A foldable element for a retractable thruster and a method for producing such element

Abstract

 A foldable element for a retractable thruster for a boat and a method for producing such element, comprising;
- moulding a first and a second element (100a, 100b) comprising thruster tunnel segments (20a, 20b) shaped as a hollow cylinder segment, and at least one arm section (41 a, 41 b) extending from said first segment (20a),
- arranging a propeller shaft (21), a drive shaft (15) and a gearbox (16) connecting said drive shaft (15) to said propeller shaft (21) in said first thruster tunnel segment (20a), wherein said drive shaft (15) is extending out of said first thruster tunnel segment (20a),
- arranging said second element (100b) relative said first element (100a) so that said first and second segments (20a, 20b) constitute a hollow cylinder (17) surrounding said propeller shaft (21), wherein said first and second arm sections (41 a, 41 b) is are in contact and fastened.

Description

Field of the invention

[0001] The present invention relates to the field of thrusters for watercrafts, such as vessels and smaller boats and sailboats. More particularly it relates to thrusters and a foldable element for a retractable thruster that can be retracted into the hull of the watercraft when they are not in use, such as thrusters used for docking a boat.

Background of the Invention

[0002] Thrusters for manoeuvring a watercraft have been known for a long time. Different types of thrusters exist, such as the azimuth thruster which is a combined propulsion and steering device that can be rotated to turn the ship or boat. The tunnel thruster used mainly for larger vessels requires a thruster tunnel to be built into the hull below the waterline. An impeller in the tunnel can create thrust in either direction. Externally mounted thrusters are mainly used for smaller boats where thrusters have to be retrofitted, and the space in the hull is limited. Such thrusters will increase the boats resistance to forward motion if they are below the water line during normal propulsion.

[0003] European patent application EP 1 876 094 A2 shows a retractable thruster for a vessels where the propeller unit and the electric motor are moved between a recessed position and an operational position. The propeller unit and the electric motor are comprised in a piston element that can be moved inside a housing by applying hydraulic pressure on the piston.

[0004] US patent 5152240 (Fontanille) describes a retractable and storable thruster for a vessel, using a trapezoidal deformable rotatable device producing rectilinear movement inside a well in the vessel's hull.

[0005] US patent application US 2006/0060127 shows a retractable thruster comprising a propulsion assembly comprising a rigid structure secured to a propeller housing and containing a motor driving the propeller inside the propeller housing via at least one rotary shaft between the motor and the propeller. Displacement means enable the propulsion assembly to be moved between retracted and deployed positions by the propulsion assembly performing uniform circular movement about an axis of rotation situated substantially at the level of the hull or beneath the hull.

[0006] European patent application EP2 246 252 A2 shows a thruster that can be tilted into a thruster housing in the hull of a boat. The thruster and the thruster motor driving the propeller are both tiltably connected to a swing frame and will rotate around an axis of the swing frame when moving from an extracted position to a retracted position and vice versa.

Short summary of the invention

[0007] In retractable thrusters, the moving part is a critical element for the behavior of the thruster. Without a proper mechanism for retracting the propeller housing, the thruster cannot work properly, and dangerous situations may occur if the thruster does not return to its retracted position inside the hull.

[0008] An object of the present invention is to disclose a foldable element of a retractable thruster that is smaller in size than comparable foldable elements according to prior art, since the available space in the hull of a watercraft is generally limited. Due to the problems related to the wear and tear of cables and components as the motor is moved during the retraction, it is also an object of the invention that the motor shall be fixed mounted relative the hull of the boat.

[0009] Another object of the present invention is to disclose a method for manufacturing the foldable part of a retractable thruster that provides a foldable element for a retractable thruster that can be made with more precision than the propeller housings of thrusters according to background art.

[0010] It is also an object of the invention to develop a method for assembling the various elements of a retractable thruster elements in a simpler and more efficient way than what is done according to prior art, and with a reduced number of molding tools.

