EP2110311A2

EP2110311A2 - Finned rudder

Info

Publication number: EP2110311A2
Application number: EP09152042A
Authority: EP
Inventors: Daisuke Mitsubishi Heavy Industries Ltd. Matsumoto; Toshinobu Mitsubishi Heavy Industries Ltd. Sakamoto
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 2008-04-18
Filing date: 2009-02-04
Publication date: 2009-10-21
Anticipated expiration: 2029-02-04
Also published as: ES2411475T3; EP2110311B1; CN101559828B; EP2110311A3; CN101559828A; JP2009255835A

Abstract

A fin that generates thrust in the forward motion direction by utilizing rising, falling, and swirling flows near a rudder surface is attached to a rudder to improve the propulsion performance. The invention provides a finned rudder, disposed aftward of a screw propeller that rotates clockwise as viewed from the stern side during forward motion, for changing the course of a ship and provided with a first fin and a second fin in respective rudder surfaces. A first end of the first fin is attached at a position higher than a center position of the screw propeller on a leading edge side of a central portion of the rudder surface, and a first end of the second fin is attached at a position lower than the center position of the screw propeller on the leading edge side of the central portion of the rudder surface. A second end of the first fin extends at an upward inclination to a position inside the rotation radius of the screw propeller where the upward flow is strong, and a second end of the second fin extends horizontally to a position inside the rotation radius of the screw propeller where the downward flow is strong.

Description

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention relates to finned rudders provided on ships to change their course.

2. DESCRIPTION OF RELATED ART

In the related art, there are known structures for converting a rotational flow from a propeller to a flow in the propulsion direction in order to improve the propulsion performance. For example, Japanese Unexamined Patent Application, Publication No. HEI-6-305487 discloses a structure in which horizontal fins whose inclination angle (twist angle) is set relative to the inflow angle, which changes according to the distance from the center of the propeller shaft, are provided on the port and starboard sides of a center axis position of a rudder bulb. In another known structure, in order to improve the propulsion performance, a bulb is added to the leading edge of the rudder near the center of the propeller shaft, and horizontal fins at the same heights on the left and right are provided integrally with this bulb (see Japanese Unexamined Patent Application, Publication No. HEI-11-139395 ).
However, as a result of detailed analysis by the present inventors of the flow field around a rudder behind the ship's propeller that runs clockwise, it was found that a water flow like that shown in Fig. 12 is generated at the leading edge of the rudder plate, and a water flow like that shown in Fig. 13 is generated at the center position of the rudder shaft. As can be understood from Figs. 12 and 13, at the port side of the ship's rudder R (the right side in Figs. 12 and 13), the upward flow is strong inside a rotation radius P of the screw propeller above a center position C, and the downward flow is strong near the rudder R. On the other hand, at the starboard side of the rudder (the left side in Figs. 12 and 13), the downward flow is strong inside the rotation radius P of the screw propeller below the center position C, and the downward flow is strong near the rudder R.
Accordingly, at the center position of the rudder shaft in the propeller wake, the flows above and below the center position, as well as the strengths of the flows, significantly differ at the port and starboard sides where the propeller rotation directions are different, as described above. Therefore, as described in Japanese Unexamined Patent Application, Publication No. HEI-6-305487 and Japanese Unexamined Patent Application, Publication No. HEI-11-139395 , there is a problem in that it is not possible to efficiently improve the propulsion efficiency in the propeller wake with horizontal fins provided at the same height on the port and starboard sides at the shaft center position and close to the center.
Figs. 12 and 13, which show the flow field analysis results obtained at a Reynold's number of 1,080,000 and a screw propeller rotational speed of 7.8 rps, illustrate forward motion as viewed from the bow side. The screw propeller, which is not shown in the figures, rotates clockwise, as viewed from the stern side, during forward motion. The directions of the arrows in these figures indicate the directions of flows in the plane thereof, and the lengths of the arrows indicate the magnitudes of the flows. In particular, the downward flow is strong below the center line of the propeller on the starboard side of the rudder.

