Description

Propulsion unit comprising two coaxial contra-rotating propellers

Technical Field

The present invention relates to a propulsion unit.

In particular, the present invention relates to a propulsion unit for boats and the like, comprising two coaxial contra-rotating propellers.

Background Art

In the technical sector of propulsion for nautical and/or aeronautical vehicles the use is known of propellers which rotate appropriately in the fluid, causing the vehicle to move, the vehicle referred to hereinafter by way of example and without limiting the scope of the invention being a boat.

Many forms of propulsion units are also known, including, in particular in the naval sector, those equipped with two coaxial propellers positioned on contra- rotating shafts. Such propulsion units, although designed to overcome some limitations of single propeller propulsion units - because they can reduce its diameter and tip speed, with a consequent reduction of the centrifugal forces, the boat reaction torques and the overall weight - have a significant limitation in terms of application, since they are suitable for operation with optimum performance only in a specific and narrow range of boat speeds.

Departing from said range of speeds, even by a small amount, greatly reduces the overall performance of the propulsion unit.

Disclosure of the Invention

Therefore, a propulsion unit with two contra-rotating propellers is required, in particular for boats, which is suitable for maintaining high performance levels at a wide range of speeds, keeping the advantages of reduced radial dimensions and weight compared with the solution with a single propeller.

Said propulsion unit must also be compact, easy and economical to make and assemble and easy to install by any user using normal standardised connecting means, as well as easily accessible for normal maintenance operations.

Said results are achieved in accordance with the present invention by a propulsion unit, in particular for boats or the like, comprising two propellers, respectively front and rear, coaxial and contra-rotating, rotationally driven by the shaft of at least a main motor through a motion transmission where the rear propeller rotates at a speed of rotation different to the speed of rotation of the front propeller.

Brief Description of the Drawings

Further details are provided in the following description of a non-limiting example of an embodiment of the present invention with reference to the accompanying drawings, in which: Figure 1 is a schematic side view of a propulsion unit in accordance with the present invention applied to the bottom of a boat;

Figure 2 is a schematic cross-section according to the plane II - II of the front part of the propulsion unit of Figure 1;

Figure 3 is a schematic cross-section according to the plane IH - HI of the rear part of the propulsion unit of Figure 1;

Figure 4 is a schematic cross-section similar to that of Figure 2 of only the part which controls rotation of the two propellers;

Figure 5 is a schematic cross-section according to the plane V - V of Figure 4; Figure 6 is a schematic cross-section similar to that of Figure 2 of only the part which controls the variation in the pitch of the two propellers, and

Figure 7 is a schematic cross-section according to the plane VII - VII of Figure 6.

Detailed description of the preferred embodiments of the invention

Figures 1 and 2 show, for a convenient description only, without having a limiting effect, a set of three axes, with directions respectively longitudinal X - X, transversal Y - Y and vertical Z - Z, as well as a front part corresponding to the bow of the boat and a rear part opposite the previous part. The propulsion unit P according to the present invention is applied to the bottom of a boat 1 and comprises a nose-shaped housing 100 (Figures 1 and 3). Inside the housing 100 there is a support 110 comprising a circular outer wall 111 fixed to the bottom Ia of the boat, by suitable means, and a circular inner wall 112. The inner wall 112 supports bearings 113 on whose inner thrust block there is axially inserted the hub 114 of a longitudinal splined shaft 210, hollow on the inside and axially held in position by a lock nut 115, easily accessible from the rear part of the housing 100.

The longitudinal shaft 210 is part of the chain for transmission of motion to two propellers, respectively a front propeller 300 (that is to say, the first to encounter the fluid in the direction of travel, indicated by the arrow Fl) and a rear propeller 400 (that is to say, the second to encounter the fluid in the direction of travel), positioned at the front of the propulsion unit P.

As illustrated in Figure 3, the chain for transmission of motion comprises a bevel gear 220 positioned in front of the support 110 and designed for connection with the vertical shaft 2a of the main motor of the boat 1. The motor is schematically illustrated with a block 2 in Figure 1. In this way, rotation of the vertical shaft 2a causes rotation of the longitudinal shaft 210 with a number of revolutions controlled by the on board instruments, which are of the conventional type and therefore not illustrated or described in detail.

