EP2263936B1

EP2263936B1 - Rudder for ship

Info

Publication number: EP2263936B1
Application number: EP09008089A
Authority: EP
Inventors: Soo Ho Lee; Tae Ik Cho; Young Man Lee; Jae Ryu Bae; Young Bok Choi
Original assignee: Daewoo Shipbuilding and Marine Engineering Co Ltd
Current assignee: Hanwha Ocean Co Ltd
Priority date: 2009-06-17
Filing date: 2009-06-19
Publication date: 2012-05-02
Anticipated expiration: 2029-06-19
Also published as: ATE555981T1; EP2263936A1

Abstract

A ship rudder is disposed behind a propeller at a stem of a ship to control a movement direction of the ship. The ship rudder is divided into an upper blade and a lower blade on an axis of the propeller. Leading edge parts and trailing edge parts of the upper and lower blades are twisted at a predetermined twist angle on the axis of the propeller to be biased towards a rotational incoming flow induced from the propeller and to form an asymmetrically dislocated part. The leading edge parts of the upper and lower blades include a rudder bulb formed around the axis of the propeller to surround the asymmetrically dislocated part and including a thrust fin formed at either side of the rudder bulb. In the trailing edge parts of the upper and lower blades, portions of the asymmetrically dislocated part adjacent to the axis of the propeller in a vertical direction are cut at a constant tilt angle. The cut portions of the upper and lower blades are connected to each other via a tilt scissors plate mounted on surfaces of the cut portions to secure structural rigidity of the rudder.

Description

BACKGROUND

Technical Field

The present disclosure relates to a rudder for ships and, more particularly, to a ship rudder, both leading-edge and trailing-edge parts of which have asymmetrical cross-sections.

Description of the Related Art

Generally, ship rudders are located behind a propeller of a ship to control a movement direction of the ship. In this case, the rudders are subjected to propeller induced velocities and propeller induced flow angles that vary along the rudder span. The induced flow generates different pressures at right and left sides of the rudders according to upper and lower locations on an axis of the propeller. Viewing the rudder from behind the ship, a right-hand rotating propeller will produce a pressure distribution on the rudder surface such that a pressure side is created at a left upper portion and a right lower portion of the rudder, and a suction side is created at a right upper portion and a left lower portion thereof. Accordingly, when a rudder having a symmetrical cross-section is located behind a high speed (20 Knots or more) or highly loaded propeller of a ship, the suction pressure peak causes cavitation on the surface of the rudder where the suction side is created. In order to suppress the cavitation on the rudder surface, asymmetrical rudders have been developed which have leading edge parts, that is, front parts of the upper and lower blades of the rudders on the axis of the propeller, or trailing edge parts, that is, back parts of the upper and lower blades thereof, twisted so as to have profiles along its entire span that are aligned with the propeller induced flow into the rudder. In other words, viewing the rudder from behind the ship with the propeller rotating in the right-hand direction about the rudder, the leading edge parts of the upper and lower blades of such a conventional asymmetrical rudder on the axis of the propeller are twisted towards the port side and the starboard side, respectively, or the trailing edge parts of the upper and lower blades thereof are twisted towards the starboard side and the port side, respectively. In this structure, the leading or trailing edge parts of the rudder are asymmetrically located on the axis of the propeller. As a result, it is possible to reduce the lower suction pressure region along the leading edge parts of the rudder that normally causes cavitation on the rudder surface, thereby solving the problems of the conventional symmetrical rudders.
Since the leading or trailing edge parts of the upper and lower blades of the rudder centered on the axis of the propeller are twisted toward the port and starboard sides, however, the conventional asymmetrical rudder has a discontinuous cross-section and must include a scissors plate for structural rigidity. Further, a leading-edge asymmetrical rudder having the discontinuous cross-section is liable to suffer cavitation on the discontinuous surfaces of the leading edge parts and scissors plate due to a hub vortex of the propeller. On the other hand, a trailing-edge asymmetrical rudder is susceptible to rudder vibration caused by generation of a strong vortex behind a discontinuous section of the trailing edge parts and suffers deterioration in structural safety due to the elongated discontinuous section in the longitudinal direction of the rudder as compared to leading-edge asymmetrical rudders.
Particularly, since the scissors plate of the conventional rudder is perpendicularly disposed on a discontinuous surface of an asymmetrically dislocated part formed between the twisted leading or trailing edge parts of the upper and lower blades, the scissors plate undergoes severe cavitation damage.
Examples of the leading-edge asymmetrical rudder having a discontinuous cross-section are disclosed in Korean Patent Laid-open Publication No. 10-2005-0103137 , Japanese Utility Model Laid-open Publication No. S62-031000 , and the like.
Examples of the trailing-edge asymmetrical rudder having a discontinuous cross-section are disclosed in DE 20 2007 017448 U1 , Japanese Patent Laid-open Publication No. H56-063598 , and the like.
The DE 20 2007 015941 U1 shows a rudder comprising an upper blade and a lower blade, wherein leading edge parts of the upper and lower blades are twisted, the rudder further comprising a rubber bulb.