BRIEF SUMMARY OF THE INVENTION

The present invention has been conceived in light of the circumstances described above, and an object thereof is to provide a finned rudder, as well as a ship provided therewith, that can increase the wake gain by more effectively rectifying the flow of water behind a screw propeller, thereby improving the propulsion performance (propulsion efficiency).
In order to solve the problems described above, the present invention employs the solutions defined in the claims.
A first aspect of a finned rudder according to the present invention is a finned rudder, disposed aftward of a screw propeller, for changing the course of a ship and provided with a first fin in a port-side rudder surface and a second fin in a starboard-side rudder surface. When the screw propeller rotates clockwise, as viewed from a stern side, during forward motion, a first end of the first fin is attached at a position higher than a center position of the screw propeller on a leading edge side of a central portion of the port-side rudder surface, and a first end of the second fin is attached at a position lower than the center position of the screw propeller on the leading edge side of a central portion of the starboard-side rudder surface. When the screw propeller rotates anticlockwise, as viewed from the stern side, during forward motion, the first end of the first fin is attached at a position lower than the center position of the screw propeller on the leading edge side of the central portion of the port-side rudder surface, and the first end of the second fin is attached at a position higher than the center position of the screw propeller on the leading edge side of the central portion of the starboard-side rudder surface. The second end of each fin horizontally extends to a position inside a rotation radius of the screw propeller where an upward or downward flow is strong.
A second aspect of a finned rudder according to the present invention is a finned rudder, disposed aftward of a screw propeller that rotates clockwise as viewed from a stern side during forward motion, for changing a course of a ship and provided with a first fin in a port-side rudder surface and a second fin in a starboard-side rudder surface. A first end of the first fin is attached at a position higher than a center position of the screw propeller on a leading edge side of a central portion of the port-side rudder surface, and a first end of the second fin is attached at a position lower than the center position of the screw propeller on the leading edge side of a central portion of the starboard-side rudder surface; a second end of the first fin extends at an upward inclination to a position inside a rotation radius of the screw propeller where an upward flow is strong; and a second end of the second fin extends horizontally to a position inside the rotation radius of the screw propeller where a downward flow is strong.
When disposed aftward of a screw propeller that rotates anticlockwise, as viewed from the stern side, during forward motion, the positional relationship of the first fin and the second fin is inverted left-right.
With the finned rudder according to the present invention, because a fin presenting an appropriate angle relative to the falling, rising, and swirling flows near the rudder generates thrust, the hull resistance can be reduced, thus decreasing the propulsion horsepower. By moving the attachment position of the fin up or down, as shown in Fig. 7, or by inclining the attachment angle, as shown in Fig. 8, to be better adapted to the flow around the rudder, it is possible to achieve a greater reduction in the propulsion horsepower with the fin.
As shown in Fig. 13, the flow at the upper port-side of the rudder surface may approach a swirling flow formed by combining the rising flow and the inward flow. In such a case, if the attachment angle of the fin is directed at an upward inclination, it is possible to achieve a greater thrust production effect with the fin.
In the finned rudder described above, preferably, a rudder bulb formed of a raised bump is provided at a leading edge portion opposing a propeller boss of the screw propeller, and a leading edge thereof is twisted in conformance with the inflow direction of a wake from the screw propeller.
With the finned rudder having this configuration, the flow generated from the aft end of the propeller boss is made to flow along the surface of the rudder bulb, thereby attenuating the boss vortex. Therefore, it is possible to reduce the vortex resistance and to further improve the propulsion performance (propulsion efficiency).
The leading edges of the finned rudder are twisted in opposite directions at the top and bottom in conformance with the inflow direction of the wake from the screw propeller, and a gap that would normally occur at the height of the propeller shaft at the leading edges of the rudder, causing cavitation, is eliminated by the rudder bulb. It is thus possible to inhibit rudder cavitation produced at the leading edge, which makes it possible to prevent erosion of the rudder surface and paint peeling off from the rudder surface.
