CA1237607A - Ship's stern bulb - Google Patents

Abstract

ABSTRACT

This invention relates to the propulsion of self-propelled ves-sels, and in particular to the form of a ship's stern. The invention des-cribes a new and distinctive form of bulbous-shaped stern form which, unlike other stern bulbs developed to date, does not fair into the hull, instead in-tersecting it to create a sharp discontinuity between surfaces. This discon-tinuity creates a turbulent zone along the said intersection, allowing the wake to reorganize and to follow in a desired pattern the bulb's shape in the proximity of the propeller. Thus a more uniform wake field is formed within the propeller disc, consequently improving propeller efficiency and reducing propeller induced vibration. These effects, which result in improved quiet-ness and overall efficiency of a running propeller, along with other improved characteristics, have been confirmed by a preliminary series of model tests.
The form of the bulb, and its attendant benefits, are described on the follow-ing pages.

Description

7 6 lo SHIP'S STERN BULB Page 2.
Invention by Robert G. Alien, P. Erg.
Peter J. van Depone M.Sc.

Specification BACKGROUND OF INVENTION

Propeller induced vibration and all associated problems caused by propeller blades running in a non-uniform wake field have been a subject o-f research efforts ever since the propeller itself was introduced for ship pro-pollution.

Theoretically, propeller blades may be designed for a particular progressive speed of the propeller in liquid, with its parameters (area, pitch, etc.) chosen to produce maximum lift and minimum circumferential drag force when rotating in said liquid at that speed. Unfortunately, due to the influence of the ship's hull, the wake is disturbed and has a non-uniform velocity across the propeller disc area (as well as everywhere else in the hull's proximity), and usually a -turbulent character in that zone. The blade, in traveling through a liquid of changing speed during one revolution, pro-dupes a variable lift force which results in vibration of -the blade, as well as a general loss in efficiency. This problem has been addressed by many de-signers in a variety of ways, introducing either systems of fins of different ; 30 sizes and configurations, or various bulbous forms immediately forward of the propeller in an attempt to induce a more uniform wake pattern.

The concept behind this invention which distinguishes it from all other systems is, rather than attempting to force the already developed and deformed fluid flow into a new pat-tern, to introduce a bulb form which creates a turbulent zone well ahead of the propeller where the deformed flow would be disturbed and then reorganized along a properly defined geometric surface into the desired uniform pattern. The described flow disturbance is achieved by introduction of an abrupt change in hull surface and an additional volume in way of water flow, the first being the line of intersection of the said bulb ~237)6~

SHIP'S STERN BULB Page 3.
Invention by Robert G. Alien, P. Erg.
Peter J. van Deepen, M.Sc.

form and hull, the latter the bulb itself. Preliminary series of tests India gate that in order to develop the described turbulent zone, the angle between bulb and hull surface (indicated as on figure 2) should be less -than 120 degrees in forward part of the bulb and bulb should intersect with hull with-out firing or with only minimal firing. Again, tests indicate that for a I common hull size and speed the said turbulence phenomenon begins -to diminish at about 300 mm radius of firing. Furthermore, the bulb surface, purposely geometrically independent of the hull shape, would be designed in such a way that -the reorganized wake would regain its luminary character and maintain it all the way to the propeller. By so doing, an additional effect of hull resistance reduction can be expected since part of the turbulent zone which usually exists behind a ship's stern and creates significant drag force, would be eliminated. The resistance reduction thus created would at least balance or offset the additional resistance created by the bulb surface itself, erect-in an overall resistance reduction. To date, three independent series of model tests have proven this theory and, in one case, as much as a lo overall resistance reduction was demonstrated. In addition to -the features of the disclosed invention described above, in order to achieve a desired circular pattern of the now luminary wake field in the propeller disc, transverse sea-lions of the said bulb should be either circular or, if it is found for a par-titular vessel that the said circularity is deformed, of such near-circular shape that will correct the wake field pattern.

There is yet another aspect of this invention which contributes to savings in capital investment in addition to vessel operating costs. By introducing the bulb described herein, it is possible to avoid the traditional complex shape of the aft part of -the hull, introducing instead a more simple one, less expensive to construct. The bulb itself, although appearing to be of unusual shape, is also relatively inexpensive to build.

