Technical field

The present description concerns a variable pitch propeller for vessels, in particular for sailing vessels, in which the propeller blades can be rotated to different positions depending on the use.

Background of the description

Document IT 1312215 describes a variable pitch propeller, which comprises a main body supporting a plurality of blades which can rotate with respect to the main body around radial axes thanks to a mechanical transmission, in particular comprising bevel gears, which connects the blades with a hub which can rotate in the main body between two stops and which can be keyed to the propeller shaft of a vessel, in particular of a sailing vessel. When the propeller is not engine driven, hydrodynamic forces acting on the blades drive the blades into a first feathered position to reduce the hydrodynamic drag of the propeller. When the propeller is driven by an engine, the blades are rotated by the hub by means of the mechanical transmission into a second position of forward thrust or a third position of reverse thrust, so as to orient the blades in their optimum position according to the direction of rotation of the propeller shaft. Therefore, the first position of the blades is substantially stable, since it is the one in which the blades are rotated by the hydrodynamic forces, while the second and third positions are substantially unstable, since the blades are kept in this position only due to the torque imparted by the engine to the propeller shaft. However, if the propeller is not connected directly to a traditional endothermic engine with a mechanical transmission, or with other systems that prevent rotation of the propeller shaft (brakes or other mechanical locking mechanisms), for example if the propeller is connected to the engine by means of a hydraulic transmission, the propeller shaft can rotate freely when the engine is stopped. In this case, the blades can rotate with the propeller shaft around the longitudinal axis of the propeller due to hydrodynamic drag, so that the blades do not return to the feathered position, thus limiting or even eliminating the advantages of this type of propeller. To overcome this technical problem, document EP 3683135 A1 describes a variable pitch propeller similar to the propeller of IT 1312215, in which however the hub also comprises a first sleeve which is mechanically coupled with a second sleeve by means of outer profiles with opposite surfaces which can slide over each other in such a way that a reciprocal rotation of the two sleeves about the longitudinal axis can cause their reciprocal axial movement, or vice versa. In particular, the second sleeve can slide in the main body along the longitudinal axis and is urged towards the first sleeve by elastic means. The outer profiles of the first sleeve and of the second sleeve are the same and are arranged on projections which protrude from the sleeves and develop around the longitudinal axis, wherein the projections of each sleeve have the same extension in the direction of the longitudinal axis. With this arrangement, when the engine is stopped, the elastic means urge the second sleeve towards the first sleeve, causing a reciprocal rotation of the two sleeves and the automatic arrangement of the blades in the first feathered position, even if the propeller shaft is in a neutral position, while when the engine is operated in the forward or reverse direction, the torque acting on the hub causes the two sleeves to move apart, overcoming the force of the elastic means, so as to arrange the blades in a second position of forward thrust or a third position of reverse thrust. Therefore, in this known propeller, the first position of the blades is substantially stable, since it is maintained by the elastic means which urge the sleeves against each other, while the second position and the third position are substantially unstable, since they are maintained dynamically only by the torque generated by the engine which acts on the hub, overcoming the force of the elastic means.

However, the propellers described in IT 1312215 and EP 3683135 A1 cannot also be used to transmit a torque from the propeller to the propeller shaft, for example to exploit the motion of a sail vessel to generate current by means of an electric current generator connected to the propeller shaft, since the blades of these known propellers tend to move automatically to the first feathered position when the propeller shaft is free to rotate or is not driven by an engine.

Summary of the description The object of the present description is therefore to provide a propeller free from these problems. Said object is achieved with a propeller, the main features of which are specified in the attached claims, to be considered an integral part of the present description.

Thanks to the particular outer profile of the sleeves, the propeller according to the present description allows to automatically arrange the blades in a first position and a third position which are substantially stable. In the first and third position of the blades, the sleeves are urged against each other by elastic means, while the second position of the blades is substantially unstable as it is maintained by the torque of the engine which acts on the hub and overcomes the force of the elastic means. Therefore, in the first position the blades can be arranged in a feathered position, while in the third position the blades are arranged in a substantially stable manner in the forward or reverse thrust position, so that in this third position it is possible to transmit a torque not only from the propeller shaft to the propeller but also vice versa, for example to generate electric current through the movement of the vessel by exploiting the hydrodynamic forces acting on the propeller.