The second end of the leading edge is connected to the top of the rudder bulb, and the first end of the leading edge is connected to the bottom of the rudder bulb. In other words, because the first and second ends of the leading edge are connected via the rudder bulb, it is possible to simplify processing of the leading edge and to improve the manufacturability.
In the finned rudder described above, preferably, the first fin is formed so as to have a wing shape in cross section and an upward camber, and the second fin is formed so as to have a wing shape in cross section and a downward camber.
With the finned rudder having this configuration, cambers that are oriented so as to increase the dynamic lift are provided in the respective fins. Thus, because the forward component of this lift acts as a thrust that propels the hull of the ship in the forward direction, this thrust acts on the hull, and the hull resistance decreases.
Accordingly, it is possible to further improve the propulsion performance (propulsion efficiency).
With the finned rudder according to the present invention, because a fin presenting an appropriate angle relative to the falling, rising, and swirling flows near the rudder generates thrust, the hull resistance can be reduced, thus decreasing the propulsion horsepower.
The finned rudder according to the present invention affords an advantage in that, because a fin presenting an appropriate angle relative to the falling, rising, and swirling flows near the rudder generates thrust, the hull resistance can be reduced and the fuel efficiency can be improved.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Fig. 1 is a right side view, taken from the starboard side, of a stern portion of a ship equipped with a finned rudder according to a first embodiment of the present invention.
Fig. 2 is a left side view, taken from the port side, of the stern portion of the ship equipped with the finned rudder according to the first embodiment of the present invention.
Fig. 3 is an elevational view, taken from the bow side, of the finned rudder according to the first embodiment of the present invention.
Fig. 4 is a graph showing experimental results obtained by propelling a ship equipped with the finned rudder according to the first embodiment of the present invention.
Fig. 5 is a right side view, taken from the starboard side, of a stern portion of a ship equipped with a finned rudder according to a second embodiment of the present invention.
Fig. 6 is a left side view, taken from the port side, of the stern portion of the ship equipped with the finned rudder according to the second embodiment of the present invention.
Fig. 7 is an elevational view, taken from the bow side, of the finned rudder according to the second embodiment of the present invention.
Fig. 8 is an elevational view, taken from the bow side, of a finned rudder according to a third embodiment of the present invention.
Fig. 9 is a right side view, taken from the starboard side, of a stern portion of a ship equipped with a finned rudder according to a fourth embodiment of the present invention.
Fig. 10 is a left side view, taken from the port side, of the stern portion of the ship equipped with the finned rudder according to the fourth embodiment of the present invention.
Fig. 11 is an elevational view, taken from the bow side, of a finned rudder according to a fourth embodiment of the present invention.
Fig. 12 is a view, taken from the bow side, of water flow at a leading edge of a rudder plate of a ship equipped with a conventional ship's rudder.
Fig. 13 is a view, taken from the bow side, of water flow at a center position of a rudder shaft of a ship equipped with a conventional ship's rudder.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of a finned rudder according to the present invention will be described below with reference to Figs. 1 to 4.
Fig. 1 is a right side view, taken from the starboard side, of a stern portion of a ship equipped with a finned rudder according to this embodiment; Fig. 2 is a left side view, taken from the port side, of the stern portion of the ship equipped with the finned rudder according to this embodiment; Fig. 3 is an elevational view, taken from the bow side, of the finned rudder according to this embodiment; and Fig. 4 is a graph showing experimental results obtained by propelling the ship equipped with the finned rudder according to this embodiment.
As shown in Fig. 1 or 2, a finned rudder 10 according to this embodiment is a plate-shaped member which is attached to a rudder shaft 5 that extends vertically downward from a stern bottom 4 of a stern portion 3 located aftward (on the stern side) of a screw propeller 2, and is rotated about a vertical axis together with the rudder shaft 5 to change the course of a ship 1.