BRIEF SUMMARY

The present invention provides a rudder as defined by independent claim 1. Further advantageous features of the invention are identified in the subclaims.
The present disclosure is directed to solving the problems of the related art as described above, and one embodiment includes a rudder for ships that can enhance propulsion efficiency of the ship and prevent generation of a vortex on the rudder having a discontinuous cross-section while minimizing influence of an asymmetrical pressure caused by a trailing flow induced onto the rudder and rotating in one direction by a propeller rotating in one direction. In accordance with one aspect, the present disclosure provides a rudder for a ship disposed behind a propeller at a stern of the ship to control a movement direction of the ship, characterized in that the rudder is divided into an upper blade and a lower blade on an axis of the propeller, leading edge parts and trailing edge parts of the upper and lower blades are twisted at a predetermined twist angle on the axis of the propeller to be biased towards a rotational incoming flow induced from the propeller and to form an asymmetrically dislocated part, the leading edge parts of the upper and lower blades include a rudder bulb formed around the axis of the propeller to surround a discontinuous surface of the asymmetrically dislocated part of the leading edge parts and including a thrust fin formed at either side of the rudder bulb, and, in the trailing edge parts of the upper and lower blades, portions of the asymmetrically dislocated part adjacent to the axis of the propeller in a vertical direction are cut at a constant tilt angle, the cut portions of the upper and lower blades being connected to each other via a tilt scissors plate mounted on surfaces of the cut portions to secure structural rigidity of the rudder. The propeller may rotate in a right-screw direction centered on the rudder, and the leading edge parts of the upper and lower blades may be twisted towards port and starboard sides of the ship about the axis of the propeller, respectively, and the trailing edge parts of the upper and lower blades may be twisted towards starboard and port sides of the ship about the axis of the propeller, respectively.
Each of the leading edge parts of the upper and lower blades may be twisted at an angle of 2-8 degrees about the axis of the propeller, and each of the trailing edge parts of the upper and lower blades may be twisted at an angle of 2~8 degrees about the axis of the propeller. The rudder bulb may be formed to surround a width variation region of an asymmetrical cross-section comprising a port-side contour line of an upper cross-section of the discontinuous surface and a starboard-side contour line of a lower cross-section of the discontinuous surface in the leading edge parts of the upper and lower blades.
The thrust fin of the rudder bulb formed towards the port side of the ship on the axis of the propeller may be greater than the thrust fin formed towards the starboard side of the ship.
The tilt angle may be 30∼60 degrees to secure a welding angle between the tilt scissors plate and a main body of the rudder.
In the trailing edge parts of the upper and lower blades, the portions of the asymmetrically dislocated part adjacent to the axis of the propeller in the vertical direction may be cut at a constant tilt angle with reference to imaginary cross-section centerlines parallel to the axis of the propeller in the vertical direction.
A distance between the imaginary cross-section centerlines may be set to a thickness of the tilt scissors plate, in which one of the imaginary cross-section centerlines is provided as a reference line for cutting the portion of the asymmetrically dislocated part adjacent to the axis of the propeller in the vertical direction in the trailing edge part of the upper blade, and the other imaginary cross-section centerline is provided as a reference line for cutting the portion of the asymmetrically dislocated part adjacent to the axis of the propeller in the vertical direction in the trailing edge part of the lower blade.
The tilt scissors plate may include a first part corresponding to the surface of the cut portion of the upper blade, a second part corresponding to the surface of the cut portion of the lower blade, and a third part connecting the first part and the second part.
The tilt scissors plate may be welded to the rudder along circumferences of the respective surfaces of the cut portions of the upper and lower blades.
The cut portion of the trailing edge part of the upper blade may have a vertical length in the range of 20∼50% of a span of the upper blade, and the cut portion of the trailing edge part of the lower blade may have a vertical length in the range of 20∼50% of a span of the lower blade.