Therefore, the propeller according to the present description allows at least two substantially stable positions of the blades, while the propeller of EP 3683135 A1 allows only one substantially stable position and the propeller of IT 1312215 does not allow any substantially stable position.

The outer profile of the sleeves is preferably arranged on one or more series of projections which are substantially the same and/or which include particular helical surfaces and/or substantially flat surfaces, so as to optimize not only the transmission of torques from the propeller shaft to the propeller, and vice versa, but also the transition from the first position to the third position of the blades, and vice versa, by varying the torque acting on the propeller shaft and/or on the blades.

The mechanical transmission which transmits the motion of the hub to the blades comprises in particular bevel gears, so as to make the interaction more fluid not only between the blades and the hub but also between the sleeves, which are preferably not included in this mechanical transmission.

The propeller shaft can be fixed to the hub quickly and simply by means of a particular nut which can act as a guide for the sliding of a sleeve on its outer surface.

The blades preferably have particular shapes and edges in order not only to optimize the forward and reverse thrust of the propeller and to reduce the hydrodynamic drag in the feathered position of the blades but also to optimize the transmission of a torque from the propeller to the the propeller shaft when the blades are in the third position.

Brief description of the drawings

Further advantages and features of the propeller according to the present description will become apparent to those skilled in the art from the following detailed description of some embodiments, to be considered non-limiting examples of the claims, with reference to the attached drawings in which: figure 1 is a perspective view of the propeller in a first position of the blades; figure 2 is a first exploded view of the propeller of figure 1 ; figure 3 is a second exploded view of the propeller of figure 1 ; figure 4 is a side view of the propeller of figure 1 ; figure 5 is a rear view of the propeller of figure 1 ; figure 6 is a bottom view of the propeller of figure 1 ; figure 7 is a top view of the propeller of figure 1 ; figure 8 is the enlarged section A-A of figure 5; figure 9 is the enlarged section B-B of figure 6; figure 10 is the enlarged section C-C of figure 7; figure 11 is the partial detail XI of figure 6; figure 12 is a partial perspective view of the sleeves of the propeller of figure 1 ; figure 13 is the partial detail XIII of figure 6; figure 14 is the partial detail XIV of figure 7; figure 15 is a perspective view of the propeller in a second position of the blades; figure 16 is a bottom view of the propeller of figure 15; figure 17 is a top view of the propeller of figure 15; figure 18 is the enlarged section A-A of figure 15; figure 19 is the enlarged section B-B of figure 16; figure 20 is the enlarged section C-C of figure 17; figure 21 is the partial detail XXI of figure 16; figure 22 is a partial perspective view of the sleeves of the propeller of figure 16; figure 23 is the partial detail XXIII of figure 16; figure 24 is the partial detail XXIV of figure 17; figure 25 is a perspective view of the propeller in a third blade position; figure 26 is a bottom view of the propeller of figure 25; figure 27 is a top view of the propeller of figure 25; figure 28 is the enlarged section A-A of figure 25; figure 29 is the enlarged section B-B of figure 26; figure 30 is the enlarged section C-C of figure 27; figure 31 is the partial detail XXXI of figure 26; figure 32 is a partial perspective view of the sleeves of the propeller of figure 26; figure 33 is the partial detail XXXIII of figure 26; figure 34 is the partial detail XXXIV of figure 27.