The screw propeller 2 is attached to the aft end (the end at the stern side) of a propeller shaft 7 that passes through a bossing 6. The screw propeller 2 rotates together with the propeller shaft 7, clockwise (right) as viewed from the stern side during forward motion, and anticlockwise (left) as viewed from the stern side during reverse motion.
A first fin 12 is provided in (attached to) a port-side rudder surface 11 of the finned rudder 10 according to this embodiment, and a second fin 14 is provided in (attached to) a starboard-side rudder surface 13 of the finned rudder 10.
As shown in Figs. 2 and 3, the first fin 12, a first end (root) of which is towards the leading edge of a center portion of the rudder surface 11, is a small blade having a wing shape in cross-section and an upward camber and is attached higher than a center position C of the screw propeller 2 so as to extend horizontally (outward towards the port side in Fig. 3). A second end (tip) of the first fin 12 extends to a position (region) inside the rotation radius P (see Figs. 12 and 13) of the screw propeller 2 where the upward flow is strong.
As shown in Fig. 1 and 3, the second fin 14, a first end (root) of which is towards the leading edge of a center portion of the rudder surface 13, is a small blade having a wing shape in cross-section and a downward camber and is attached lower than the center position C of the screw propeller 2 so as to extend horizontally (outward towards the starboard side in Fig. 3). A second end (the tip) of the second fin 14 extends to a position (region) inside the rotation radius P (see Figs. 12 and 13) of the screw propeller 2 where the downward flow is strong.
With the finned rudder 10 according to this embodiment, an upward flow generated inside the rotation radius P of the screw propeller 2, which is produced close to the rudder surface 11, and above the center position C of the screw propeller 2 is attenuated (inhibited) by the first fin 12. Additionally, a downward flow generated inside the rotation radius P of the screw propeller 2, which is produced close to the rudder surface 13, and below the center position C of the screw propeller 2 is attenuated (inhibited) by the second fin 14.
Thus, it is possible to reduce hull resistance by rectifying the upward and downward flows near the rudder surfaces. In other words, by utilizing a lift generated by the fins on the rudder surfaces from the upward and downward flows, it is possible to improve the propulsion performance (propulsion efficiency).
Fig. 4 is a graph of experimental results obtained by propelling a ship equipped with the finned rudder 10 according to this embodiment, showing ship speed (kn: knots) on the horizontal axis and horsepower (kW) on the vertical axis. The broken line drawn from the bottom left to the top right in this figure is data obtained from a ship that is not equipped with the finned rudder 10 according to this embodiment, and the solid line drawn from the bottom left to the top right is data obtained from the ship equipped with the finned rudder 10 according to this embodiment.
As shown in Fig. 4, the ship equipped with the finned rudder 10 according to this embodiment requires less horsepower to attain the same speed as a ship that is not equipped with the finned rudder 10 according to this embodiment, and the speed is higher (increased) when applying the same horsepower as the ship that is not equipped with the finned rudder 10 according to this embodiment. Accordingly, a reduction in fuel consumption of about 2% compared with the conventional case can be achieved, which is an effective experimental result demonstrating the above-described advantages of the finned rudder 10 according to this embodiment.
Furthermore, with the finned rudder 10 according to this embodiment, the fin 12 is given an upward camber by attaching it to the corresponding rudder surface 11 so that the leading edge thereof is located lower than the trailing edge, and the fin 14 is given a downward camber by attaching it to the corresponding rudder surface 13 so that the leading edge thereof is located higher than the trailing edge. Therefore, it is possible to generate upward and downward lifts at the bow side of the respective fins 12 and 14. Because the forward component of these lifts serves as thrust propelling the hull of the ship 1 in the forward direction, this thrust acts on the hull, reducing the hull resistance.
Accordingly, the propulsion performance (propulsion efficiency) can be improved.
A finned rudder according to a second embodiment of the present invention will be described with reference to Figs. 5 to 7. Fig. 5 is a right side view, taken from the starboard side, of a stern portion of a ship equipped with a finned rudder according to this embodiment, Fig. 6 is a left side view, taken from the port side, of the stern portion of the ship equipped with the finned rudder according to this embodiment, and Fig. 7 is an elevational view, taken from the bow side, of the finned rudder according to this embodiment.