[0011] The invention is in an embodiment a method for manufacturing a foldable element of a retractable thruster for a boat, wherein said method comprises the following steps;

• moulding a first element comprising a first thruster tunnel segment shaped as a hollow cylinder segment, and at least one first arm section extending from said first thruster tunnel segment,
• moulding a second element comprising a second thruster tunnel segment shaped as a hollow cylinder segment, and at least one second arm section extending from said second thruster tunnel segment,
• arranging a propeller shaft with one or more propellers, a drive shaft perpendicular to said propeller shaft and a gearbox connecting said drive shaft to said propeller shaft in said first thruster tunnel segment, wherein said drive shaft is extending out of said first thruster tunnel segment,
• arranging said second element relative said first element so that said first thruster tunnel segment and said second thruster tunnel segment constitutes a hollow cylinder surrounding said propeller shaft, wherein said first arm section is in contact with said respective second arm section, and fastening said first arm section (41a) to said respective second arm section.

[0012] The invention is also a A foldable element of a retractable thruster for a boat, comprising

• a first element comprising a first thruster tunnel segment shaped as a hollow cylinder segment, and at least one first arm section extending from said first thruster tunnel segment,
• a second element comprising a second thruster tunnel segment shaped as a hollow cylinder segment, and at least one second arm section extending from said second thruster tunnel segment, wherein
• said second element is arranged relative said first element so that said first thruster tunnel segment and said second thruster tunnel segment constitutes a hollow cylinder, and wherein said first arm section is in contact with, and fastened to said respective second arm section,
• said foldable element further comprising a propeller shaft with one or more propellers, a drive shaft perpendicular to said propeller shaft and a gearbox connecting said drive shaft to said propeller shaft arranged inside said hollow cylinder, wherein said drive shaft is extending through an opening in the wall of said a hollow cylinder.

[0013] In an embodiment the first element (100a) and said second element (100b) are identical. When the elements are identical they may be moulded in the same form and there is need for only one set of tools for performing the operation.

Short figure captions

[0014] The attached figures illustrate some embodiments of the claimed invention.

Fig. 1 illustrates in an exploded view a foldable thruster element (100) according to an embodiment of the invention.

Fig. 2 illustrates in a side view a retractable thruster (1) according to an embodiment of the invention, with the foldable element (100) and the propeller unit (20) in an extracted position.

Fig. 3 illustrates in a partly exploded top perspective view the retractable thruster of Fig. 2.

Fig. 4 illustrates in a simplified drawing the foldable element (100) with the propeller unit (20) and the door (30) of the retractable thruster in a retracted position (A) and an extended position (B). It also illustrates the second direction of rotation (s) of the door (30) relative the first direction of rotation (r) of the propeller housing (20) between these two positions.

Fig. 5 illustrates in a simplified drawing a housing of the retractable thruster.

Fig. 6 illustrates a foldable double cardan joint drive shaft that may be used in a foldable thruster. A first universal joint (150) of the foldable double cardan joint drive shaft comprising an offset cross shaft (151) is shown.

Embodiments of the invention

[0015] With reference to the attached drawings the device and system according to the invention will now be explained in more detail.

[0016] In Fig. 1 it is shown a foldable element (100) of a retractable thruster for a boat according to the invention. The foldable element comprises a first element (100a) comprising a first thruster tunnel segment (20a) shaped as a hollow cylinder segment, and at least one first arm section (41a) extending from said first thruster tunnel segment (20a). The foldable element (100) further comprises a second element (100b) comprising a second thruster tunnel segment (20b) shaped as a hollow cylinder segment, and at least one second arm section (41b) extending from said second thruster tunnel segment (20b). The second element (100b) is arranged relative said first element (100a) so that said first thruster tunnel segment (20a) and said second thruster tunnel segment (20b) constitutes a hollow cylinder (17), and wherein said first arm section (41a) is in contact with, and fastened to said respective second arm section (41b).

[0017] The foldable element further comprising a propeller shaft (21) with one or more propellers (22), a drive shaft (15) perpendicular to said propeller shaft (21) and a gearbox (16) connecting said drive shaft (15) to said propeller shaft (21) arranged inside said hollow cylinder (17), wherein said drive shaft (15) is extending through an opening (12) in the wall of said a hollow cylinder (17).