Exemplary embodiments

Figures 1 -14 show a first embodiment of the propeller according to the present description in a first position which can correspond to the feathered position, namely with the blades oriented in the position of minimum hydrodynamic drag. This propeller comprises a main body 1 which is suitable to be fixed to a propeller shaft 2 to rotate around a longitudinal axis A and which supports a plurality of blades 3 which protrude in a radial direction from the main body 1 and can rotate with respect to the main body 1 around a radial axis R which forms an angle of 45°-90° with the longitudinal axis A, in particular which is substantially perpendicular to the longitudinal axis A of the main body 1 . The present embodiment of the propeller preferably comprises three blades 3 with radial axes R mutually arranged to form angles of 120° with each other. Preferably, the main body 1 has a substantially ogival shape tapering towards its rear end. With particular reference to figures 4-5, the blades 3 may comprise a base portion 3a which is connected to the main body 1 and a main portion 3b which extends outwards from the base portion 3a and has an outer edge which corresponds substantially to the outer edge of the blade 3. Preferably, the main portion 3b of the blades 3 is substantially flat, namely has a cross section which does not have significant variations in curvature or thickness except for a tapering towards its outer edge. Furthermore, the outer edge of the main portion 3b of the blade 3 preferably comprises a leading portion 3c which has a substantially convex outline and a trailing portion 3d which has a substantially concave outline. The leading portion 3c of the outer edge of the main portion 3b of the blade 3 is longer than the trailing portion 3d and/or has a radius of curvature which decreases towards the inflection point 3e between the two portions 3c, 3d. Preferably, the tangent lines t1 arranged along the leading portion 3c of the outer edge of the main portion 3b of the blade 3 cover an angle greater than 200°, in particular greater than 220°, while the tangent lines t2 arranged along the trailing portion 3d cover an angle less than 60°, in particular less than 30°. Preferably, the blade 3 is substantially symmetrical with respect to a plane of symmetry (corresponding to plane A-A of figure 5). In the first position of the blades 3, the leading portion 3c of the outer edge of the main portion 3b of the blade 3 faces forward, namely towards the propeller shaft 2 and/or the bow of the vessel, and the trailing portion 3d faces backward, while the plane of symmetry is substantially parallel to the longitudinal axis A. Still in the first position of the blades 3, the chord line C of the blades 3 is inclined with a pitch angle b of approximately 90°, namely is substantially parallel to the longitudinal axis A.

In other embodiments, the blades 3 may not be flat but have a profile of a different type, symmetrical or asymmetrical, for example a wing, Naca, B.Troost, Ogival, Aerofoil or Crescent profile, in order to improve the Venturi effect.

The main body 1 may comprise a front portion 4, namely suitable to face the propeller shaft 2, and a rear portion 5, which are joined together by axial screws 6. A terminal 7 may be joined to the rear portion 5 by means of a coaxial screw 8 screwed into a coaxial hole 9 of a threaded plug 10 in turn screwed into a coaxial hole 1 1 of the rear portion 5. A hub 12 is arranged in the main body 1 , in particular mainly in the front portion 4, and can rotate with respect to the main body 1 around the longitudinal axis A through an angle h of less than 360°, namely the hub 12 can rotate less than one full turn around the longitudinal axis A in the main body 1. The hub 12 is suitable to be keyed to the propeller shaft 2, for example by means of a keyed conical coupling.

A nut 13, in particular having a substantially cylindrical outer surface, is arranged in the main body 1 , in particular mainly in the rear portion 5 of the main body 1 , and can rotate with respect to the main body 1 and to the hub 12 around the longitudinal axis A when the propeller shaft 2 is not fixed to the hub 12. The nut 13 may be provided with an inner thread 14 for screwing the nut 13 onto an outer thread of the propeller shaft 2, thus fixing the propeller shaft 2 to the hub 12, and vice versa. A toothed washer 15 may be arranged between the hub 12 and the nut 13 to prevent inadvertent reciprocal rotations. The nut 13 can be screwed to the propeller shaft 2 or unscrewed from the propeller shaft 2 by removing the terminal 7 and the threaded plug 10, inserting a tool, for example a hex key, through the coaxial hole 1 1 of the rear portion 5, coupling the tool with the rear end of the nut 13 opposite the propeller shaft 2, for example by means of a form-fit with a shaped axial hole 16 of the nut 13, and by rotating the nut 13 with respect to the propeller shaft 2 and to the hub 12 by means of the tool.

