GB2073689A - A Ship with Stern Ring - Google Patents

Abstract

A ship has a ring-shaped structure 1 which is arranged at the stern portion thereof and in front of a propeller 3. The rear end surface 1a of the ring is arranged perpendicular to the propeller axis. The front end surface of the ring has, below the level of the propeller axis, a lower portion 1b perpendicular to the propeller axis, and further has, above the level of the propeller axis, an upper portion 1c inclined to be gradually farther as it extends upward. <IMAGE>

Description

SPECIFICATION A Ship with Stern Ring The present invention relates to a ship provided at the stern portion thereof with a ring-shaped structure for controlling water flow.

In recent years, ships, such as a tanker, designed to sail at low speeds have become large with a result that Froude number has reduced, whereby viscous resistance has come to occupy a large proportion in the total resistance acting on the ships. Accordingly, reduction in the viscous resistance and the effective utilization of viscous wake for the improvement of propulsive efficiency are required. Further, propeller efficiency has decreased by the increment of propeller load coefficient as another result of the enlarged sizes of the ships. Improvement in the propeller efficiency, therefore, is also required. In order to meet these requirements, a ship has been developed which is provided at the stern portion thereof with a ring-shaped structure (hereinafter referred to as "stern ring") as viewed in the longitudinal direction of the ship.

Before describing the present invention, the general effects of the stern ring will firstly be explained briefly. A ship, when sailing along, brings forth friction wake due to the viscosity of sea water. The friction wake near the ship's stern comprises shear flow, in the boundary layer, running in the longitudinal direction of the ship and bilge vortex produced under the ship's bilge by three dimensional separation and having its rotational axis in said longitudinal direction. Since the friction wake diffuses due to the viscosity of the sea water, a propeller receives only a part of the wake.The stern ring prevents the diffusion of the wake and guides most of the wake to the propeller disc in addition to the fact that it per se produces thrust through its suction of the water, whereby the required thrust of the propeller is lessened and the propeller has an increased propulsive efficiency. Further, the stern ring stabilizes unstable flow in the vicinity of the stern, thereby reducing the resistance against the ship.

Still further, since the wake, when passing through the stern ring, is accelerated and uniformized, there would be reduced fluctuations in the thrust and torque of the propeller.

The above-mentioned effects of the stern ring, the hydrodynamic mechanism of which will be described later with reference to the accompanying drawings, are derived from circulation r produced around the stern ring. The circulation r acts to suck the water ahead of the ring. It, therefore, is required that the ring be so designed as to maximize the circulation r.

Additionally, if the circulation r is kept constant at any location around the ring, free vortex would not flow out and a possible resistance caused by the ring can be avoided. The extension of the length of the ring in the longitudinal direction of the ship may contribute to afford a larger circulation F. However, if the length is unduly extended, the frictional resistance on the ring increases, hence undersirable.

In the meantime, since large wake zone exists around the upper stern portion of the hull, the velocity of flow into the ring tends to be high near the stern bottom portion and low near the upper portion. It, therefore, is conceivable to shape the stern ring in an inverted triangle, as viewed widthwise of the ship, to make the circulation r constant around the ring. However, as the shape of the stern frame line varies, the wake distribution will inevitably change.For example, in case the stern frame is full at its bottom portion in front of the propeller, the flow velocity is increased near the upper portion of the stern and reduced near the lower portion in comparison with an ordinary wake distribution, whereby variations in the velocity distribution around the propeller axis becomes small and the stern ring of the inverted triangular shape as above will not serve to maintain the circulation r constant around the ring.

The main object of the present invention is to provide a stern ring which acts to keep the circulation r constant at any locations therearound and enhance the propulsive efficiency of a propeller when the shape of a stern frame line is such that it produces a wake distribution less variable circumferentially around the propeller axis.

To fulfil this object, the present invention provides a ship provided, at the stern portion thereof and in front of a propeller, with a stern ring having a rear end surface perpendicular to the propeller axis and a front end surface, whose lower portion below the level of the propeller axis is parallel with said rear end surface, and whose upper portion above the axis level is inclined to be farther from said rear end surface as it extends upward.

Other numerous features and effects of the present invention will bye readily understood from the description of a preferred embodiment given with reference to the accompanying drawings, in which: Figure 1 is a diagram illustrating the hydrodynamic effects of a stern ring; Figure 2 is a sectional view showing a flow distribution in the vicinity of a propeller at a stern portion; Figure 3 is a diagram showing the shapes of two typical stern frame lines; Figures 4a and 4b are diagrams respectively showing wake distributions near the corresponding sterns in Figure 3; Figure 5 is a graphic diagram showing flow velocity distributions near the stern portion; Figure 6 is a fragmentary side elevation of a ship with a stern ring according to an embodiment of the present invention; and Figure 7 is a view showing the stern ring in Figure 6 as seen from the aft side of the ship.