[0018] The invention is also a method for producing a foldable element as described above. The method comprises the following steps;

• moulding a first element (100a) comprising a first thruster tunnel segment (20a) shaped as a hollow cylinder segment, and at least one first arm section (41a) extending from said first thruster tunnel segment (20a),
• mould ing a second element (100b) comprising a second thruster tunnel segment (20b) shaped as a hollow cylinder segment, and at least one second arm section (41b) extending from said second thruster tunnel segment (20b),
• arranging a propeller shaft (21) with one or more propellers (22), a drive shaft (15) perpendicular to said propeller shaft (21) and a gearbox (16) connecting said drive shaft (15) to said propeller shaft (21) in said first thruster tunnel segment (20a), wherein said drive shaft (15) is extending out of said first thruster tunnel segment (20a),
• arranging said second element (100b) relative said first element (100a) so that said first thruster tunnel segment (20a) and said second thruster tunnel segment (20b) constitutes a hollow cylinder (17) surrounding said propeller shaft (21), wherein said first arm section (41a) is in contact with said respective second arm section (41b), and fastening said first arm section (41a) to said respective second arm section (41b).

[0019] The first and second elements may be moulded with injection moulding or any suitable moulding technique.

[0020] The foldable element according to the invention may be used in a retractable thruster as described in Fig. 2, 3, 4, 5 and 6, but it may also be used in any other retractable thruster with a foldable element. In Fig. 2 the retractable thruster (1) for a watercraft with a hull (2) comprises an opening (3), wherein the retractable thruster (1) comprises;

• a motor (10) arranged for being fixed inside the hull (2) of the watercraft,
• a propeller unit (20) arranged for moving along an arc (AB) with a first direction of rotation (r) between a retracted position (A) inside the hull (1) and an extended position (B) outside the hull (1),
• a door (30) arranged for closing the opening (3) when the propeller unit (20) is in the retracted position (A), the door (30) further arranged for moving with the propeller unit (20) in the first direction of rotation (r) along the arc (AB) and for rotating with a second direction of rotation (s) opposite of the first direction of rotation (r). It can be seen that the actuator (60) can rotate the foldable element (100) about the end of the foldable element (100) that is located opposite to the propeller housing (20) end.

[0021] The watercraft where the retractable thruster (1) is installed or is to be installed has an opening in the hull (2) where the retractable thruster (1) can be mounted as seen in Fig. 3. However, as will be described below, the thruster comprises in an embodiment of the invention a housing (70) that is mounted into the hull (2). The housing (70) allows simpler and easier adaption to different hull (2) shapes.

[0022] The motor (10) is fixed relative the hull (2), preferably to the housing (70) that itself is arranged for being fixed to the hull (2). The motor may be e.g. electric or hydraulic and connected by electrical cables or hydraulic tubes. The fixed motor (10) simplifies wiring and reduces the wear and tear of cables and tubes. In the embodiment where the motor (10) is fixed inside the housing (70) simple bushings can be used in the wall of the housing (70) between the motor (10) and the cabling in the hull (2).

[0023] The propeller unit (20) is arranged for moving between the retracted position (A) inside the hull (1) and the extended position (B) outside the hull (1) as shown in Fig. 4 along an arc (AB). In an embodiment of the invention the arc (AB) is a part of a circumference of a circle with centre (O) inside the hull (2). Preferably the centre (O) is close to the bottom of the hull (2). When the propeller unit (20) travels from the retracted position (A) to the extended position (B) it rotates around the centre (O) with a direction of rotation (r) as indicated by an arrow in Fig. 4. When the propeller unit (20) travels from the extended position (A) to the retracted position (B) it rotates around the centre (O) opposite the first direction of rotation (r) indicated by the arrow.

[0024] In an embodiment the arc (AB) is a part of a circumference of an ellipse with centre (O). The arc (AB) may in other embodiments of the invention be any kind of arc.

[0025] The retractable thruster (1) may comprise a door (30). The door (30) is arranged for moving with propeller unit (20) when the propeller unit (20) moves along the arc (AB) in the first direction of rotation (r). When the propeller unit (20) is in the retracted position (A) the door (30) is arranged for closing the opening (2), and when the propeller unit (20) travels from the retracted position (A) to the extended position (B) the door (30) is arranged to follow the propeller unit. In the extended position (B) the door is then moved away from the opening (2) to allow the propeller unit (2) to be in the extended position (B). From the extended position (B) to the retracted position (A) the propeller moves along the arc (AB) opposite the first direction of rotation (r).