The hub 12 is connected to the blades 3 by a mechanical transmission, so that a relative rotation of the hub 12 with respect to the main body 1 around the longitudinal axis A can cause a rotation of the blades 3 around their radial axis R. Preferably, said mechanical transmission comprises bevel gears 17, 18, wherein the hub 12 comprises a first bevel gear 17, in particular made in one piece with the hub 12. The first bevel gear 17 is coaxial with the hub 12, namely with the longitudinal axis A, and is meshed with second bevel gears 18 joined to radial shafts 19, in particular made in one piece with one of the second bevel gears 18. Each blade 3 can be keyed to each radial shaft 19. The radial shafts 19 are suitable to rotate around the radial axes R in seats 20 which are obtained in the main body 1 . The seats 20 can be provided with bushings 21 and/or bearings to reduce the friction between the radial shafts 19 and the main body 1 . Preferably, the seats 20 have a substantially semi-cylindrical shape and are obtained along adjacent edges of the front portion 4 and of the rear portion 5 of the main body 1 , so that the radial shafts 19 are arranged between the front portion 4 and the rear portion 5 of the main body 1 . The bushings 21 can also have a substantially semi-cylindrical shape.

The hub 12 comprises a first sleeve 22, in particular joined in a substantially coaxial manner to the hub 12 by means of radial set screws 23. The first sleeve 22 can be arranged in a substantially coaxial manner between the first bevel gear 17 and the nut 13. The nut 13 may be provided with an annular relief 24 arranged in an annular groove 25 of the first sleeve 22, so that the nut 13 can rotate about the longitudinal axis A and move along the longitudinal axis A only within the annular groove 25 of the first sleeve 22.

The first sleeve 22 is mechanically coupled with a second sleeve 26, arranged substantially coaxially to the first sleeve 22, by means of outer profiles 27, 28 with opposite surfaces which can slide over each other so that a reciprocal rotation of the two sleeves 22, 26 about the longitudinal axis A can cause their reciprocal axial movement, or vice versa. In particular, the outer profile 27 of the first sleeve 22 develops along the rear edge of the first sleeve 22 and the outer profile 28 of the second sleeve 26 develops along the front edge of the second sleeve 26. In other embodiments, the mechanical transmission which connects the hub 12 to the blades 3 for the rotation of the blades 3 may also include the first sleeve 22 and/or the second sleeve 26 and/or may not include bevel gears.

The second sleeve 26 can slide along the longitudinal axis A, in particular on the outer surface of the nut 13, and/or is urged towards the first sleeve 22 by elastic means, in particular one or more helical springs, preferably three helical springs 29, 29', 29" which are arranged coaxially around the longitudinal axis A between the rear portion 5 of the main body 1 and the second sleeve 26. The thrust of the helical springs 29, 29', 29" allows to keep in contact with each other, slidingly, one or more opposite surfaces of the helical profiles 27, 28 of the first sleeve 22 and of the second sleeve 26. Preferably, the extensions of the helical springs 29, 29', 29" along the longitudinal axis A are inversely proportional to their diameter, while the second sleeve 26 and/or the main body 1 , in particular the rear end of the second sleeve 26 and/or the rear portion 5 of the main body 1 , comprise seats 31 , 32 with coaxial circular steps to accommodate one end of the helical springs 29, 29', 29" and compensate for their different lengths.

Preferably, the first sleeve 22 can rotate together with the hub 12 in the main body 1 around the longitudinal axis A but cannot slide longitudinally with respect to the main body 1 , while the second sleeve 26 can slide longitudinally in the main body 1 but cannot rotate with respect to the main body 1 around the longitudinal axis A, or vice versa in alternative embodiments. For this purpose, the second sleeve 26 may be provided with a prismatic extension 33 having a shape, for example with a substantially hexagonal outer profile, complementary to the shape of a prismatic cavity 34 of the main body 1 , in particular of the rear portion 5 of the main body 1 , so that the prismatic extension 33 can slide along the longitudinal axis A in the prismatic cavity 34 but cannot rotate with respect to the prismatic cavity 34. The prismatic extension 33 may be provided with axial holes to avoid overpressure in the main body 1 caused by the axial movement of the second sleeve 26 in the prismatic cavity 34.

