Marine tunnel thruster

1. A marine thruster comprising:

a tubular duct ( 1 ) with a propeller ( 2 ) fitted therein which is operatively connected to a drive system allowing the propeller to be rotated about an axis parallel to a longitudinal axis of said duct ( 1 ),

wherein said tubular duct ( 1 ) is composed of three sections comprising a first central section ( 12 ) extending between two end sections ( 11 , 13 ), said first central section ( 12 ) housing said propeller ( 2 ) and having a first axial length (L) and a first diameter (A), the two end sections ( 11 , 13 ) having a second axial length (M) and a second diameter (B) which is greater than the first diameter (A) of the central section ( 12 ), said end sections ( 11 , 13 ) being connected to said central section ( 12 ) by a radial annular enlargement ( 132 ) having an inner surface that is inclined by at least 30° in relation to the longitudinal axis of the duct, and

wherein the two end sections ( 11 , 13 ) are connected to a first end side ( 121 ) and to a second end side ( 123 ) of the first central section ( 12 ) respectively by an annular connecting surface ( 132 ) causing an increase in diameter in a direction of a longitudinal axis of the duct ( 1 ), and

wherein a length of the annular connecting surface is defined by:

a =k ( B −A )

where:

a=the length of the annular connecting surface,

k=1 if the annular connecting surface has a lateral profile shaped as a quarter of an ellipse having a 2:1 ration of major and minor diameters, otherwise k=0.04−4.0.

2. The marine thruster according claim 1 , wherein the second diameter (B) of each of said two end sections ( 11 , 13 ) is a function of the first diameter (A) of said central section ( 12 ).

3. The marine thruster according to claim 1 , wherein the second diameter (B) of each of said two end sections ( 11 , 13 ) has a value falling within a range defined by a size of the first diameter (A) of said central section ( 12 ) multiplied by a first factor defining a limit of said range and multiplied by a second factor defining another limit of the range, said first factor and said second factor being defined according to a length of said duct ( 1 ) or according to a velocity of a fluid flowing into said duct ( 1 ).

4. The marine thruster according to claim 1 , wherein said annular connecting surface is shaped as a step connection, wherein an annular enlargement connecting the end sides ( 121 , 123 ) of the central section ( 12 ) and of the corresponding end section ( 11 , 13 ) is an annular surface perfectly radial and perpendicular to the longitudinal axis of the tubular duct ( 1 ).

5. The marine thruster according to claim 1 , wherein said annular connecting surface is shaped as a step connection, said step connection having a rectangular shape with reference to a sectional view taken along a diametric plane, and end side of said central section ( 12 ) being connected to one of the ends of each of said two end sections ( 11 , 13 ) by two surfaces perpendicular to each other.

6. The marine thruster according to claim 1 , wherein said annular connecting surface is a frustum conical surface.

7. The marine thruster according to claim 1 , wherein the annular connecting surface is curved and has a concavity faced towards an inner or outer side of the tubular duct ( 1 ).

8. The marine thruster according to claim 1 , wherein said tubular duct ( 1 ) is provided within a hull ( 3 ) of a marine floating working system or a boat ( 4 ) or a vessel, said two end sections ( 11 , 13 ) coming out from the hull ( 3 ) at sides of the hull, through two respective apertures ( 31 , 32 ) provided in said sides, said apertures ( 31 , 32 ) coinciding with the end sides of the two end sections ( 11 , 13 ).

9. The marine thruster according to claim 1 , wherein said tubular duct ( 1 ) is provided into a hull ( 3 ) of a boat ( 4 ) or a vessel, said duct ( 1 ) being fitted to be oriented with its longitudinal axis transversely to the fore-and-aft axis of said boat ( 4 ) or vessel.

10. The marine thruster according to claim 1 , wherein said tubular duct ( 1 ) has one or more fixed vanes ( 5 ) fitted therein, said vanes having an airfoil section and an elongated shape, each vane ( 5 ) being arranged radially to the longitudinal axis of said duct ( 1 ), with a first end connected to an inner surface of said duct ( 1 ), while a second end radially faces the longitudinal axis of said duct ( 1 .

11. A marine thruster comprising:

a tubular duct ( 1 ) with a propeller ( 2 ) fitted therein which is operatively connected to a drive system allowing the propeller to be rotated about an axis parallel to a longitudinal axis of said duct ( 1 ),

wherein said tubular duct ( 1 ) is composed of three sections comprising a first central section ( 12 ) extending between two end sections ( 11 , 13 ), said first central section ( 12 ) housing said propeller ( 2 ) and having a first axial length (L) and a first diameter (A), the two end sections ( 11 , 13 ) having a second axial length (M) and a second diameter (B) which is greater than the first diameter (A) of the central section ( 12 ), said end sections ( 11 , 13 ) being connected to said central section ( 12 ) by a radial annular enlargement ( 132 ) having an inner surface that is inclined by at least 30° in relation to the longitudinal axis of the duct,

wherein said tubular duct ( 1 ) is provided into a hull ( 3 ) of a marine floating working vehicle, a boat ( 4 ) or a vessel, said duct ( 1 ) extending longitudinally to define an additional duct ( 7 ) having a longitudinal axis coinciding with or parallel to a fore-and-aft axis of the boat, said additional duct ending at the bow with an aperture ( 71 ),

wherein said additional duct ( 7 ) is connected to one of said two end sections ( 11 , 13 ) through a connecting surface causing an increase in diameter in a direction of a longitudinal axis of the duct ( 1 ), and

wherein a length of the connecting surface is defined by:

a=k ( B−A )

where:

a=the length of the connecting surface,

k=1 if the connecting surface has a lateral profile shaped as a quarter of an ellipse having a 2:1 ration of major and minor diameters, otherwise k=0.04−4.0.

12. The marine thruster according to claim 1 , wherein said tubular duct ( 1 ) has an outer covering shell ( 6 ), said covering shell ( 6 ) being connected to an outer surface of said tubular duct ( 1 ) by polymeric elements ( 61 ) of visco-elastic nature configured to absorb vibrations caused by the propeller.

13. A watercraft comprising:

at least a maneuvering propeller ( 2 ), housed in an intermediate location into a tubular duct ( 1 ) oriented with a specific inclination with respect to the fore-and-aft axis of said watercraft and at such a level that an axis of rotation of the propeller is below or at a waterline thereof,

wherein said tubular duct ( 1 ) is composed of three sections comprising a first central section ( 12 ) and two end sections ( 11 , 13 ), the first central section ( 12 ) having a specific length (L) and a specific diameter (A), and the two end sections ( 11 , 13 ) having a specific length (M) and a specific diameter (B), the specific diameter (B) of the two end sections being greater than the specific diameter (A) of the central section ( 12 ), said end sections ( 11 , 13 ) being connected to said central section ( 12 ) by a radial annular enlargement having a inner surface that is inclined by at least 30° in relation to a longitudinal axis of said tubular duct, and

wherein the two end sections ( 11 , 13 ) are connected to a first end side ( 121 ) and to a second end side ( 123 ) of the first central section ( 12 ) respectively by an annular connecting surface ( 132 ) causing an increase in diameter in a direction of the longitudinal axis of the duct ( 1 ), and

wherein a length of the annular connecting surface is defined by:

a=k ( B−A )

where:

a=the length of the annular connecting surface,

k=1 if the annular connecting surface has a lateral profile shaped as a quarter of an ellipse having a 2:1 ration of major and minor diameters, otherwise k=0.04−4.0.