[0026] The door (30) may be arranged for rotating with a second direction of rotation (s) opposite of the first direction of rotation (r). As an example, the propeller unit (2) starts extending from the retracted position (A) in an embodiment of the invention by moving along the arc (AB) in the direction of rotation (r). The door (30) will move with the propeller unit (2) as described above, and in addition the door (30) will rotate with a second direction of rotation (s) that is opposite the first direction of rotation (r) until the propeller unit (2) reaches its extended position (B). In Fig. 4 it is illustrated the position that a door (30') would have had in the extended position (B) if it had not rotated with the second direction of rotation (s). This would clearly not be a good solution. On the contrary it is also shown the position of the door (30) when the door has rotated with the second direction of rotation (s). Even though this is for illustration purpose only, it shows that with the retractable thruster according to the invention, a more compact design can be obtained.

[0027] The extent of the rotation of the door (30) in the second direction of rotation (s) depends on the design of the retractable thruster (1). However, according to an embodiment of the invention the door (30) should in the extended position constitute a shield for the propeller unit (20) in the forward direction of the boat. In this embodiment the top of the door is close to the hull (2) of the boat and the bottom of the door is inclined backwards with respect to the longitudinal direction of the boat. The door (30) may in this embodiment protect the propeller unit (20) against underwater obstacles, such as underwater rocks, that may be located just below the hull and therefore are hard to discover. This is especially important when retractable thrusters are used during docking in shallow water.

[0028] According to an embodiment the propeller unit (20) comprises a propeller shaft (21) with one or more propellers (22) and wherein the motor (10) is arranged for driving the propeller shaft (21). In a preferred embodiment the propeller shaft (21) is arranged perpendicular to the longitudinal direction of the boat, and the one or more propellers (22) on the propeller shaft (21) will then arranged in the transverse direction of the boat and provide a lateral thrust. The force and direction of the lateral thrust will depend on the shape of the propellers (22) and the speed and direction of the rotation of the propeller shaft (21). In an embodiment the motor (10) is an electric motor, and the direction of the lateral thrust can be changed by changing the polarity of the DC voltage of the motor (10).

[0029] In an embodiment the propeller shaft (21) is arranged for being aligned with the longitudinal direction of the boat. The retractable thruster may then also be used as an alternative or backup propulsion device.

[0030] According to an embodiment of the invention the door (30) is arranged for rotating about the propeller shaft (21) with the second direction of rotation (s). However, other centres of rotation for the door may also be used to achieve the same functionality.

[0031] According to an embodiment of the invention the door (30) is fixed to the propeller unit (20). Accordingly the door and propeller unit will in this embodiment rotate with the same rotational speed around the same centre of rotation, which may be the rotational axis of the propeller shaft (21).

[0032] According to an embodiment of the invention the retractable thruster (1) comprises a drive shaft (15) between a motor shaft (11) of the motor (10) and the propeller shaft (21). The drive shaft (15) is arranged for the transfer of rotational motion from the motor shaft (15) to the propeller shaft (21) when the motor (10) is running. The drive shaft (15) may be connected to the propeller shaft (21) by a gear, such as a bevel gear, a worm gear or any other suitable gear.

[0033] The motor shaft (11) may also be connected to the propeller shaft (21) by e.g. sprockets and chains, hydraulic transmission, etc.

[0034] In an embodiment the drive shaft (15) is a foldable double cardan joint drive shaft as illustrated in Fig. 6 comprising;

• a driving shaft (140) with a driving rotational axis (d),
• an intermediate shaft (180) with an intermediate rotational axis (e),
• a driven shaft (170) with a driven rotational axis (f),
• a first universal joint (150), and
• a second universal joint (160),
  wherein the first universal joint (150) is arranged between the driving shaft (140) and the intermediate shaft (180), and comprises an offset cross shaft (151) with an offset axis (g) comprising;
• a first pivot shaft (151a) with a first pivot axis (a) connected to the intermediate shaft (180) and
• a second pivot shaft (151b) connected to the driving shaft (140), wherein the first and second pivot axes (a, b) are both perpendicular to the offset axis (g) and to each other, and wherein the first pivot axis (a) is offset an offset distance (o) relative the second pivot axis (b) in the direction of the offset axis (o).

