GB2111007A

GB2111007A - Rudder bulb

Info

Publication number: GB2111007A
Application number: GB08234877A
Authority: GB
Inventors: Akira Tanaka; Tadao Yamano; Yasuki Yamashita; Hiroshi Okamoto
Original assignee: Kawasaki Heavy Industries Ltd; Kawasaki Jukogyo KK
Current assignee: Kawasaki Heavy Industries Ltd; Kawasaki Motors Ltd
Priority date: 1981-12-08
Filing date: 1982-12-07
Publication date: 1983-06-29
Also published as: NO154262B; DE3245125C2; NO154262C; NO824120L; DE3245125A1; GB2111007B

Abstract

A bulb 6 is provided aft of a ship's propeller, fixed to the rudder 2 in axial alignment with the propeller. The propeller is located forward of the rudder and has a cap on the aft end of the propeller boss, and the rudder is supported at the top by a bracket 4 which extends down from the ship's hull but terminates above the bulb. The bulb has a maximum diameter larger than the maximum diameter of the propeller boss, and its forward end is adapted to closely approach, or to receive within a recess in its forward end, the cap on the propeller boss. <IMAGE>

Description

SPECIFICATION Rudder bulb This invention relates to a bulb provided on the rudder aft of the screw propeller of a ship or marine vessel.

A rudder bulb properly shaped and sized and located just aft of a propeller, improves the ship propulsion efficiency. In general, such a bulb has a maximum diameter larger than that of the propeller boss.

Figure 1 shows a G stern with a rudder 2 supported adjacent its top by a horn 4 and at its bottom. A conventional costa bulb 6 is fixed to the rudder 2 in axial alignment with the propeller 8.

It is known that the power-saving effect by providing a rudder bulb increases as the bulb approaches the propeller.

Figure 2 shows another type of stern with a rudder 2a supported by a post 4a, which extends between the rudder 2a and propeller 8a. Another conventional costa bulb 6a is provided close to the propeller 8a by dividing the bulb into a rear or main part 10 fixed to the rudder 2a and a front part 12 fixed to the post 4a.

However, in a balanced rudder such as a G-type rudder (Figure 1) or a semi-balanced rudder such as a mariner-type rudder, which have no rudder post such as 4a in Figure 2, considering the bulb arrangement and sizes adopted in the conventional costa bulb, and the necessity to avoid the contact of the bulb with the propeller cap when the rudder is operated, it has not been possible to provide the bulb close to the propeller.

Figures 3(A) and 3(B) show conventional radial reaction fins 14 attached to an enlargement 6b on a rudder 2b, to absorb the fin thrust converted from the energy of the rotating water flow remaining aft of the propeller 8b in the same direction as the propeller rotation. However, the flow field aft of the propeller is complicated. As shown in Figure 4, the flow is asymmetric about the center line. Especially on the starboard side, as a result of interaction between the rotating flow induced by the propeller and the flow containing vortices coming from forward of the propeller, a comparatively strong rotating flow in the reverse direction to the propeller rotation exists around the propeller blade tips, and inside this flow, a zone without any rotating flow also exists. In such a flow field, the conventional fins 14 function as a drag as well as generating no thrust.

According to the present invention, there is provided a bulb fixed to a rudder substantially in axial alignment with a ship's propeller, which is located forward of the rudder and has a cap on the aft end of the propeller boss, the rudder being supported at least adjacent its upper portion by a member which extends downwardly from the ship's hull only to above said bulb, said bulb having a maximum diameter larger than that of the propeller boss, said bulb having a forward end adapted to be located at or forward of the aft end of the propeller cap.

In a preferred embodiment, such a bulb is provided with reaction fins thereon for further propulsion efficiency.

Preferred embodiments of this invention will now be described with reference to the accompanying drawings, wherein: Figures 1 and2 are schematic fragmentary side views each of a stern with a conventional costa bulb; Figures 3(Aj is a schematic fragmentary side view of a stern with conventional reaction fins; Figure 3(B) is a front view of parts in Figure 3(A); Figure 4 is a rear view of a flow field just aft of a propeller; Figure 5(A) is a schematic side view of a stern, showing an embodiment of this invention; Figure 5(B) is a front view of parts in Figure 5(A); Figures 6 and7 are schematic side views showing other embodiments of this invention; Figures 8(A), 8(B) are views similar to Figures 5(A), 5(B), but showing still another embodiment;; Figure 9 is a schematic side view showing a different type of rudder bulb.

Referring to Figures 5(A)/(B) for the first embodiment applied to a G stern, a rudder 2c is supported steerably adjacent its top by a horn 4c secured to the ship's hull, and at its bottom by a shoe piece 16 on the hull.