With reference to Figure 1, the hydrodynamic effects of a stern ring 1 is described firstly. At one section of the ring 1 , fluid attacks the ring with an angle of incidence a with respect to the zero lift angle line X-Y of the ring, at which time a flow having circulation r is produced around the ring 1.

The flow in turn affords a lift L perpendicular to the incident flow. The forward component of the lift L adds up to the thrust of a propeller and thereby apparently reduces hull resistance. The flow having circulation r also acts to suck fluid, thereby preventing the diffusion of the friction wake. These effects are promoted by so designing the ring 1 as to maximize and equalize the circulation r therearound, as stated before.

With reference to Figures 2 through 5, wake distribution in the vicinity of the stern ring is described. In case of a ship, such as a tanker or a bulk carrier, comprising a full stern portion, a pair of bilge vortices having their rotational axes in the longitudinal direction of the ship are formed on both sides of the center line of a hull 2, as shown in Figure 2. (Figure 2 is a cross-sectional view showing a flow distribution adjacent to the propeller of a model ship). The bilge vortices are brought about by three dimensional separation of the flow running adjacent to the stern bilge portion, and their intensity and locations are affected by the shape of the stern frame line. The vortices, in turn, greatly affect the condition of the wake distribution near the stern portion.

Figure 3 shows two typical shapes of stern frame line, A being named V-shaped frame line and B U-shaped frame line. In the case of the V shaped frame line, the centers of the bilge vortices are located above the level of the propeller axis and the intensity of the vortices is not high. Contrary, in the case of the U-shaped frame line, the centers of the bilge vortices are located at about the same level as the propeller axis and the intensity of the vortices is higher than in the case of the V-shaped frame line.

Wake distribution under the influence of the bilge vortices is such that eyes corresponding to a portion where the wake is the largest (the flow velocity is the smallest) exist near the centers of the vortices. Figures 4a and 4b shows wake distributions respectively corresponding to the frame lines A and B in Figure 3. Figure 5 shows flow velocity distributions A and B near the stern ring 1 obtained on the basis of the respective wake distributions in Figures 4a and 4b. In Figure 5, 6 indicates an angular position around the propeller axis with respect to a vertical reference line (see Figure 7), and V(1-w) indicates a flow velocity at an angle 6, where V is the ship speed and w is the wake coefficient.In the case of the wake distribution shown in Figure 4a, V(1-w) is small above the level of the propeller axis and large below the same level as elucidated by a broken line A in Figure 5. In the case of the wake distribution given in Figure 4b on the other hand, the outer fast flow is guided into the upper part (S is small) due to the intense bilgevortices and V(1 w) is increased in that part as indicated by a solid line B, while the flow velocity in the lower part (8=90"-1 800) is decreased since the inner fast flow is shifted outward. Under the flow velocity distribution such as the line A in Figure 5, aforementioned stern ring having an inverted triangular shape as viewed sidewise is effective, whereas the ring is required to have a different shape in the case of the flow velocity distribution B.

Figures 6 and 7 illustrate an example of a stern ring effective under the flow velocity distribution such as B is Figure 5. A ring-shaped structure (ring) 1 is mounted in front of a propeller 3 under the water line to overlap the both sides of the stern portion of a hull 2. The diameter of the ring 1 approximately equals the propeller diameter at the rear end thereof and gradually increases forwardly. The rear end surface 1 a of the ring 1 is arranged perpendicular to the propeller axis 0.

The front end surface of the ring 1 has, below the level of the axis 0, a lower portion 1 b parallel with the rear end surface 1 a, and further has, above the level of the axis 0, an upper portion 1 c arranged to be gradually farther from the rear end surface, i.e., inclined to give larger ring length, as it extends upward. Alternatively, the ring 1 may be installed within a range between a location where the rear end surface 1 a of the ring 1 is disposed in front of the propeller tip and a location immediately before where the rear end of the ring 1 surrounds the propeller tip. Preferably, the ratio of a distance I between the ring rear end surface 1 a and the propeller tip to the propeller diameter Dp is within the range of 0.05 to 0.15.

The length of the ring 1 is desirably 30% to 60% of the propeller diameter Dp at the level of the propeller axis 0. Indicated at 4 is a rudder.

According to the above construction, the length of the ring 1 is so adapted, in consideration of the wake distribution under the influence of the U-shaped frame, as to equalize circulation r around the ring 1, thereby maximizing the effects of the ring 1.

Claims (3)

Claims

1. A ship comprising a stern ring at the stern portion thereof and in front of a propeller, wherein characterized that the rear end surface of the stern ring is arranged perpendicular to the propeller axis, and that the front end surface has a lower portion, below the level of the propeller axis, parallel with said rear end surface and an upper portion, above the level of the propeller axis, inclined to be gradually farther from said rear end surface as it extends upward.

2. A ship as set forth in claim 1, wherein characterized that a distance between the rear end surface of the ring and the propeller tip is 0.05 to 0.15 times the propeller diameter.

3. A ship substantially as described herein with reference to and as illustrated in Figures 6 and 7 of the accompanying drawings.