The longitudinal shaft 210 and the bevel gear 220 form, for the propulsion unit P, respective means 200 for mechanical transmission of motion from the motor 2 to the propellers 300, 400.

With reference to Figures 2 and 4, the front propeller 300 comprises a hub 310 on which the blades 301 are mounted and secured by screws 301a. In this

way, advantageously the blades can be produced as a single piece with high precision technologies and applied and/or substituted very easily. The hub 310 has a spherical extension 311, facing inwards, whose function is described below, and is integral with a rotor 320, in turn connected to a tube 321 connected, by a key 211, to the driving shaft 210.

Therefore, the propeller 300 rotates about its own axis Al with the same number of revolutions and with the same direction of rotation as the longitudinal shaft 210.

With reference to Figures 2 and 4, the rear propeller 400 in turn comprises a hub 410 on which the blades 401 are mounted and secured by screws 401a. The hub 410 has a spherical extension 411, facing inwards, whose function is described below, and is integral with a rotor 420, in turn connected to a ring gear

421 in an epicyclic gear train 430.

As is also illustrated in Figure 5, the epicyclic gear train 430 comprises a ring gear with teeth on the inside 421, integral with the rotor 420, a gear with teeth on the outside 436, coaxial with the ring gear 421 and integral with a hub 221 in turn integral with the bevel gear 220 and the shaft 210, and a plurality of planet gears 433 inserted between the ring gear 421 and the gear 436. The planet gears

433, consisting of idle gears 434, rotate on respective shafts 433a supported by a planet carrier or spider 431. The spider 431 is fixed relative to the axis of rotation

Al of the longitudinal shaft 210, being stably connected to the circular outer wall

111 by an annular portion 437 not described in detail.

The shafts 433a of the gears 434 are mounted cantilever-style on the spider 431 and are connected to one another, at respective outer ends, by a ring 432 coaxial with the axis Al.

The gears 434 forming the planet gears 433 rotate about their own shafts 433a.

According to the preferred, non-limiting embodiment illustrated, the gear train 430 is only apparently epicyclic because the shafts 433a of the planet gears 433 are stationary relative to the central axis Al of the gear train. Therefore, it is

an ordinary gear train. In spite of this, for the sake of clarity, hereinafter said gear train 430 is referred to as epicyclic.

In detail, with reference to Figures 2 and 4, the spider 431 is mounted on a bearing 435 keyed on a hub 221, integral with the bevel gear 220 and the longitudinal shaft 210, so that it is stationary relative to the rotary shaft 210. The planet gears 433, meshing with the gear 436 integral with the shaft 210, are rotationally driven about their own axes and, also meshing with the ring gear with teeth on the inside 421, transmit to the latter a rotating motion which is opposite to the rotating motion of the gear 436. Since the ring gear 421 is integral with the rotor 420 of the rear propeller 400, the latter is also rotationally driven. The idle gears 434 forming the planet gears 433 therefore cause an inversion of the direction of rotation of the propeller 400 relative to the direction of rotation of the longitudinal shaft 210.

The epicyclic gear train 430 may be sized to give a reduction or multiplication ratio of the motion of the longitudinal shaft 210 to the rear propeller 400 which therefore rotates with a number of revolutions that is lower or higher, but in any case different, compared with that of the front propeller 300 and in the opposite direction to the latter which is driven directly by the longitudinal shaft 210. According to preferred embodiments of the present invention, the number

NpA of blades 301 of the front propeller 300 is greater than the number NpP of blades 401 of the rear propeller 400 and the epicyclic gear train 430 reduction ratio R is equal to the ratio of the number of front and rear blades

R = NpA / NpP thus producing a propulsion unit P with coaxial propellers contra-rotating at different speeds.

