half"vertebrae 36a ·and ·36bc
15 Once assembled against one anotl:ler, the two naif-vertebrae nave an
exterior surface tl:lat is substantially cylindrical of axis X, of the same
diameter as that of the exterior surface of the running vertebrae 30.
Each half-vertebra 36a and 36b comprises a slot 40a and 40b
respectively, recessed into tile exterior cylindrical surface and extending
20 along the axis Z. After the vertebra 36 has been assembled, the two slots are
symmetrical about a vertical plane passing through the axes X and Z.
The finger 23b terminates in a fork 41 intended to collaborate with
the two slots 40a and 40b. The fork 41 comprises two tines 41 a and 41 b
each intended to enter one of the slots 40a and 40b respectively. The interior
25 dimensions of the slots 40a and 40b are substantially equal, to within a
functional tolerance, to the exterior dimensions of the tines 41a and 41b so
that the tines 41 a and 41 b slide in the slots 40a and 40b in a translational
movement along the axis Z during the mechanical connection of the towfish
13 to the termination 18. The two half-vertebrae 36a and 36b as well as the
30 fork 41 are pierced so as to allow the passage of a rod 42 that allows the
position of the fork 41 on the vertebra 36 to be locked. A pin 43 may prevent
the rod 42 from disengaging.
Figure 7 depicts a second embodiment of the termination 18, in
35 which all the vertebrae have a cylindrical surface with the same diameter.
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More specifically, in the embodiment visible in figures 3 and 4, the vertebra
35 has a diameter greater than that of the other vertebrae. This change in
diameter generates additional drag in the water when the sonar is being
towed. During launch or recovery, the change in diameter may also give rise
5 to jolts and knocks, notably as the termination 18 passes through the fairlead
15. In the embodiment of figure 7, this change in diameter is eliminated.
In addition, in the embodiment visible in figures 3 and 4 the lugs
38 and 39 protrude relative to the cylindrical exterior surface of the running
vertebrae 30. This protrusion has been eliminated in· the embodiment of
1 o figure 7 and the free portion of the electrical cable 27 is arranged inside the
cylindrical exterior surface of the running vertebrae 30 from where it exits the
finger 23b as far as the vertebra 35. For this purpose. the running vertebrae
30 situated between the vertebrae 35 and 36 comprise a slot 45 oriented
along the axis X and created inside the cylindrical exterior surface oHhe
15 running vertebrae 30 concerned. The slot 45 is intended to contain the
electrical cable 27. The slot rnay grip the electrical cable 27 slightly so that it
remains in the slot 45 when the various vertebrae move relative to one
another at their pivot connections. Inside the slot 45, the electrical cable 27
may nevertheless move along the axis X so as to conform to the movement
20 of the vertebrae 30 relative to one another as they move relative to one
another by means of their pivot connections. This movement makes it
possible to avoid premature wearing of the electrical cable 27 through pulling
thereof along the axis X.
Figure 8 depicts in greater detail a part of the termination 18 at the
25 level of the slot 45. Figure 9 depicts this same part with the electrical
connection of the electrical cable 27 made. Advantageously, a removable
electrical extension 47 is arranged between the plug 26 that forms the end of
the electrical cable 27 and the fixed connector 25. The fitting and removal of
the towfish 13 with respect to the termination 18 are performed electrically
30 between the plug 26 and the extension 47. In this way, the fixed connector 25
is not handled. The electrical extension 47 constitutes a wearing component
that can easily be replaced. This wearing component limits the risk of
damage to the fixed connector 25 during handling of the towfish 13. The
electrical extension 47 is arranged inside the cylindrical exterior surface of
35 the vertebra 35.
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Figures 1 Oa to 1 Oj depict, for the second embodiment, how the
towfish 13 is mechanically retained on the termination 18. The fingers 23a
and 23b terminate in forks 50 which differ from the forks 41. Figures 1 Oa to
5 1 Oj depict just one fork 50 facing a vertebra 52 intended to accept it. The two
forks 50 on either side of the two vertebrae intended to accept the forks 50
are advantageously identical. The vertebra 52 differs from the vertebra 36 in
terms of the shape of its slots 52a and 52b into which the fork 50 is inserted.
