VESSEL WITH A SINGLE SCREW HULL

Abstract: A vessel equipped with a single screw comprising a hull being symmetrical with regard to the hull cener line and a propeller shaft (3) being positioned eccentrically from the hull center line (2). The propeller shaft is positioned at a ratio (d/D) of 5 to 25%, where d represents a distance between the propeller shaft and the hull center line and D represents a diameter of the propeller.

Description: The present invention relates to a hull form of a vessel, and more particularly to a position where a propeller shaft is installed. A body plan of a single-screw hull equipped with a conventional symmetric type stern is shown in Fig. 1. Referential numeral 1 denotes a transverse sectional shape, 2; the hull center line, 3; a propeller shaft, 4; a propeller disc plane and WL; load waterline. It is well known that the propeller shaft is usually provided, on the hull center line, for a conventional type of a single-screw vessel. When the propeller shaft is installed in such a position, water inflows to the propeller disc plane are shown in Fig. 2. Fig. 2 represents graphically water inflow speed to the propeller disc plane. Fig. 2(A) is a representation of wake distribution, and Fig. 2(B) is a vector diagram for transverse velocity of water. Curved line (a) shows a ratio of wake speed generated on the propeller disc plane in relation to vessel speed, and vector (b) shows transverse direction of wake velocity generated on every point of the propeller disc plane. As clearly understood from these representations, inflows to the propeller disc plane are formed into symmetrical flows with regard to propeller shaft 3. In this manner, complicated distribution of wakes are generated while the vessel is sailing. As shown in Fig. 3, the wakes become symmetrical with regard to propeller shaft 3 positioned on hull center line 2. Vessels with high block coefficient and wide breadth have been increasing in number to raise loading capacity. Owing to this high blockage coefficient and wide breadth, vertical vortices around longitudinal axes are generated on the propeller disc plane, from the aforementioned wakes. These vertical vortices are generated in pairs by both sides of a vessel, unbalancing the wakes on the propeller disc plane. This results in reducing efficiency in propulsion and increasing hull-resistance. In those circumstances, there has been demanded a reduction of ratio of fuel consumption for sailing as well as improvement in loading capacity. To satisfy this demand, improvement in propulsive efficiency is indispensable. A prior art that is of interest is that disclosed in FR-A-2 320 859 wherein a boat hull having a single propeller is disclosed. The hull is so shaped as to define a heliciodal canal. This canal is positioned at the stern to one side of the centre line of the hull and because of this the cross-section of the hull changes to some extent at the stern according to where a cross section of the hull is taken. The single propeller of boat is arranged on that side of the centre line of the hull where the heliciodal canal is positioned. The object of the present invention is to overcome existing difficulties in reducing the loss of efficiency in propulsion due to the vertical vortices that are brought about by the wakes generated during the sailing of a vessel. FR-A-2 320 859 makes no reference to these vortices. In accordance with the present invention there is provided a vessel having a single screw propulsion system, the vessel comprising a hull which is approximately symmetrical with respect to a vertical center plane containing the center line of the hull; a propeller positioned laterally offset from said vertical center line of the hull on a propeller shaft for propelling the vessel, said propeller having a diameter D, the center of said propeller being laterally offset by a distance d from said vertical plane such that the ratio d/D is from 5% to 25%. The invention and its embodiments are described more fully in the attached detailed description with reference to the drawings, in which: Fig. 1 is a body plan showing an afterbody of a prior art vessel viewed from the stern. Fig. 2 is a graphic representation showing water inflow speed to a propeller disc plane for a prior art vessel; Fig. 3 is a graphic representation showing a vector diagram of water inflows on the propeller disc plane for a prior art vessel; Fig. 4 is a body plan showing an afterbody of a vessel viewed from the backward side, according to the present invention; Fig. 5 is an elevational view showing vector diagram of water inflows on the propeller disc plane, according to the present invention; Fig. 6 is a schematic illustration showing relationship between water inflows to a propeller disc plane and rotating direction of the propeller according to the present invention; Fig. 7 is a schematic representation showing plane views of embodiments of the present invention; and Fig. 8 is a graphic representation showing relation of a distance between a propeller shaft and the hull center line to relative propulsive power ratio against a prior art vessel according to the present invention. Referring now to the drawings, wherein like reference characters designate like parts or corresponding parts throughout the several views, Fig. 4 shows an afterbody of a hull body plan viewed from the backward side, according to the present invention. As shown in Fig. 4, the hull construction is symmetrical with regard to hull center line 2 and the propeller shaft is positioned eccentrically from the hull center line. Consequently, the only parts at which the propeller shaft is installed are asymmetrical. The work of this positioning of the propeller shaft will now be described. With reference specifically to the drawing, Fig. 5 represents vector diagram illustrating movements of water inflows on the propeller disc plane of a vessel. As shown in Fig. 5, water inflow vector (b) is transverse component of velocity which is symmetrical about hull center line 2. Propeller blades are rotated clockwise, on the axis of propeller shaft 3 which is positioned horizontally on the starboard side of the hull center line. Relation of the direction of the water inflows to the direction of the rotation of the propeller, is shown in Fig. 6. In Fig. 6, arrow 5 represents a direction of the water inflows which is indicated by vector (b) shown in Fig. 5. Arrow 6 represents a rotating direction of the propeller. As apparently understood from Fig. 6, the propeller constantly receives the water inflows that circulate reverse to the direction to which the propeller shaft is rotated. This gives such an effect as if the rotating speed of the propeller shaft were increased. In other words, increase of propulsion efficiency can be attained by this positioning of the propeller shaft. As described in the above, increase of propulsion efficiency is attained by rotating the propelling shaft clockwise when the propeller shaft is positioned on the starboard side of the hull center line, and by rotating the propeller shaft counterclockwise when positioned on the port side. Otherwise, for example, when the propeller shaft is positioned on the starboard side and rotated counterclockwise, the rotating direction of the propeller shaft becomes same with the circulating direction of water inflows. Consequently, the propulsion efficiency is lowered. When the propeller shaft is positioned on the port side and rotated clockwise, the propulsion efficiency is lowered as well. With reference now specifically to the drawing, Fig. 7 shows plane views of examples of the present invention. A rudder at the stern is positioned on the hull center line. Type (A) is a schematic representation illustrating propeller shaft 3 being positioned horizontally in parallel off the hull center line 2, without horizontal rake. (B) is a schematic representation illustrating propeller shaft 3 being constructed with horizontal rake angle to the hull center line. It depends on the space of an engine room and the capacity of a main engine of which types (A) or (B) is adopted. According to the test results, there was no difference between types (A) and (B) with respect to steering ability and propulsion efficiency. In addition, there were no differences between one vessel equipped with a propeller shaft positioned off the hull center line and another vessel equipped with a propeller shaft conventionally positioned, with respect to the steering ability. Fig. 8 graphically shows relation of a distance between propeller shaft 3 and hull center line 2 to relative propulsive power ratio efficiency which was obtained through a water tank test of propelling a 200,000 DWT ore carrier. In Fig. 8, the ordinate shows a ratio of W(O)/W(C) [HP(O)/HP(C)] where W(O)[HP(O)] represents propulsive watts [horse powers] generated by an engine in the case of a propeller shaft positioned off the hull center line and W/(C) [HP(C)] represents propulsive watts [horse powers] generated in the case of a propeller shaft positoned on the hull center line, and the abscissa represents a ratio of d/D where d represents a distance between the propeller shaft and the hull center line and D represents a diameter of a propeller. As apparently recognized from Fig. 8, the relative propulsive power ratio shown by the W(O)/W(C) [HP(O)/HP(C)] is remarkably improved when the d/D ranges from 5 to 25%. If the ratio is less than 5%, the propulsive efficiency does not increase. On the other hand, if the ratio is over 25%, the propulsive efficiency does not increase, either. The ratio ranges from 10 to 15% most preferably. Other test results proved that rudder position was not required to be restricted owing to this positioning of the propeller shaft; the rudder position was not unfavorably affected. The present invention enabled the propulsive efficiency to be improved (by 10% approximately) by making use of vertial vortices which had caused a conventional vessel with the wide breadth and high blockage to reduce the propulsive efficiency. Moreover, the present invention also enabled to keep a hull structure symmetrical on both sides of the vessel.

