Ship's rudder

Abstract: The invention relates to a ship's rudder having a main rudder body (1) with a pair of fins (2) horizontally extending from opposite vertical surfaces thereof and vertically extending winglets (3) on the tip ends of the fins (2).

Description: The present invention relates to ship's rudder according to the preamble of claim 1. A ship's rudder according to the preamble of claim 1 is known from GB-A-2,156,297 and includes a vertical rudder plate from each side of which a transverse auxiliary wing projects. As means for enhancing a propulsive efficiency of a ship, Japanese Patent Publication Laid Open No.15895/72 discloses the provision of a fin extending horizontally from each side of a rudder body, the fins being symmetrically mounted relative to the center plane of the rudder, whereby rotational energy in a propeller slip stream is recovered and converted to forward thrust. A fin of an improved form is disclosed in Japanese Utility Model Publication Laid Open No.28000/90. In this example, as shown in Figures 5 (A) to (C) herewith attached, a leading edge of a rudder 1 coincides with a leading edge of a fin 2 on the center line of the rudder 1, and the leading edge of the fin has a sweepback angle of ϑ. The chord length of the fin 2 is set so that the length ℓr at the root of the fin is one and a half times the length ℓt at the tip of the fin, or more, and a plane form of taper fin, wherein the chord length of the fin decreases from the above-described root of the fin toward the tip of the fin, is adopted. In the form of fin of the above-described prior art rudder, the leading edge of the fin has a sweepback angle of 20 to 50°, and the plane form of the taper fin, wherein a fin span decreases as it comes near to the tip of the fin, is adopted. In consequences, when an inflow attack angle of a current flowing into the fin is regarded as large to some extent, a strong fin tip vortex tends to occur at the vicinity of the fin tip compared with a case where a fin has a plane form without any sweepback angle.( see Fig.6 ). The inflow attack angle ( αt ) at the fin tip is decreased ( αt < αr ) by an induced velocity of the strong fin tip vortex, whereby lift caused to the fin is decreased and drag ( referred to as an induced drag ) is increased. Accordingly, thrust, which is caused by the fin and which is a forward component of the lift, is decreased, with an adverse affect on propulsive efficiency. The present invention is intended, at least to an extent, to solve the above-described problem. It is an object of the present invention to provide a rudder structure which enhances the propulsive efficiecy of the rudder by decreasing the induced drag of the fin by diffusing and weakening the vortex generated from the aforementioned fin tip and by utilizing some of energy of the fin tip vortex, which rotates fluid, as forward thrust. This object is solved according to the present invention by a ship's rudder including the features of claim 1. Other objects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the appended drawings Figure 1 is a perspective view of a rudder according to the present invention; Figure 2 is a top plan view of the rudder showing a fin on one side of the rudder; Figure 3 (A) is an elevation view of the rudder and surrounding parts of a ship; Figure 3 (B) is a top plan view of the rudder and the surrounding ship parts; Figure 3 (C) is a side elevation showing only the rudder seen from the rear; Figure 4 (A) is an elevation showing the prior art rudder and its surrounding ship parts; Figure 4 (B) is a top plan view showing the prior art rudder and its surrounding ship parts; Figure 4 (C) is a side elevation showing only the prior art rudder seen from the rear; and Figure 5 is a perspective view showing the rudder of Figure 4 and accompanying current flow. The winglet at the tip of the fin in the present invention diffuses a strong fin tip vortex, which is generated from the tip of an energy saving fin mounted on a rudder and which extends in a rolled-up vortex filament toward the downstream, in the plane form. Thereby, the attack angle even at the tip of the fin is not decreased, and the induced drag of the fin is decreased. Due to this effect, the lift generated in a fin body is increased, and the thrust as the forward component of the lift, which is generated on the fin, also is increased. Further, since there is a rolled-up flow from the side of a positive pressure to the negative pressure side around the tip of the fin, an inflow into the winglet at the tip of the fin becomes a flow from the outside of said winglet at the tip of the fin to the inside thereof. The lift is generated on the winglet itself by adopting a blade section in the winglet at the tip of the fin, and the forward component of the lift is converted to thrust. Since a flow, which has components directed toward the hull center line under the influence of a hull positioned forward, is dominant due to the position of the rudder of a ship, the flow further increases the thrust generated by the winglet itself. An embodiment of the present invention will now be described with specific reference to the appended drawings of Figures 1 to 4 of the drawins. Figure 1 is a perspective view showing a rudder relative to the right-handed turning propeller, which is seen from the forward side of the starboard and shows the principle of the present invention. In the drawing, reference numeral 1 denotes a rudder body, which is positioned in a propeller slip stream. A fin 2 extending in the horizontal direction is mounted on said rudder 1 on the horizontal plane passing through the center of the propeller as in the prior art rudder. Fin 2 has a sweepback angle of ϑ at the leading edge thereof. A winglet 3 extending in the longitudinal direction ( in the upward direction in the example as shown in Figure 1 ) is mounted on the tip of the fin 2. In the case of the fin 2 having such winglet 3, a vortex at the tip of the fin 2 is depressed and weakened by the above-mentioned winglet 3 and thereafter flows in the form of a plane. Accordingly, the attack angle α t at the tip of the fin is not much decreased. There is little difference between the attack angle α t and the attack angle α r at the root of the fin. The lift generated on the fin 2 yet obtains the forward component, that is, the thrust. Since the winglet 3 has an attack angle α w due to a converging flow along the hull form of the stern, a lift L w having a forward component T w as shown in Figure 2 is generated. Such forward component T w is converted to thrust. As described above, in the present invention, the thrust at the tip of the fin 2 is not decreased. Moreover, a new or additional thrust can be obtained on the winglet 3. Thereby, the propulsive efficiency of the rudder is increased. It was confirmed in a tank test that horse power was decreased by about 2 % independent of the ship load conditions. The above-mentioned winglet has an effect in any of the upward direction and downward direction relative to the fin. For example, as shown in Figure 3 (A) to (C), taking into consideration that cambers of the fin are reversed on the starboard and on the port side, the winglet can produce the symmetrical effects to the left and the right by causing the cambers to extend in a certain direction. Further, the winglet may be divided into a front portion and a rear portion. In any of the cases, it can be expected that the thrust at the leading edge of the fin is prevented from being decreased and a new thrust at the leading edge of the fin is generated.