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SHIP'S STERN BULB Page 4.
Invention by Robert G. Alien, P. Erg.
Peter J. van Deepen, M;Sc.

BRIEF DESCRIPTION OF THE DRAWINGS
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For better understanding of this invention, reference is made to the drawings, wherein:

Figure 1 Shows the outside elevation of a ship's hull fitted with the bulb, which is the subject of this invention.

Figure 2 Shows the plan view of a horizontal section waterline through the center of the bulb and adjacent part of ship's hull.

Figure 3 Shows a typical transverse section of -the bulb.

DETAILED DESCRIPTION OF TIE INVENTION

;75 The bulb described herein may have a certain range of shapes, sizes and locations necessary to achieve desired angle of intersection with the hull and is dependent upon the hull shape, design speed, installed power and a number of other parameters of the topic vessel. For sake of clarity, only the most typical example is presented on drawings 1, 2 and 3 attached hereto. In general terms, the bulb which is the subject of this invention and is shown on figure 1, is a separate body outlined by dotted line (1), partly built into the hull without firing, and intersecting the hull along the locus

(2). Also shown are hull profile at centerline (3) and propeller (5). In order to stabilize the wake between the hull and the bulb, a connecting fin (4) is introduced. An additional short box-section keel (6) is optional if required for docking purposes. An additional advantage of fitting the said bulb is the possibility of building the basic shape along the simplified centerline (3) as opposed to the more traditional and complex shape indicated on figure 1 by dotted line (7).

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I So SHIP'S STERN Bull Page 5.
Invention by Robert G. Alien, P. En.
Peter J. van Deepen, M.Sc.

Figure 2 shows again the outline of the bulb (1) and simplified (thus easier to build) waterline (8) as opposed to traditional bulbous water-line (9). The turbulent zone (10) of the water flowing along the hull is then transformed into the luminary zone (11) in which the propeller works, as opposed to traditional location of turbulent zone (12). The critical angle, I between bulb and hull surface is also indicated thereon.

Figure 3 shows a section through the largest diameter of the bulb, where (13) is the outline of the bulb and (14) the outline of the simplified hull after-body, while (15) represents the section of a "traditional" stern bulb, complex and difficult to build. A typical Frame layout is shown on the 100 left hand side of the section and illustrates additional usable space (16) created by the bulb.

The bulb's profile (longitudinal section I shown on figures 1 and 2), is of particular importance and should be designed for a particular hull size, propeller shaft depth and most of all for design speed, and as 105 such, it cannot be claimed by the authors of this invention. In both series of model tests previously referred to, a typical NIKKO 0030 profile was used.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

(1) In a ship having a propeller as means of propulsion, the improve-ment comprising a portion of said ship's hull located immediately forward of said propeller and having a form of a bulbous shape of circular transverse sections, the said bulbous shape being elon-gated horizontally between its leading and trailing edges and intersecting the non-bulbous part of hull surface without fairing forward of the largest transverse section of the bulbous shape, the said bulbous shape increasing in transverse section radius between the leading edge and an intermediate section and decreas-ing in transverse section radius between the intermediate section and the trailing edge immediately forward of the propeller, the said bulbous shape being of such profile and size relative to ad-jacent hull that there is an abrupt increase in hull width aft of the leading edge of intersection line between said hull and bulb, and that there are points along said intersection line where the angle between the hull and bulb surfaces, measured from outside in a plane perpendicular to the intersection line at said points, is between 0 and 120 degrees.

(2) The bulb as defined in claim 1, which is faired into the hull along their intersection lines with a curvature into which a circle of radius equal to or less than 300 mm would fit without creating a gap between the said circle and the surface.

(3) A bulb as defined in claim 1, which has every transverse section between points of intersection with the hull exactly or approxi-mately circular, which is defined as any line located entirely between the smallest possible circle tangent from outside to the said line and another concentric circle of radius equal to 75% of radius of the said tangent circle.

(4) A marine vessel that incorporates one or more bulbs as defined in claims 1, 2 or 3.

(5) A marine vessel that incorporates one or more bulbs as defined in claims 1, 2 and 3 and a connecting vertical fin between the aft pa of at least one of the said bulbs, in the area where the said bulb is separate from the vessel's hull, and the hull.