[0035] Thus, when the drive shaft (15) is folded into the collapsed position, it is able fold in three dimensions due to the flexibility of the first universal joint (150), and therefore the drive shaft (15) can be folded independently of the rotational position of the driving shaft (140) and the second pivot axis (b).

[0036] In an embodiment the intermediate shaft (180) is a fixed length shaft. However a spring loaded telescopic shaft may in an embodiment also be used.

[0037] To further understand the function of the foldable double cardan joint, it can be added that the offset axis (g) will coincide with the rotational and longitudinal axis of the intermediate shaft (180) when the retractable thruster in the extended position (A). In other positions, the offset axis (g) will wobble if the drive shaft is rotated, but since no energy needs to be transferred to the propeller axis (21) from the motor shaft (11) in these positions, only rotational movement related to the rotational alignment of the propeller axis (21) with the motor shaft (11) needs to be considered when the retractable thruster (1) is not in the extended position (A)

[0038] It is important to transfer energy efficiently between the motor (10) and the propeller (22) when the retractable thruster (1) is running in the extended position (B). In an embodiment of the invention the drive shaft (15) is mainly rectilinear in its longitudinal direction and coincident with a rotational axis (11a) of the motor shaft (11) when the propeller unit (20) is in the extended position (B). This ensures that the drive shaft (15) has no bends and that energy is transferred optimally.

[0039] One of the advantages of the invention, is the possibility to perform a sufficient retraction and extension, and at the same time keeping the motor fixed to the hull and obtaining a compact and robust design. The basis for this is the rotation along the arc (AB). According to an embodiment of the invention the arc (AB) is a circular arc and the propeller unit (20) is arranged for moving between the retracted position (A) and the extended position (B), along the arc (AB) where a central angle (θ) of the arc (AB) is at least 90 degree, as shown in Fig. 4.

[0040] The retractable thruster (1) may comprise a seal (190) around the driven shaft (170) for tightening against the propeller unit. The seal (190) may be flexible in the longitudinal direction of the driven shaft (170), and expanded in the length direction by a spring that pushes one end of the seal (190) towards the propeller unit (20) and the other end towards a fixed point of the driven shaft (170), such as a flange, to take up small length variations in the drive shaft (15) between the retracted position (A) and the expanded position (B).

[0041] Arms with rotating joints may be used to restrict the movement of the propeller unit (20) along the arc (AB) as illustrated in Fig. 6. According to an embodiment of the invention the retractable thruster (1) comprises at least one first arm (40) comprising a first revolute joint (41) in a first end of the first arm (40) and a second revolute joint (42) in a second end of the first arm (40), where the first revolute joint (41) is rotatably connected in the origo (O) of a sector defined by the arc (AB), and the second revolute joint (42) is rotatably connected to the propeller unit (20) at a rotary point (ab) along the arc (AB). Thus, the first revolute joint (41) is centred in the centre of rotation of a point along the arc AB, and the second revolute joint (42) is fixed to the propeller unit (20) at a point that should follow the arc (AB). According to this embodiment the propeller unit (20) can now move between the retracted position (A) and the extended position (B). Stoppers may be used to restrict the end positions of the movement of the first arm (40).

[0042] The rotary point (ab) may in an embodiment of the invention coincide with a rotational axis of the propeller shaft (21).

[0043] According to the invention the door rotates in the opposite direction of the movement of the propeller unit around the centre (O). In an embodiment the retractable thruster comprises at least one track rod (50), as illustrated in Fig. 6, being rotatably connected to a first track rod point (51) fixed relative the motor (10) in a first end of the track rod (50) and rotatably connected to a second track rod point (52) fixed relative the propeller unit (20) in a second end of the track rod (50).

[0044] By proper design and fastening of the track rod (50) to the motor (10) or a point fixed to the motor (10) or hull (2) in one end, and to the propeller unit (20) in the other end, the desired rotational effect of the door (30) can be obtained.

[0045] There are alternative ways of obtaining the same rotation of the door, such as e.g. the use of actuators in the propeller unit (20) to rotate the propeller unit (20) relative e.g. the first arm (40). In such an embodiment the rotation may be performed non-uniformly over the movement between the retracted position (B) and the extended position (A), e.g. all the rotation could be done in the extended position (B).

[0046] However, a passive mechanical solution such as a track rod has several advantages related to reduced wear and maintenance.