Claims: A ship's rudder, comprising: a main rudder body (1) having a cross-sectional shape which is streamlined and symmetrical relative to the vertical center plane of the rudder (1); and a pair of fins (2) extending horizontally from opposite vertical surfaces of the rudder body (1), symmetrically arranged relative to the center plane of the rudder (1), having the form of a taper fin wherein a chord length of the fin (2) decreases from a root of the fin (2) toward the tip of the fin (2), having respective leading edges, which coincide at the center plane of the main rudder body (1), and located at the same level as the center line of a propeller of the ship: characterized by comprising: a pair of winglets (3), each of which is mounted on the outer end of a respective one of each of said pair of fins (2) and one of which is directed in the upward direction from the outer end of one of said pair of fins (2) and the other of which is directed in the downward direction from the outer end of the other of said pair of fins (2). The rudder of claim 1, characterized in that each said fin has a sweepback angle of 20 to 50° formed by a line extending along a leading edge of the fin and each said fin is perpendicular relative to the center plane of the rudder. The rudder of claim 1, characterized in that said chord length at the root of the fin is at least one and a half times the length ℓt at the tip of the fin. The rudder of claim 1, characterized in that said winglet has an attack angle, lift having a forward component which with said attack angle converts to a forward thrust. The rudder of claim 1, characterized in that said winglets each have a front portion and a rear portion, the front portion being directed vertically in one direction and the rear portion being directed vertically in the opposite direction.

Inventors: KASAHARA YOSHIKAZU C O NKK COR, OKAMOTO YUKIHIKO C O NKK CORPO, KASAHARA, YOSHIKAZU, C/O NKK CORPORATION, OKAMOTO, YUKIHIKO, C/O NKK CORPORATION
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1991
Main classification: B63H25
IPC codes: B63H25