With particular reference to figures 11 -12, the outer profiles 27, 28 of the first sleeve 22 and/or of the second sleeve 26 are arranged on one or more series of projections, in particular substantially cusp-shaped, which protrude from the sleeves 22, 26 and develop around the longitudinal axis A. In particular, the outer profiles 27, 28 of the first sleeve 22 and/or of the second sleeve 26 comprise two series of projections 27a, 28a, 27b, 28b which develop one after the another along approximately 180° around the longitudinal axis A. Each series of projections 27a, 27b and/or 28a, 28b of the first sleeve 22 and/or of the second sleeve 26 preferably comprises a first projection 27a or 28a and a second projection 27b or 28b, wherein the first projection 27a or 28a is more extended in the direction of the longitudinal axis A than the second projection 27b or 28b. The first projections 27a or 28a of a series are substantially the same, and/or the second projections 27b or 28b of a series are substantially the same. In particular, the outer profiles 27, 28 of the first sleeve 22 and of the second sleeve 26 are substantially the same. The first projection 27a, 28a of the sleeves 22, 26 has a helical surface 27c, 28c and/or a substantially flat surface 27d, 28d which is substantially parallel to the longitudinal axis A. Preferably, the first projection 27a or 28a is substantially triangle-shaped, in particular has the shape of right triangle. The second projection 27b, 28b has one or more helical surfaces 27e, 28e that extend less than the helical surface 27c, 28c of the first projection 27a or 28a. The second projection 27b, 28b may also have a substantially flat surface 27f, 28f which is substantially perpendicular to the longitudinal axis A and is arranged between the two helical surfaces 27e, 28e. Preferably, the second projection 27b, 28b has a substantially trapezoidal shape, in particular a shape of an isosceles trapezium. In the first position of the blades 3, the helical surfaces 27c, 28c of the first projections 27a, 28a are in contact with each other and are not in contact with the helical surfaces 27e, 28e of the second projections 27b, 28b. In particular, with respect to a plane P passing through the longitudinal axis A, the helical surfaces 27c, 28c and/or 27e, 28e of the first projection 27a, 28a and/or of the second projection 27b, 28b develop at an angle a of 35°-55° and/or the substantially flat surface 27d, 28d of the first projection 27a, 28a develops at an angle of 0°-10°, in particular it lies on said plane P. For simplicity, figures 1 1 -12 show only the intersection lines of some planes P with the outer edge of the outer profiles 27, 28.

The hub 12 is locked axially with respect to the main body 1 by a locking ring 35 which is screwed onto a threaded portion 36 of the hub 12 and is in turn locked by a locking screw 37. An adjustment dial 38 is arranged around the main body 1 , in particular around its front portion 4, and can rotate with respect to the main body 1 to move along the longitudinal axis A a mobile stop 39 which is provided with an outer thread engaged with an inner thread of the adjustment dial 38 The rotation of the adjustment dial 38 can be locked for example by a radial or axial screw, in particular an axial screw 6 which differs from the other axial screws 6 in that it is provided with an extension 6a which is arranged in an axial hole which passes through the front portion 4 and into an axial hole of a series of axial holes 38a arranged along the rear edge of the adjustment dial 38. The adjustment dial 38 further includes a graduated scale 38b to show its setting position, namely the pitch angle b of the blades 3 in the second position, for example the forward thrust position. The mobile stop 39 can slide axially in an opening

40 formed in the main body 1 , in particular in its front portion 4, to limit the rotation of the hub 12 in the main body 1 up to the contact between the mobile stop 39 and a tooth

41 which protrudes from the hub 12, so as to limit the rotation of the blades 3 with respect to the main body 1 in the forward thrust position. The adjustment dial 38 is locked axially by a spacer ring 42 and a locking ring 43. A bushing 44 can be arranged between the locking nut 35 and the adjustment dial 38. One or more bushings 45 can also be arranged between the hub 12 and the main body 1. A fixed stop 46 can be arranged in the main body 1 , in particular in its front portion 4, to further limit the angle h of the relative rotation of the tooth 41 of the hub 12 in the main body 1 , namely limiting the rotation of the blades 3 with respect to the main body 1 in the third position, in particular the reverse thrust position.