[0047] To move the propeller unit (20) between the retracted position (B) and the extended position (A) the retractable thruster comprises in an embodiment of the invention an actuator (60), as can be seen illustrated in Fig. 2 and Fig. 3, arranged for moving the propeller unit along the arc (AB) between the retracted position (A) and the extended position (B) by applying a force to at least the first arm (40) relative the hull (2). The actuator may be rotatably connected in one or both ends to a point fixed relative the hull in one end, and to a point fixed relative the first arm (40) in the other end. Different types of actuators may be used such as electrical, hydraulic, etc.

[0048] It is an objective of the invention to provide a retractable thruster that easily can be retrofitted into a boats hull. In an embodiment of the invention the retractable thruster (1) comprising a housing (70), wherein the motor (10) is fixed relative the housing (70) inside the housing (70), and an adapter frame (71) comprising the opening (2), wherein the adapter frame (71) is arranged for being mounted between the hull (2) and the housing (70). The separation of the adapter frame (71) from the housing (70) allows different frames to be made for the different hull types, where the adapter frame (71) is adapted to the specific hull (2) underneath, and to the housing (70) on the top. Thus the same housing may be used for a large number of hull types. In the embodiment where the housing (70) and adapter frame (71) is used, the door (30) fits to the opening (2) of the housing (70) or adapter frame (71) when it is in the retracted position (A).

[0049] According to an embodiment of the invention, the retractable thruster (1) comprises one or more security devices (not shown) to protect the retractable thruster (1) and the boat against damages if the retractable thruster (1) should be run into an obstacle when not in the retracted position (A). The security device may e.g. be a breakable pin in one of the ends of the actuators so that the pin breaks and the propeller unit (20) and the door (30) are allowed to be pushed into the hull (2) in such cases. Instead of a breakable pin, it could also be any kind of flexible device that will allow the actuator to loose the grip in one of its ends above a certain force, and to allow refitting without spare parts.

[0050] In an embodiment the method comprises the step of arranging an end plate (18) arranged for holding said first thruster tunnel segment (20a) and said second thruster tunnel segment (20b) together on each end of said cylinder (17) and fastening said end plates (18) fixed relative each other.

[0051] In an embodiment the method comprises the step of pushing said second element (100b) towards said first element (100a) to engage one or more snap locks (19 a) arranged on the edge of said first thruster tunnel segment (20a) with corresponding one or more snap lock counterparts (19 b) arranged on the edge of said second thruster tunnel segment (20b).

[0052] In an embodiment the first element (100a) and said second element (100b) are identical. When the elements are identical they may be moulded in the same form and there is need for only one set of tools for performing the operation.

[0053] In an embodiment the first element (100a) and said second element (100b) comprises two first arm sections (41a) and two second arm sections (41b), respectively, and wherein said method comprises the step of arranging said drive shaft (15) between said two first arm sections (41a).

[0054] In an embodiment the first arm section (41a) and said second arm section (41b) comprises slits (42a, 42b) in a longitudinal direction of said first arm section (41a) and said second arm section (41b), and wherein said method comprises the step of arranging a reinforcing element (43) in said slits (42a, 42b), wherein said reinforcing element (43) comprises a hole with a centre, where said foldable element is arranged to pivot about said centre.

[0055] In an embodiment the reinforcing element (43) is arranged to be fixed to an axel parallel to said propeller axis (21) going through said hole (44), and wherein said axis is arranged to rotate to fold and unfold said foldable element (100).

Claims

1. A method for manufacturing a foldable element (100) of a retractable thruster for a boat, wherein said method comprises the following steps;

- moulding a first element (100a) comprising a first thruster tunnel segment (20a) shaped as a hollow cylinder segment, and at least one first arm section (41a) extending from said first thruster tunnel segment (20a),

- moulding a second element (100b) comprising a second thruster tunnel segment (20b) shaped as a hollow cylinder segment, and at least one second arm section (41b) extending from said second thruster tunnel segment (20b),

- arranging a propeller shaft (21) with one or more propellers (22), a drive shaft (15) perpendicular to said propeller shaft (21) and a gearbox (16) connecting said drive shaft (15) to said propeller shaft (21) in said first thruster tunnel segment (20a), wherein said drive shaft (15) is extending out of said first thruster tunnel segment (20a),

- arranging said second element (100b) relative said first element (100a) so that said first thruster tunnel segment (20a) and said second thruster tunnel segment (20b) constitutes a hollow cylinder (17) surrounding said propeller shaft (21), wherein said first arm section (41a) is in contact with said respective second arm section (41b), and fastening said first arm section (41a) to said respective second arm section (41b).