According to preferred embodiments of the propulsion unit P, the gear ratio between the speeds of the two contra-rotating propellers 300, 400 is equal to the ratio of the number of blades on the front and rear propellers. For example, according to values one after another of 3/2, 5/3, 7/5, 9/5, 11/7, 13/9, 15/11 with

an increase in the diameter of the propellers, in this way causing an improved propulsive performance because the tangential increase in the speed of the rear propeller 400 is reduced.

In addition to this, the configuration described above results in a reduction in the vibrations and noise and a counter-balancing of the torques around the propeller.

Moreover, the chord CP of the rear propeller 400 blades 401 is much smaller than the chord CA of the front propeller 300.

The average distance DA between the front propeller blades is preferably the same as the distance DP between the rear propeller blades, with a ratio of between

0.8 and 1.2, giving a hydrodynamic condition designed to avoid propeller suffocation, also delaying the appearance of cavitation phenomena even for very high fluid current speeds.

With reference to Figures 2, 6 and 7, according to the present invention, the propulsion unit also comprises means 500 for moving and controlling the pitch of the blades 301, 401 of the respective propellers 300, 400.

In more detail, said means 500 comprise a front ring 510 and a rear ring 520 with vertical axis, having respective spherical slots 511, 521 designed to house the above-mentioned spherical extensions 411, 511 of the two hubs 310, 410 of the propellers 300, 400. The rings 510, 520 are connected to a respective disk 512,

522 with longitudinal axis, rotating with the respective hub and they are connected to one another in the longitudinal direction by tie rods 530. The rear end 530a of each tie rod 530 is integral with a drum 531 connected to a transversal control element 532 in turn integral with a screw 533, coaxially inserted in the longitudinal shaft 210 and engaged on a female screw 534 axially fixed and connected to a front driving motor 535.

In this way, the rotation of the motor 535 in one direction or the other causes the corresponding rotation of the female screw 534, which forces the screw 533 to rotate, resulting in the movement of the transversal element 532 in one direction or the other and therefore axial movement of the disks 511, 521 in the same

direction along the axis Al. The axial movement of the disks 511, 521 forces the blades 301, 401 of the respective propeller 300, 400 to rotate through the ball joint

311, 511 and 411, 521.

According to a preferred embodiment of the present invention, the motor 535 comprises a stepping driving part 535a and an epicyclic transmission unit

535b, so that the drive unit provides great precision and compact dimensions.

Therefore, the propulsion unit disclosed allows high performance, reduced overall dimensions and user-friendliness. In particular, the possibility of inserting the tapered shaft 210 longitudinally, securing it at the rear with a lock nut 115, allows very easy and rapid maintenance, since the entire unit can be removed from the nose-shaped housing and substituted with a replacement unit, without the risk of inaccuracy, and performing any maintenance work necessary on the replaced unit in the workshop.

According to another embodiment of the propulsion unit P disclosed, in the rear part of the nose-shaped housing 100 there is a distributor 600 for fluid with a low friction coefficient, shown in Figure 3. This is pumped through the cavity

210a in the longitudinal shaft 210 to the front part of the nose-shaped housing where it is dispensed to the outside through pipes 610, shown in Figure 2, which direct the fluid onto the nose-shaped housing, reducing its liquid friction to the flow of the fluid in which it is immersed and allowing the boat to reach very high speeds.

For example, the propulsion unit P may give performance that is substantially constant, or in any case very high, for a range of speeds between 5 and 75 knots. The cavity 210a in the longitudinal shaft 210 can also house the electrical cables, not illustrated, used to supply power to the motor which drives the variation in the pitch of the blades.

The propulsion unit P disclosed can also rotate through 360° about the vertical axis Z - Z of the boat main motor 2 shaft 2a. This means that with a suitable control unit the propulsion unit P can allow the boat 1 to be manoeuvred,

with consequent elimination of the rudder and connected linkages which, as well as being additional parts, are also immersed obstructions which may hinder the best positioning of the propellers.

Moreover, the two propellers, front and rear, may rotate at different speeds, thanks to the use of respective independent motors, preferably located in the housing 100.