The main body 1 , in particular its front portion 4, may comprise an auxiliary opening 47 to facilitate its manufacture, in particular to obtain the fixed stop 46 and a channel in which the tooth 41 can slide. The tooth 41 of the hub 12 may comprise a helical surface suitable for coming into contact with the mobile stop 39 and a flat surface suitable for coming into contact with the fixed stop 46. In other embodiments, the tooth 41 of the hub 12 can rotate between two fixed stop or between two mobile stops.

Figures 15-24 show the propeller in a second position which may correspond to the forward thrust position, namely with the blades 3 oriented with a pitch angle b in which the propeller supplies a forward hydrodynamic thrust when the shaft propeller 2 is rotated by an engine. When started, the engine generates a torque on the propeller shaft 2, whereby an opposite resistant torque is generated on the propeller due to the effect of the hydrodynamic force acting on the surfaces of the blades 3 initially arranged in the first position. This resistant torque, combined with the inertia of the blades 3, allow the main body 1 to remain almost stationary for an instant during which the hub 12 rotates in the main body 1 until it reaches the stop position on the mobile limit switch 39 with the tooth 41 The rotation of the hub 12 in the main body 1 causes the rotation of the blades 3 due to the mechanical transmission 17, 18, as well as the relative rotation between the sleeves 22, 26, which in turn causes the axial displacement of the second sleeve 26 and the compression of the elastic means 29, 29', 29", due to the reciprocal sliding of the helical surfaces 27c, 28c of the first projections 27a, 28a of the sleeves 22, 26.

When the tooth 41 of the hub 12 reaches the stop position on the mobile stop 39, the hub 12, namely the propeller shaft 2 driven by the engine, begins to rotate the main body 1 around the longitudinal axis A. In this position the chord line C of the blades 3 is inclined with a pitch angle b, for example adjustable at 10°-40°, which is determined by the position of the mobile stop 39 in the main body 1 , in turn determined by the adjustment dial 38. In the second position of the blades 3, the helical surfaces 27c, 28c of the first projections 27a, 28a are still in contact with each other, albeit over a smaller area than in the first position, and are not in contact with the helical surfaces 27e, 28e of the second projections 27b, 28b.

When the engine is switched off, the hydrodynamic force acting on the blades 3 and/or the elastic means 29, 29', 29" urging the second sleeve 26 towards the first sleeve 22 rotate the hub 12 in the main body 1 , so as to automatically return the blades 3 from the second position to the first position shown in figures 1 -14.