2. The method of claim 1 comprising the step of arranging an end plate (18) arranged for holding said first thruster tunnel segment (20a) and said second thruster tunnel segment (20b) together on each end of said cylinder (17) and fastening said end plates (18) fixed relative each other.

3. The method of claim 1 comprising the step of pushing said second element (100b) towards said first element (100a) to engage one or more snap locks (19 a) arranged on the edge of said first thruster tunnel segment (20a) with corresponding one or more snap lock counterparts (19 b) arranged on the edge of said second thruster tunnel segment (20b).

4. The method of claim 1 wherein said first element (100a) and said second element (100b) are identical.

5. The method of claim 1, wherein said first element (100a) and said second element (100b) comprises two first arm sections (41a) and two second arm sections (41b), respectively, and wherein said method comprises the step of arranging said drive shaft (15) between said two first arm sections (41a).

6. The method of claim 1, wherein said first arm section (41a) and said second arm section (41b) comprises slits (42a, 42b) in a longitudinal direction of said first arm section (41a) and said second arm section (41b), and wherein said method comprises the step of arranging a reinforcing element (43) in said slits (42a, 42b), wherein said reinforcing element (43) comprises a hole with a centre, where said foldable element is arranged to pivot about said centre.

7. The method of claim 6, wherein said reinforcing element (43) is arranged to be fixed to an axel parallel to said propeller axis (21) going through said hole (44), and wherein said axis is arranged to rotate to fold and unfold said foldable element (100).

8. A foldable element (100) of a retractable thruster for a boat, comprising

- a first element (100a) comprising a first thruster tunnel segment (20a) shaped as a hollow cylinder segment, and at least one first arm section (41a) extending from said first thruster tunnel segment (20a),

- a second element (100b) comprising a second thruster tunnel segment (20b) shaped as a hollow cylinder segment, and at least one second arm section (41b) extending from said second thruster tunnel segment (20b), wherein

- said second element (100b) is arranged relative said first element (100a) so that said first thruster tunnel segment (20a) and said second thruster tunnel segment (20b) constitutes a hollow cylinder (17), and wherein said first arm section (41a) is in contact with, and fastened to said respective second arm section (41b),

- said foldable element further comprising a propeller shaft (21) with one or more propellers (22), a drive shaft (15) perpendicular to said propeller shaft (21) and a gearbox (16) connecting said drive shaft (15) to said propeller shaft (21) arranged inside said hollow cylinder (17), wherein said drive shaft (15) is extending through an opening (12) in the wall of said a hollow cylinder (17).

9. The foldable element of claim 8 comprising an end plate (18) on each end of said cylinder (17) arranged for holding said first thruster tunnel segment (20a) and said second thruster tunnel segment (20b), wherein said end plates (18) are fixed relative each other.

10. The foldable element of claim 8 comprising one or more snap locks (19a) arranged on the edge of said first thruster tunnel segment (20a) with corresponding one or more snap lock counterparts (19b) arranged on the edge of said second thruster tunnel segment (20b).

11. The element of claim 8, wherein said first element (100a) and said second element (100b) are identical.

12. The element of claim 8, wherein said first element (100a) and said second element (100b) comprises two first arm sections (41a) and said opening (12) and said drive shaft (15) is arranged between said two first arm sections (41a).

13. The element of claim 8, wherein said first arm section (41a) and said second arm section (41b) comprises slits (42a, 42b) in a longitudinal direction of said first arm section (41a) and said second arm section (41b), and a reinforcing element (43), wherein said reinforcing element (43) is extending into said slits (42a, 42b), wherein said reinforcing element (43) comprises a hole with a centre, where said foldable element (100) is arranged to pivot about said centre.

14. The method of claim 13, wherein said reinforcing element (43) is arranged to be fixed to an axel parallel to said propeller axis (21) going through said hole (44), and wherein said axis is arranged to rotate to fold and unfold said foldable element (100).

Drawing