In the trailing edge parts of the upper and lower blades, a sum of the vertical lengths of the cut portions may be 30∼60% of a radius of the propeller.
The rudder bulb may be integrally formed with the thrust fins by a casting process.
The tilt scissors plate mounted on the surfaces of the cut portions of the trailing edge parts of the upper and lower blades may be an insert plate that completely covers the cut portions.
In accordance with another aspect, the present disclosure provides a rudder for a ship disposed behind a propeller at a stem of the ship to control a movement direction of the ship, characterized in that the rudder has a discontinuous cross-section while minimizing influence of an asymmetrical pressure caused by a trailing flow induced onto the rudder and rotating in one direction by a propeller rotating in one direction, leading edge parts of the rudder include a rudder bulb formed around an axis of the propeller to surround the discontinuous surface and including thrust fins, and trailing edge parts of the upper and lower blades are provided with a tilt scissors plate on the axis of the propeller.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a partial side view of a ship including a rudder according to one embodiment of the present disclosure;
Figure 2 is a perspective view of the rudder with a scissors plate separated therefrom according to the embodiment of the present disclosure;
Figure 3 is a perspective view of the rudder with the scissors plate coupled thereto according to the embodiment of the present disclosure;
Figure 4 is a partially sectioned plan view of the rudder according to the embodiment of the present disclosure;
Figure 5 is a front view of the rudder according to the embodiment of the present disclosure;
Figure 6 is a rear view of the rudder according to the embodiment of the present disclosure;
Figure 7 is a partially enlarged view of Figure 6; and
Figure 8 is a plan view of the scissors plate provided to the rudder according to the embodiment of the present disclosure.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings.
A ship rudder according to one embodiment of the disclosure is a leading-edge asymmetrical rudder.
Referring to Figure 1, the ship rudder 4 according to the embodiment of the disclosure is provided behind a propeller 2 located at the stern of a ship 1 to control a movement direction of the ship 1.
In this embodiment, a full-spade rudder will be illustrated as one example of the rudder 4. The rudder 4 is provided to a rudder trunk 3 located at the stem of the ship 1. Figure 1 shows the rudder connected to the rudder trunk 3, and Figures 2 to 4 show only the rudder.
Recently, full-spade rudders have been developed for large vessels.
The full-spade rudder is formed at an upper surface thereof with a rudder stock, which is inserted into a lower surface of the rudder trunk at the stem via bearings such that the full-spade rudder can be rotatably supported by the rudder trunk. Such a full-spade rudder is well-known in the art and details thereof are not shown in Figure 1.
The rudder 4 is generally divided into an upper blade 4a and a lower blade 4b on an axis L1 of the propeller 2. Each of the upper and lower blades 4a and 4b is also divided into leading edge parts 41', 41" corresponding to a front part of the rudder and trailing edge parts 42', 42" corresponding to a back part of the rudder. Referring to Figure 2, the terms "leading edge part" and "trailing edge part" herein refer to the front and back parts of the rudder 4 with reference to a maximum thickness widthwise centerline L3, respectively.
As shown in Figures 2 to 6, in the rudder 4 according to this embodiment, the leading edge parts 41', 41 " of the upper and lower blades 4a and 4b are twisted at a predetermined twist angle about the axis L1 of the propeller 2 to be biased towards a rotational incoming flow induced from the propeller 2, thereby forming an asymmetrically located part in the leading edge parts 41', 41" of the upper and lower blades 4a and 4b.
Viewing the rudder 4 according to this embodiment from behind the ship with the propeller 2 rotating in the right-screw direction about the rudder 4, the leading edge part parts 41', 41" of the upper and lower blades 4a and 4b are twisted towards the port and starboard sides of the ship about the axis L1 of the propeller 2, respectively. Further, the trailing edge parts 42', 42" of the upper and lower blades 4a and 4b are twisted towards the starboard and port sides of the ship about the axis L1 of the propeller 2, respectively.