Claims: A vessel having a single screw propulsion system, comprising: a hull which is approximately symmetrical with respect to a vertical center plane containing the center line of the hull; and a propeller positioned laterally offset from said vertical center line of the hull on a propeller shaft for propelling the vessel, said having a diameter D, characterized by the center of said propeller being laterally offset by a distance d from said vertical center plane such that a ratio d/D is from 5 % to 25 %. The vessel of claim 1, characterized in that said ratio d/D is from about 10 to 15 %. The vessel of Claim 1, characterized in that said propeller shaft is positioned on the starboard side of said vertical center plane of said hull, and means is provided for rotating said propeller shaft in a clockwise direction. The vessel of claim 1, characterized in that said propeller shaft is positioned on the port side of said vertical center plane of said hull, and means is provided for rotating said propeller shaft in a counterclockwise direction. The vessel of claim 1, characterized in that said propeller shaft is arranged substantially horizontally and substantially parallel to said vertical center plane of said hull. The vessel of claim 1, characterized in that said propeller shaft is arranged at an angle to said vertical center plane of said hull. The vessel of claim 6, characterized in that said propeller shaft is arranged substantially horizontally.

Inventors: MATSUMOTO, NORIHIRO PATENT & LICENCE, Matsumoto, Norihiro Patent &amp; Licence
Applicants: NIPPON KOKAN KABUSHIKI KAISHA
Application year: 1987
Main classification: B63H5
IPC codes: B63H5, B63B1