As shown in Figs. 5 to 7, a finned rudder 20 according to this embodiment differs from that in the first embodiment described above in that it includes a rudder bulb 21, and as shown in Fig. 7, leading edges 22 and 23 thereof are inclined with respect to the perpendicular axis (vertical axis). The other components are the same as those in the first embodiment described above, and a description thereof is thus omitted here.
The rudder bulb 21 is a raised bump provided at the leading edge of the finned rudder 20 at a position facing the propeller boss 2a (see Fig. 1 or Fig. 2) of the screw propeller 2 (see Fig. 1 or Fig. 2). As shown in Figs. 5 and 6, it steeply increases in diameter at the leading edge of the finned rudder 20 from the bow-side end face opposing the propeller boss 2a, and gently reduces in diameter from the leading edge of the finned rudder 20 towards the trailing edge. The second fin 14 is provided below the rudder bulb 21 at the wake side of the leading edge of the finned rudder 20, as shown in Fig. 5. The first fin 12 is provided above the rudder bulb 21 on the wake side of the leading edge of the rudder 20, as shown in Fig. 6. Accordingly, in view of the flow field near the rudder, it is possible to obtain the most effective thrust by providing the first fin 12 and the second fin 14 in the vicinity of the rudder bulb 21 at the wake side of the leading edge of the finned rudder 20. As shown in Fig. 7, the cross-sectional shape of the rudder bulb 21 is substantially circular, and the cross section thereof has the same shape at all positions from the bow side to the stern side.
This rudder bulb 21 inhibits a boss vortex produced from the aft end of the propeller boss 2a; in other words, it attenuates (collects) the boss vortex by making the flow produced from the aft end of the propeller boss 2a flow along the surface of the rudder bulb 21, thus reducing vortex resistance and improving the propulsion performance (propulsion efficiency).
Regarding the leading edge 22 which is located above the rudder bulb 21, that is to say, at the top end of the rudder bulb 21, as shown in Fig. 7, a first end thereof (the top end) is located at the same position as the center position C of the screw propeller 2 or on the starboard side of the center position C of the screw propeller 2, and a second end thereof (the bottom end) is located on the port side of the center position C of the screw propeller 2. Additionally, the leading edge 22 is formed so as to form a substantially straight line from the first end to the second end thereof. The second end of the leading edge 22 is connected to the top of the rudder bulb 21.
On the other hand, regarding the leading edge 23 which is located below the rudder bulb 21, that is to say, at the bottom end of the rudder bulb 21, a first end thereof (the top end) is located on the starboard side of the center position C of the screw propeller 2, and a second end thereof (the bottom end) is located at the same position as the center position C of the screw propeller 2 or on the starboard side of the center position C of the screw propeller 2. Additionally, the leading edge 23 is formed so as to form a substantially straight line from the first end to the second end thereof. The first end of the leading edge 23 is connected to the bottom of the rudder bulb 21.
In other words, the leading edges 22 and 23 are twisted in conformance with the inflow direction of the wake from the screw propeller 2, thereby reducing the inflow directions at the finned rudder 20 (inflow angle) from the screw propeller 2, and inhibiting rudder cavitation produced by the leading edges 22 and 23 of the finned rudder 20.
With the finned rudder 20 according to this embodiment, by means of the rudder bulb 21, the flow produced from the aft end of the propeller boss 2a is made to flow along the surface of the rudder bulb 21, thereby attenuating boss vortex. Therefore, it is possible to decrease vortex resistance and to further improve the propulsion performance (propulsion efficiency).
Because the leading edges 22 and 23 of the finned rudder 20 are twisted in conformance with the inflow direction of the wake from the screw propeller 2, thereby reducing the inflow directions (inflow angle) of the wake from the screw propeller 2 at the finned rudder 20, it is possible to inhibit rudder cavitation produced by the leading edges 22 and 23, and to prevent erosion of the rudder surfaces 11 and 13 and peeling of paint from the rudder surfaces 11 and 13. In particular, with high-speed ships such as container ships, there is a possibility of erosion and paint peeling due to rudder cavitation. There is a significant reduction in rudder cavitation when the present invention is applied to high-speed ships such as container ships.