When the leading edge parts 41', 41 " and the trailing edge parts 42', 42" of the upper and lower blades 4a and 4b are twisted at a predetermined angle about the axis L1 of the propeller 2, the leading edge parts 41', 41" of the upper and lower blades 4a and 4b must be twisted towards the port and starboard sides of the ship and the trailing edge parts 42', 42" of the upper and lower blades 4a and 4b must be twisted towards the starboard and ports sides of the ship, respectively, in order to counterbalance the asymmetrical pressure acting on the rudder surface due to the trailing flow induced onto the rudder and rotating in one direction by the propeller rotating in one direction (right-screw direction).
As illustrated in Figure 4, the leading edge part 41' of the upper blade 4a and the leading edge part 41" of the lower blade 4b may be twisted at an angle (α, β) of 2~8 degrees about the axis L1 of the propeller 2, and the trailing edge part 42' of the upper blade 4a and the trailing edge part 42" of the lower blade 4b may be twisted at an angle (α, β) of 2~8 degrees about the axis L1 of the propeller 2. Here, it should be noted that the angle α may be the same as or different from the angle β.
Further, in the rudder 4 according to this embodiment, the leading edge parts 41', 41" and the trailing edge parts 42', 42" of the upper and lower blades 4a and 4b are twisted with respect to imaginary cross-section centerlines parallel to the axis L1 of the propeller 2 in the vertical direction at intersection points between the imaginary cross-section centerlines parallel to the axis L1 of the propeller 2 and the maximum thickness widthwise centerline L3 of the rudder 4, respectively.
In the leading edge parts 41', 41" of the upper and lower blades 4a and 4b, a rudder bulb 50 is formed around the axis of the propeller 2 to surround a discontinuous surface of the asymmetrically dislocated part formed by twisting the leading edge parts 41', 41" of the upper and lower blades 4a and 4b.
The rudder bulb 50 may be formed to surround a width variation region of an asymmetrical cross-section comprising a port-side contour line of an upper cross-section 41a of the discontinuous surface and a starboard-side contour line of a lower cross-section 41 b of the discontinuous surface in the leading edge parts 41', 41" of the upper and lower blades 4a and 4b.
In the leading edge parts 41', 41" of the upper and lower blades 4a and 4b, a horizontal scissors plate (not shown) is provided between the upper cross-section of the discontinuous surface and the lower cross-section of the discontinuous surface to horizontally connect the upper and lower cross-sections to each other in order to secure structural rigidity. It can be understood that the rudder bulb 50 also surrounds the horizontal scissors plate provided to the leading edge parts 41', 41" of the upper and lower blades 4a and 4b.
The rudder bulb 50 is formed at opposite sides thereof with thrust fins 60, respectively.
The rudder bulb 50 may be integrally formed with the thrust fins 60 by a casting process.
Alternatively, the thrust fins 60 may be welded to outer surfaces of the upper and lower blades 4a and 4b and the horizontal scissors plate, although not shown in the drawings. In this case, with the thrust fins 60 welded to the outer surfaces of the upper and lower blades 4a and 4b and the horizontal scissors plate, insertion slits (not shown) for inserting the thrust fins 60 are formed in the rudder bulb 60 from a central portion to a rear end at a middle height of the rudder bulb 50. Then, with the thrust fins 60 inserted into the insertion slits, the rudder bulb 50 is welded to the outer surfaces of the upper and lower blades 4a and 4b, and the thrust fins 60 and the insertion slits of the rudder bulb 50 are welded to each other.
Viewing the rudder 4 according to this embodiment from behind the ship with the propeller 2 rotating in the right-screw direction about the rudder 4, the thrust fin 60 of the rudder bulb 50 formed towards the port side of the ship about the axis L1 of the propeller 2 may be greater than the thrust fin formed towards the starboard side of the ship.
As shown in Figures 2 and 3, in the trailing edge parts 42', 42" of the upper and lower blades 4a and 4b, portions of the asymmetrically located part adjacent to the axis L1 of the propeller 2 in the vertical direction are cut at a constant tilt angle, and a tilt scissors plate 45 is mounted on surfaces of the cut portions 42a, 42b of the trailing edge parts 42', 42" of the upper and lower blades 4a and 4b to secure structural rigidity by connecting the cut portions 42a, 42b to each other.