The fork 50 is articulated with respect to the finger 23b so as to
1 o allow the fork 50 to rotate about a horizontal axis Y. Figures 1 Oa to 1 Og
depict various positions of the fork 50 with respect to the vertebra 52. This
articulation allows the fork 50 to slide along the axis X on the various
vertebrae of the termination 18 without the risk of damaging the fork 50. It is
possible to foresee, dtming !he maneuver of attaching the towfish 13, keeping
15 the towfish 13 fixed on the deck of the ship 10. Bringing the cable 12 closer
to its position in which the towfish 13 is attached, is achieved chiefly by
operating the winch 11. In the first embodiment it is possible to foresee a
pivot connection about the axis Y between the fork 41 and the finger that
bears it. This pivot connection has a small amplitude of movement and allows
20 the fingers 41 a and 41 b to rotate to make them easier to insert into the
corresponding slots. More specifically, in the second embodiment, when the
towfish 13 is being fitted onto the termination, the arms 22a and 22b can be
positioned on the cable 12 upstream of the termination 18, namely on a part
54 of the cable 12. The operator fitting the towfish 13 then operates the winch
25 11 in such a way as to wind in the cable 12 in order to bring the arms 22a
and 22b closer to their respective host vertebra. As long as the forks 50 do
not reach the host vertebrae, they remain in the retracted position as
depicted in figure 1 Oa. As the operator continues to operate the winch 11, the
slots 52a and 52b of the vertebra 52 come to face the fork 50 which can
30 rotate freely about its pivot connection and insert itself under gravity into the
corresponding slots. Alternatively, the operator may engage the forks 50 in
their respective vertebra 52 by hand. This operation marks the transition from
the position of figure 1 Oa to that of figure 1 Oc.
The operator then resumes operation of the winch 11 in order to
35 move on from the position of figure 1 Octo that of figure 1 Og.
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The slots 52a and 52b differ from the slots 40a and 40b in terms of
their width defined along the axis X so as to allow the fork 50 to rotate inside
the corresponding slots.
To make it easier to fit the fork 50 into the slots 52a and 52b, the
5 fork 50 has a shape that curves perpendicular to the. axis Y. More
specifically, the fork 50 comprises two tines 50a and 50b each able. to
become inserted in one of the slots 52a and 52b respectively. The shape of
the fork 50 is curved so that a concave part 54 of the curved shape comes to
bear against one of the flanks 55a and 55b of the corresponding slot 52a and
10 52b. The free end 56a and 56b of each of the tines 50a and 50b comes to
bear against a flank 57a and 57b of the corresponding slot, opposite to the
flank 55a, 55b.
In the position of figure 1 Og, the forks 50 are in abutment on the
corresponding vertebrae 52. The rod 42 and the.· pin ·.43 can then·be used:as
15 before to lock the position of the fork 50 on the vertebra 52. After locking, the
towfish 13 can be launched into the water by operating the winch 11, use of
the sonar then follows.
At the end of the mission, the sonar is brought back on board the
deck of the ship 10 using the winch 11. When the towfish 13, towed by the
20 cable 12, arrives on the deck of the ship 10, the towfish 13 is immobilized on
the deck and the operation of detaching the towfish may be begun.
Detaching the towfish 13 from the termination 18 is performed by
reversing the order of the attachment operations. More specifically, the
removal of the rod 42 is performed in the position of figure 1 Oh toward that of
25 figure 1 Og. The operator then operates the winch 11 to move the termination
18 with respect to the towfish 13 in order to move on from the position of
figure 1 Og to the position of figure 1 Oc. The operator then manually lifts the
fork 50 to move the fingers 50a and 50b away from the corresponding
vertebra. The cable 12 can then be taken away from the towfish 13 and
30 continue to be wound in.
The slots 52a and 52b may be passed through the vertebra 52
along the axis Z. Thus, the termination 18 may be inverted by 180° about the
axis X and still accept the insertion of the fork 50. The fact that the slots are
35 through-slots may also apply to the first embodiment.
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The various figures 1 Oa to 1 Oj show two holes 61 and 62 able to
accept the rod 42. One or other of the two holes is used according to the
orientation of the termination 18 with respect to the towfish 13.
In addition, to make the insertion of the fork 50 easier, the slots
5 52a and 52b each have an entry bevel 63a and 63b respectively. This bevel
allows a tolerance on the rotational positioning of the termination about the
axis X with respect to the fork 50. The bottom 64 of the fork 50 may have a
hollowed shape conforming to that of the bevels 63a, 63b so as to immobilize
the fork 50 in terms of rotation about the axis X when the fork is fully inserted
1 o into the slots 52a and 52 b. The bevel may also apply to the first embodiment.
15
In addition, when insertion of the fork 50 is complete, the fingers
50a and 50b are arranged inside the exterior cylindrical surface of the
vertebra 52.