The second end of the leading edge 22 is connected to the top of the rudder bulb 21, and the first end of the leading edge 23 is connected to the bottom of the rudder bulb 21. One problem with this kind of reaction rudder is that, because the twist direction is reversed at the center of the propeller shaft, a step occurs at the leading edge of the rudder, causing cavitation at this step portion and making the processing difficult. With the present invention, however, because the second end of the leading edge 22 and the first end of the leading edge 23 are connected via the rudder bulb 21, no step occurs at the leading edge of the rudder, thus preventing the occurrence of cavitation. In addition, processing of the leading edge can be simplified and the manufacturability improved.
The other operation and effects are identical to those of the first embodiment described above, and a description thereof is thus omitted here.
A finned rudder according to a third embodiment of the present invention will now be described with reference to Fig. 8. Fig. 8 is an elevational view, taken from the bow side, of a finned rudder 30 according to this embodiment.
The finned rudder 30 according to this embodiment differs from that in the first embodiment described above in that a first fin 31 is provided instead of the first fin 12. The other components are the same as those in the first embodiment described above, and therefore, a description thereof will be omitted here.
As shown in Fig. 8, the first fin 31, a first end (root) of which is towards the leading edge of a center portion of the rudder surface 11, is a small blade having a wing shape in cross-section and an upward camber and is attached higher than the center position C of the screw propeller 2 so as to extend upward at an angle (towards the upper right in Fig. 8). A second end (tip) of the first fin 31 extends to a position (region) inside the rotation radius P (see Figs. 12 and 13) of the screw propeller 2 where the upward flow is strongest.
As shown in Fig. 13, a flow that approaches a swirling flow formed as a combination of the rising flow and the inward flow may occur at the upper port side of the rudder surface; in such cases, it is possible to further increase the thrust producing effect of the fin by directing the attachment angle thereof at an upward inclination, as with the first fin 31 according to this embodiment.
The other operation and effects are the same as those in the first embodiment described above, and a description thereof is thus omitted here.
A finned rudder according to a fourth embodiment of the present invention will now be described with reference to Figs. 9 to 11. Fig. 9 is a right side view, taken from the starboard side, of a stern portion of a ship equipped with a finned rudder according to this embodiment, Fig. 10 is a left side view, taken from the port side, of the stern portion of the ship equipped with the finned rudder according to this embodiment, and Fig. 11 is an elevational view, taken from the bow side, of the finned rudder according to this embodiment.
A finned rudder 40 according to this embodiment differs from that in the second embodiment described above in that a first fin 41 is provided instead of the first fin 12. The other components are identical to those in the second embodiment described above, and therefore, a description thereof is omitted here.
As shown in Fig. 10 and Fig. 11, the first fin 41 according to this embodiment, a first end (root) of which is towards the trailing edge of the top of the rudder bulb 21, is a small blade having a wing shape in cross-section and an upward camber and is attached higher than the center position C of the screw propeller 2 so as to extend at an upward inclination (towards the upper right in Fig. 11). A second end (tip) of the first fin 41 extends to a position (region) inside the rotation radius P (see Figs. 12 and 13) of the screw propeller 2 where the upward flow is strongest.
The operation and effects of the finned rudder 40 according to this embodiment are the same as those of the third embodiment described above, and therefore, a description thereof is omitted here.
Because the flows shown in Figs. 12 and 13 differ from ship to ship, it is preferable to perform flow field analysis for each ship to identify regions where the first fin and the second fin are most effective. The present invention is not limited to the embodiments described above; various modifications are permissible as required, so long as they do not depart from the technical idea of the present invention.
In addition, the finned rudder according to the present invention can be applied to ships and commercial vessels such as gas carriers, tankers, container ships, ferries, roll-on roll-off (RORO) ships, car carriers, bulk carriers, and passenger ships. It is possible to thereby reduce the propulsion horsepower and improve fuel efficiency, thus holding promise for low-energy ships.