Figures 25-34 show the propeller in a third position which may correspond to the reverse thrust position, namely with the blades 3 oriented with a pitch angle b in which the propeller provides a reverse hydrodynamic thrust when the propeller shaft 2 is rotated by the engine in the direction opposite to forward travel. When rotated in the opposite direction, the engine always generates a driving torque on the propeller shaft 2, whereby an opposite resistant torque is generated on the propeller due to the effect of the hydrodynamic force acting on the surfaces of the blades 3 initially arranged in the first position. This resistant torque, combined with the inertia of the blades 3, allow the main body 1 to remain almost stationary for an instant during which the hub 12 rotates in the main body 1 until it reaches the stop position on the fixed stop 46 with the tooth 41 The rotation of the hub 12 in the main body 1 causes the rotation of the blades 3 due to the mechanical transmission 17, 18, as well as the relative rotation between the sleeves 22, 26, which in turn causes at first the axial displacement of the second sleeve 26 and the compression of the elastic means 29, 29', 29", due to the sliding of the first projections 27a, 28a of the sleeves 22, 26 along the helical surfaces 27e, 28e of the second projections 27b, 28b of the sleeves 22, 26. However, at a later time, after the tips of the first projections 27a, 28a of the sleeves 22, 26 have passed the helical surfaces 27e, 28e of the second projections 27b, 28b of the sleeves 22, 26, the tips of the first projections 27a, 28a can slide along the substantially flat surfaces 27f, 28f, if present, of the second projections 27b, 28b of the sleeves 22, 26 and reach a third position, shown in the figures, in which the helical surfaces 27c, 28c of the first projections 27a, 28a are in contact with the helical surfaces 27e, 28e of the second projections 27b, 28b and/or the substantially flat surfaces 27d, 28d of the first projections 27a, 28a are in contact with each other, so as to stabilize and restore the third position in the event of a perturbation outside of the propeller. In the third position, the first projections 27a, 28a and the second projections 27b, 28b of the sleeves 22, 26 are urged against each other by the elastic means 29, 29', 29" in a substantially stable coupling capable of maintaining the propeller in this position even if contrasted by a relatively low torque, for example caused by the opposition of an electric current generator connected to the propeller through the hub 12 and the propeller shaft 2 to generate electric current thanks to the rotation of the propeller caused by the hydrodynamic forces acting on the blades 3 in the third position.

To unlock the blades 3 from the third position and return them to the first position, it is sufficient to activate the engine in forward gear for a few seconds so as to impart to the first projections 27a, 28a of the sleeves 22, 26 a torque sufficient to overcome the second projections 27b, 28b, overcoming the force of the elastic means 29, 29', 29". If the engine is kept in forward gear, the blades 3 pass from the first position to the second position in the manner described above, to then return to the first position when the engine is switched off.

Therefore, the mutual interaction of the outer profiles 27, 28 of the sleeves 22, 26 determines a first position of the blades 3, for example corresponding to the feathered position of the blades 3, and a second position of the blades 3, for example corresponding to the forward thrust of the blades 3, which is obtained by imparting a torque to the hub 12 in a first direction of rotation around the longitudinal axis A, for example by means of an engine connected to the propeller shaft 2. The first position is more stable than the second position since the elastic means 29, 29', 29" tend to bring the blades 3 from the second position to the first position when the torque applied to the hub 12 is zero or less than that generated by the elastic means 29, 29', 29" acting on the second sleeve 26. Thus, the blades 3 are automatically moved from the second position to the first position when torque is not applied to the hub 12, as occurs in the propeller of EP 3683135 A1 . However, the mutual interaction of the outer profiles 27, 28 of the sleeves 22, 26 can also determine the third position of the blades 3, for example corresponding to the reverse thrust of the blades 3. The third position is obtained by imparting a torque to the hub 12 in a second direction of rotation which is opposite to the first direction of rotation and which allows the first projections 27a, 28a of the sleeves 22, 26 to overcome the second projections 27b, 28b during their mutual rotation, overcoming the torque generated by the elastic means 29, 29', 29" acting on the second sleeve 26. The blades 3 therefore do not automatically pass from the third position to the first position, given that the elastic means 29, 29', 29" tend to keep the sleeves 22, 26 in the third position, which is therefore relatively stable like the first position, but by imparting a torque to the hub 12 in the first direction of rotation around the longitudinal axis A. In an alternative embodiment, the second position may correspond to the reverse thrust of the blades 3 and the third position can correspond to the forward thrust of the blades 3. Other embodiments can include further projections and/or further helical surfaces to obtain further substantially stable and/or unstable positions of the blades 3.

Variants or additions may be made by those skilled in the art to the embodiments described and illustrated herein while remaining within the scope of the following claims. In particular, further embodiments may include the technical features of one of the following claims with the addition of one or more technical features described in the text or illustrated in the drawings, taken individually or in any reciprocal combination and including their equivalent features.

Furthermore, the terms "a/an/one", "two", etc. in the description and claims respectively mean "at least one", "at least two", etc., unless otherwise specified. Similarly, angles, proportions and values mentioned in the specification and/or shown in the drawings include a tolerance of at least 5%, unless otherwise specified.