CLAIM
1. An active towed sonar comprising an emission antenna
integrated into a submersible object called a towfish (13), a submersible
receive antenna called a streamer (14) and a tow cable (12) intended to tow
the towfish (13) and the streamer (14), characterized in that the tow cable
5 (12) comprises a termination (18) connected mechanically and electrically to
the streamer (14), in that the termination (18) comprises means of removable
mechanical and electrical connection of the towfish (13) to the tow cable (12),
independently of the connection of the st~eamer (14) and in that the
termination (18) comprises several vertebrae (30, 36, 37; 52) articulated to
10 one another in series, the vertebrae (30, 36, 37; 52) having a substantially
cylindrical exterior surface extending along a main axis X of the tow cable
(12) when the tow cable (12) is taut.
2. The sonar as claimed in claim 1, characterized in that a first and
15 a second of the vertebrae (36, 37; 52) are intended each to collaborate with a
fork (41; 50) of the towfish (13) so as to ensure the mechanical connection of
the towfish (13) to the termination (18).
3. The sonar as claimed in claim 2, characterized in that a third of
20 the vertebrae (35) comprises at least one electrical fixed connector (25)
allowing the towfish (13) to be connected to the two cable (12), in that the
towfish (13) comprises an arm (22b) terminating in one of the forks (41; 50),
in that an electrical cable (27) allowing electrical connection of the towfish
(13) extends inside the arm (22b) and in that the electrical cable (27)
25 comprises a free portion extending outside the arm (22b) and ending in a
plug (26) intended to be connected to the fixed connector (25).
4. The sonar as claimed in claim 3, characterized in that the
vertebrae (30) referred to as the running vertebrae situated between the first
30 vertebra (36; 52) and the third vertebra (35) comprise a slot (45) oriented
along the axis X and produced inside the cylindrical exterior surface of the
running vertebrae (30) concerned and in that the slot (45) is intended to
contain the free portion of the electrical cable (27).
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5. The sonar as claimed in claim 4, characterized in that it
comprises a removable electrical extension (47) arranged between the plug
(26) forming the end of the electrical cable (27) and the fixed connector (25)
and in that the electrical extension (47) is arranged inside the cylindrical
5 exterior surface of the third vertebra (35).
6. The sonar as claimed in one of claims 2 to 5, characterized in
that the fork (41; 50) comprises two tines (41a, 41b; 50a, 50b), in that the first
and the second vertebra (36, 37; 52) comprise slots (40a, 40b; 52a, 52b), it
10 being possible for the tines (41a, 41b; 50a, 50b) each to become inserted in
one of the slots (40a, 40b; 52a, 52b).
7. The sonar as claimed in claim 6, characterized in that the slots
'·"". , " , (40a,40b; 52a, 52b) p3ss thmugl1 the vertebra concerned (36, 37; 52) along
15 a vertical axis Z perpendicular to the axis X.
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8. The sonar as claimed in one of claims 6 and 7, characterized in
that the slots (40a, 40b; 52a, 52b) have an entry bevel (63a, 63b) making it
easier to insert the fork (41; 50) in the vertebra (36, 37; 52) concerned.
9. The sonar as claimed in one of claims 2 to 8, characterized in
that the fork (50) is articulated with respect to the towfish (13) in such a way
as to allow the fork (50) to rotate about a horizontal axis Y perpendicular to
the axis X.
10. The sonar as claimed in claim 9, characterized in that the fork
(50) has a shape that curves perpendicular to the axis Y.
11. The sonar as claimed in claim 10, characterized in that the
30 shape of the fork (50) is curved such that a concave part (54) of the curved
shape comes to bear against a first flank (55 a, 55b) of the corresponding slot
(52a, 52b) and in that a free end (56a, 56b) of each of the tines (50a, 50b)
comes to bear against a second flank (57a, 57b) of the corresponding slot,
opposite to the first flank (55a, 55b).
35
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12. The sonar as claimed in one of claims 6 to 11, characterized in
that the first and second vertebrae (36, 37; 52) and the forks (41, 50) are
pierced in such a way as to allow the passage of a rod (42) aHowing the fork
(41, 50) to be locked in position on the corresponding vertebra (36, 37; 52).
13. The sonar as claimed in claim 12, characterized in that it
comprises a pin (43) making it possible to prevent the disengagement of the
rod (42) that locks the fork (41; 50).
14. The sonar as claimed in one of the preceding claims,
characterized in that the mechanical connection ensures the positioning of
the towfish (13) with respect to the termination (18), keeps it in position and
reacts the forces applied by the towfish (13) to the termination (18) .