Particularly, in the trailing edge parts of the upper and lower blades 4a and 4b, the portions of the asymmetrically dislocated part adjacent to the axis L1 of the propeller 2 in the vertical direction are cut at a constant tilt angle with reference to the imaginary cross-section centerlines parallel to the axis L1 of the propeller 2 in the vertical direction. In other words, as shown in Figures 6 and 7, a distance between an imaginary cross-section centerline L1a and an imaginary cross-section centerline L1b is set to the thickness of the tilt scissors plate 45, in which the imaginary cross-section centerline L1a is provided as a reference line for cutting the portion of the asymmetrically dislocated part adjacent to the axis L1 of the propeller 2 in the vertical direction in the trailing edge part 42' of the upper blade 4a, and the imaginary cross-section centerline L1b is provided as a reference line for cutting the portion of the asymmetrically dislocated part adjacent to the axis L1 of the propeller 2 in the vertical direction in the trailing edge part 42" of the lower blade 4b. This is because it is necessary to form a space for mounting the tilt scissors plate 45 between the cut portion 42a of the trailing edge part 42' of the upper blade 4a and the cut portion 42b of the trailing edge part 42" of the lower blade 4b.
The tilt angle may be 30~60 degrees to secure a welding angle between the tilt scissors plate 45 and a main body of the rudder 4.
Referring to Figure 8, the tilt scissors plate 45 is composed of a first part 45a corresponding to the surface of the cut portion 42a in the trailing edge part 42' of the upper blade 4a, a second part 45b corresponding to the surface of the cut portion 42b in the trailing edge part 42" of the lower blade 4b, and a third part 45c connecting the first part 45a and the second part 45b.
The tilt scissors plate 45 is welded to the rudder along circumferences of the respective surfaces of the cut portions 42a, 42b in trailing edge parts 42', 42" of the upper and lower blades 4a and 4b, as clearly shown in Figure 7.
The tilt scissors plate 45 mounted on the surfaces of the cut portions 42a, 42b of the trailing edge parts 42', 42" of the upper and lower blades 4a and 4b may be an insert plate that completely covers the cut portions 42a, 42b.
In Figure 6, a black dot indicates the axis L1 of the propeller 2, and L2 indicates a longitudinal centerline of the rudder 4 intersecting the axis L1 of the propeller 2. Here, the trailing edge part 42' of the upper blade 4a is set to a section D₁ from a top of the rudder 4' to the axis L1 of the propeller 2, and the trailing edge part 42" of the lower blade 4b is set to a section D₂ from a bottom of the rudder 4' to the axis L1 of the propeller 2. Further, a rear end of the cut portion 42a of the trailing edge part 42' of the upper blade 4a has a vertical length indicated by a section d₁, and a rear end of the cut portion 42b of the trailing edge part 42" of the lower blade 4b has a vertical length indicated by a section d₂.
In the trailing edge parts 42', 42" of the upper and lower blades 4a and 4b, the rear end of the cut portion 42a may have a vertical length (the length of the section d₁) in the range of 20∼50% of the span (the length of the section D₁) of the upper blade 4a, and the rear end of the cut portion 42b may have a vertical length (the length of the section d₂) in the range of 20∼50% of the span (the length of the section D₂) of the lower blade 4b. Further, in the trailing edge parts 42', 42" of the upper and lower blades 4a and 4b, the sum (the length of the section d₃) of the vertical lengths of the cut portions 42a, 42b may be 30∼60% of a radius of the propeller 2.
Here, the trailing edge parts 42', 42" of the upper and lower blades 4a and 4b and the vertical lengths of the cut portions 42a, 42b may be symmetrically or asymmetrically formed according to locations of the rudder 4 and the propeller 2.
As apparent from the above description, according to one embodiment of the disclosure, the rudder 4 includes the rudder bulb 50 formed on the leading edge parts 41', 41" of the upper and lower blades 4a and 4b to surround the discontinuous surface of the asymmetrically dislocated part formed by twisting the leading edge parts 41', 41 ", thereby reducing the risk of cavitation damage caused by the discontinuous surface of the asymmetrically dislocated part formed between the leading edge parts 41', 41" of the upper and lower blades 4a and 4b.
Further, according to one embodiment of the disclosure, the thrust fins 60 are formed on the opposite sides of the rudder bulb 50, thereby enhancing propulsion efficiency of the ship.
Moreover, according to one embodiment of the disclosure, in the trailing edge parts 42', 42" of the upper and lower blades 4a and 4b of the rudder, the portions of the asymmetrically dislocated part adjacent to the axis L1 of the propeller 2 in the vertical direction are cut at a constant tilt angle and are provided on the surfaces thereof with the tilt scissors plate 45 such that the scissors plate meets the rudder at a gentle angle, thereby reducing risk of cavitation damage on the scissors plate.