Fixed to the rudder 2c is a bulb 6c, which consists of a main or rear part 10c and a pair of upper and lower, functional or front parts 12e formed integrally with the main part.

The main part 1 Oc has a known form similar to the bulb in Figure 1, and a spherical front end projecting forward of the front end edge of rudder 2c and spaced from the propeller cap 18 covering the rear end of propeller boss 20.

The front parts 1 2c project forward from the periphery of main part 1 Oc at the maximum diameter of the front part, which is larger than the diameter of propeller boss 20, beyond the spherical end of main part 10c. The curved outer surfaces of front parts 12c continue from the surface of main part 1 Oc. The front parts 1 2c are symmetric with each other overlapping the propeller cap 18 or, in other words, forming a space therebetween into which the rear end of cap 18 extends.

Figure 6 shows another form of bulb 6d comprising a rear part 10d of the known shape, which is fixed to a rudder 2d of the same type as in Figures 5(A)/(B).

The bulb 6d further comprises a bowl-like functional front part 12d, which has a plane bottom fixed to the plane rear end of propeller cap 18d, and a rear concave recess 22d into which the front end of rear part 1 2d extends with a gap for the head of main part 10d to move in the recess 22d. The front part 12d has a spherical outer surface for rough or approximate continuity with the outer surface of main part 1 Od.

Figure 7 shows a mariner stern with a rudder 2e supported at its upper portion by a horn 4e. Fixed to the rudder 2e is a bulb 6e lying just below the lower end of horn 4e. The bulb 6e is formed with a front end recess or concavity 1 2e, into which the rear end of propeller cap 18e rotatably extends.

It has been proved experimentally that the optimum sizes of the bulbs in the above embodiments are as follows: Relationships of the maximum bulb diameter A, propeller boss diameter B and propeller diameter C: A/B = 1.5 to 2.0 A/C = 0.25 to 0.35 Relationship of the radius R of curvature of the outer surface of the functional bulb part and the maximum bulb diameter A: R/(A/2) = 0.80 to 0.9 Figures 8(A)/(B) show a G stern with a bulb 6f, which has substantially the same structure as in Figure 7, but is provided with a plurality of radial reaction fins 14f on its outer periphery adjacent the location of its maximum diameter.

By suitably combining a large bulb and reaction fins, the disadvantages of the bulb and fins can be mutually cancelled. The fins unify the rotating water flow from the propeller, as indicated by arrows in Figure 8(A), to reduce the resistance to the bulb. By fitting the large bulb just aft of the propeller, the flow field near the propeller blade tips on the starboard side, where the fins are not effective, i.e., where there is a flow reverse to the propeller rotation and a weak rotating flow, is displaced beyond the circumference of the propeller disc. At the same time, the rotating flow around the bulb is made uniform.

Accordingly, the fins can more efficiently generate the thrust.

By model tank tests of various ships, it has been proved that the results of the fins are the best when the relation between the fin diameter G and propeller diameter F is as follows: G/F = 0.5 to 0.8 The fins should be positioned forward of the after perpendicular of the hull as well as adjacent the location of the maximum bulb diameter.

Apart from this invention, Figure 9 shows a rudder 29 of the semi-balanced hanging type (mariner type), which is supported by a horn 49 extending downwardly beyond the axis of propeller 89. A bulb 69 has a rear part 10g and a front part 12g both of substantially the same construction as in Figure 2, but the front part 12g has radial fins 149 thereon. The front part 12g is fixed to the horn 4g.

Claims

1. A bulb fixed to a rudder substantially in axial alignment with a ship's propeller, which is located forward of the rudder and has a cap on the aft end of the propeller boss, the rudder being supported at least adjacent its upper portion by a member which extends downwardly from the ship's hull only to above said bulb, said bulb having a maximum diameter larger than that of the propeller boss, said bulb having a forward end adapted to be located at or forward of the aft end of the propeller cap.

2. A bulb according to Claim 1, wherein said fore end of the bulb comprises a pair of upper and lower forward projections from said bulb, said projections forming a space therebetween into which the aft end of the propeller cap extends.

3. A bulb according to Claim 1, wherein the propeller cap has a bowl-like member fixed at its bottom to the aft end of the propeller, and said forward end of the bulb extends movably into said member.

4. A bulb according to Claim 1, wherein said forward end of the bulb is formed with a recess, into which the aft end of the propeller cap rotatably extends.

5. A bulb according to any one of the preceding Claims, and further comprising radial reaction fins on its outer periphery adjacent the location of its maximum diameter.

6. A bulb fixed to a rudder, substantially as described with reference to Figures 5A and 5B, or Figure 6, or Figure 7, or Figures 8A and 8B, or Figure 9, of the accompanying drawings.