Claims

A finned rudder (10;20) for changing the course of a ship (1), comprising:
a first fin (12) provided at a port-side rudder surface (11) and a second fin (14) provided at a starboardside rudder surface (13),
wherein,
when the finned rudder (10;20) is intended to be disposed in a vertical orientation aftward of a screw propeller (2) which is designed to rotate clockwise, as viewed from a stern side, during forward motion of the ship (1), a first end of the first fin (12) is arranged at a position higher than a center position (C) of the screw propeller (2) and toward a leading edge side of a central portion of the port-side rudder surface (11), and a first end of the second fin (14) is arranged at a position lower than the center position (C) of the screw propeller (2) and toward the leading edge side of a central portion of the starboard-side rudder surface (13); or
when the finned rudder (10;20) is intended to be disposed in a vertical orientation aftward of a screw propeller (2) which is designed to rotate anticlockwise, as viewed from the stern side, during forward motion of the ship (1), a first end of the first fin (12) is arranged at a position lower than a center position (C) of the screw propeller (2) and toward a leading edge side of a central portion of the port-side rudder surface (11), and a first end of the second fin (14) is arranged at a position higher than the center position (C) of the screw propeller (2) and toward the leading edge side of the central portion of the starboard-side rudder surface (13); and
a second end of each fin (12,14) extends horizontally to a position inside a rotation radius (P) of the screw propeller (2) where an upward or downward flow is strong.
A finned rudder (30;40) for changing a course of a ship (1), comprising:
a first fin (31;41) provided at a port-side rudder surface (11) and a second fin (14) provided at a starboard-side rudder surface (13),
wherein, when the finned rudder (30;40) is intended to be disposed in a vertical orientation aftward of a screw propeller (2) which is designed to rotate clockwise, as viewed from a stern side, during forward motion of the ship (1), a first end of the first fin (31;41) is arranged at a position higher than a center position (C) of the screw propeller (2) and toward a leading edge side of a central portion of the port-side rudder surface (11), and a first end of the second fin (14) is arranged at a position lower than the center position (C) of the screw propeller (2) and toward the leading edge side of a central portion of the starboard-side rudder surface (13);

a second end of the first fin (31;41) extends at an upward inclination to a position inside a rotation radius (P) of the screw propeller (2) where an upward flow is strong; and

a second end of the second fin (14) extends horizontally to a position inside the rotation radius (P) of the screw propeller (2) where a downward flow is strong.
The finned rudder according to claim 1 or 2, wherein a rudder bulb (21) formed of a raised bump is provided at a leading edge portion of the finned rudder (20;40) opposing a propeller boss (2a) of the screw propeller (2), and a leading edge (22,23) thereof is twisted in conformance with the inflow direction of a wake from the screw propeller (2).
The finned rudder according to one of claims 1 to 3,
wherein the first fin (12;31;41) is formed so as to have a wing shape in cross section and an upward camber, and the second fin (14) is formed so as to have a wing shape in cross section and a downward camber.
A ship comprising a finned rudder (10;20;30:40) according to one of claims 1 to 4.