Claims

A rudder (4) for a ship disposed behind a propeller (2) at a stern of the ship to control a movement direction of the ship,
wherein the rudder is divided into an upper blade (4a) and a lower blade (4b) on an axis of the propeller (2), leading edge parts (41', 41 ") and trailing edge parts (42', 42") of the upper and lower blades are twisted at a predetermined twist angle on the axis of the propeller to be biased towards a rotational incoming flow induced from the propeller and to form an asymmetrically dislocated part, the leading edge parts (41', 41 ") of the upper and lower blades include a rudder bulb (50) formed around the axis of the propeller to surround a discontinuous surface of the asymmetrically dislocated part of the leading edge parts and including a thrust fin formed at either side of the rudder bulb, and, in the trailing edge parts of the upper and lower blades, portions of the asymmetrically dislocated part adjacent to the axis of the propeller in a vertical direction are cut at a constant tilt angle, the cut portions of the upper and lower blades being connected to each other via a tilt scissors plate (45) mounted on surfaces of the cut portions to secure structural rigidity of the rudder (4).
The rudder (4) according to claim 1, characterized in that the propeller (2) rotates in a right-screw direction centered on the rudder, and the leading edge parts of the upper and lower blades are twisted towards port and starboard sides of the ship about the axis of the propeller, respectively, and the trailing edge parts of the upper and lower blades are twisted towards starboard and port sides of the ship about the axis of the propeller, respectively.
The rudder (4) according to claim 2, characterized in that each of the leading edge parts of the upper and lower blades is twisted at an angle of 2~8 degrees about the axis of the propeller, and each of the trailing edge parts of the upper and lower blades is twisted at an angle of 2~8 degrees about the axis of the propeller.
The rudder (4) according to claim 2, characterized in that the rudder bulb is formed to surround a width variation region of an asymmetrical cross-section comprising a port-side contour line of an upper cross-section of the discontinuous surface and a starboard-side contour line of a lower cross-section of the discontinuous surface in the leading edge parts of the upper and lower blades.
The rudder (4) according to claim 2, characterized in that the thrust fin of the rudder bulb formed towards the port side of the ship about the axis of the propeller is greater than the thrust fin formed towards the starboard side of the ship.
The rudder (4) according to claim 1, characterized in that the tilt angle is 30~60 degrees to secure a welding angle between the tilt scissors plate and a main body of the rudder.
The rudder (4) according to claim 1, characterized in that, in the trailing edge parts of the upper and lower blades, the portions of the asymmetrically dislocated part adjacent to the axis of the propeller in the vertical direction are cut at a constant tilt angle with reference to imaginary cross-section centerlines parallel to the axis of the propeller in the vertical direction.
The rudder (4) according to claim 7, characterized in that a distance between the imaginary cross-section centerlines is set to a thickness of the tilt scissors plate, in which one of the imaginary cross-section centerlines is provided as a reference line for cutting the portion of the asymmetrically dislocated part adjacent to the axis of the propeller in the vertical direction in the trailing edge part of the upper blade, and the other imaginary cross-section centerline is provided as a reference line for cutting the portion of the asymmetrically dislocated part adjacent to the axis of the propeller in the vertical direction in the trailing edge part of the lower blade.
The rudder (4) according to claim 1, characterized in that the tilt scissors plate comprises a first part corresponding to the surface of the cut portion of the upper blade, a second part corresponding to the surface of the cut portion of the lower blade, and a third part connecting the first part and the second part.
The rudder (4) according to claim 1 or 9, characterized in that the tilt scissors plate is welded to the rudder along circumferences of the respective surfaces of the cut portions of the upper and lower blades.
The rudder (4) according to claim 1 or 6, characterized in that the cut portion of the trailing edge part of the upper blade has a vertical length in the range of 20∼50% of a span of the upper blade, and the cut portion of the trailing edge part of the lower blade has a vertical length in the range of 20∼50% of a span of the lower blade.
The rudder (4) according to claim 11, characterized in that, in the trailing edge parts of the upper and lower blades, a sum of the vertical lengths of the cut portions is 30∼60% of a radius of the propeller.
The rudder (4) according to claim 1, characterized in that the rudder bulb is integrally formed with the thrust fins by a casting process.
The rudder (4) according to claim 1 or 9, characterized in that the tilt scissors plate mounted on the surfaces of the cut portions of the trailing edge parts of the upper and lower blades is an insert plate that completely covers the cut portions.