BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to ship propulsion in general and in particular to a method and apparatus for permitting an increased ducted propeller size for a given draft.

2. Description of Related Art

Many ships are propelled by propellers having airfoil shaped blades. Such propellers are typically rotated about an axis of a shaft to create thrust against the surrounding water. Many attempts have been made to increase the efficiency of such propellers and to increase their thrust for a given required input power from a motor or engine. One such attempt has been to provide a shroud or nozzle around the propeller as is commonly known as a ducted propeller or Kort nozzle.

As illustrated in Figure 1 , an illustration of a conventional ducted propeller is shown generally at 10 in which the propeller 12 is rotated about a shaft 14. The shroud or nozzle 16 surrounds the propeller to provide the aforementioned increase of thrust and efficiency, especially at low speeds.

One difficulty with conventional ducted propellers is that the nozzle or shroud which surrounds the propeller increases the overall diameter of the propeller assembly. In the case of conventional or Kort Nozzle, the shroud may be substantially larger than the propeller itself so as to catch as much water as possible thereby directing this additional water to the propeller. Accordingly ships fitted with such ducted propellers may have increased draft or may have the size of propellers that may be fitted thereto limited by the depth in which the ship is to operate. Such size of the propeller which may be used thereby limits the thrust that can be provided by the propulsion system for the ship. SUMMARY OF THE INVENTION

The new truncated nozzle alleviates this problem, by allowing to fit within the same draft limit a larger diameter shroud and propeller, which will produce higher thrust, or conversely, allow for less power and fuel consumption for the same thrust as conventional ducted propeller.

According to a first embodiment of the present invention there is disclosed a nozzle for a ship propeller having an axis of rotation. The nozzle comprises an annular shroud having a diameter and is adapted to surround the propeller. The shroud extends along the axis of the ship propeller and has inner and outer surfaces forming a foil. The outer surface of the shroud has a truncating surface extending along at least one of a top or bottom thereof.

The truncating surface may be located on a bottom of the shroud. The truncating surface may be located a top of the shroud.

The truncating surface may be substantially planar. The truncating surface may be substantially horizontal. The truncating surface may be convex having a radius of curvature of at least 10 times the diameter of the shroud. The truncating surface may be concave having a radius of curvature of at least 10 times the diameter of the shroud. The truncating surface may have a rounded edge with the outer surface of the shroud.

According to a further embodiment of the present invention there is disclosed an apparatus for propelling a ship comprising a screw propeller having an axis of rotation and a nozzle surrounding the screw propeller. The nozzle comprises an annular shroud having a diameter and is adapted to surround the propeller. The shroud extends along the axis of the ship propeller and has inner and outer surfaces forming a foil. The outer surface of the shroud has a truncating substantially horizontal surface extending along at least one of a top or bottom thereof. The screw propeller may be rotated by a shaft. The shaft may extend from a hull of the ship. The shroud may be supported by the hull.

The shaft may extend from a pod suspended below the ship. The pod may be rotatable about a vertical or close to vertical axis. The shroud may be supported by the pod.

Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate embodiments of the invention wherein similar characters of reference denote corresponding parts in each view,

Figure 1 is a cross-sectional view of a conventional ducted propeller.

Figure 2 is a bottom perspective view of an apparatus for propelling a ship according to a first embodiment of the present invention.

Figure 3 is a partial cross sectional view of the nozzle of Figure 2.

Figure 4 is a side view of the propulsion system of Figure 2 supported by a ship's hull.

Figure 5 is a side view of the propulsion system of Figure 2 supported from a rotating pod.

Figure 6 is a detailed rear view of the nozzle of Figure 2 having a concave truncating surface.

Figure 7 is a detailed rear view of the nozzle of Figure 2 having a convex truncating surface.

Figure 8 is a detailed front view of a propulsion system according to a further embodiment of the present invention.

DETAILED DESCRIPTION

Referring to Figure 2, an apparatus for propelling a ship according to a first embodiment of the invention is shown generally at 20. The apparatus comprises a screw propeller 22 surrounded by a nozzle or shroud 30. The propeller 22 may be of any known type and size as desired by a user for the required application. The propeller 22 is rotated and supported by a shaft 24 having an axis of rotation 26.

The nozzle 30 comprises an annular member extending between leading and trailing edges, 32 and 34, respectively along an axis 26 of the propeller 22. The nozzle is formed between inner and outer surfaces, 36 and 38, respectively forming a foil shape therebetween. It will be appreciated that any foil profile may be utilized for the nozzle.

With reference to Figures 2 and 3, the nozzle includes a truncating surface 40 extending across one or both of the top and bottom edges of the nozzle 30. The truncating surface 40 is positioned to extend substantially horizontally across the nozzle. The truncating surface 40 may be spaced apart from the nominal diameter of the nozzle 30 by a setback distance 42 which may be selected to reduce the height of the nozzle as much as possible while still providing sufficient strength to the nozzle. By way of non-limiting example, the setback distance may be selected to be between 10 and 100 percent of the nozzle foil profile. As illustrated in Figures 2 and 3, it will be appreciated that the truncating surface reduces the overall height of the nozzle 30 thereby correspondingly reducing the overall draft required for such a nozzle used with a propeller of a given size. Similarly, it will be appreciated that the reduced height of the nozzle 30 will permit a larger diameter propeller to be utilized thereby providing greater thrust for a given input power to the propeller. It will be appreciated that the truncating surface 40 will slightly reduce the efficiency of the nozzle in the location of the truncating surface due to the disruption of the flow path over the foil profile. However it will also be appreciated that the larger nozzle possible for a given permitted draft will also provide an additional thrust exceeding the losses due to the disrupted flow over the truncating surface. With reference to Figure 2 and 3, in a preferred embodiment, the truncating surface 40 may be substantially planar although the truncating surface may also be curved as illustrated in Figures 6 and 7. In particular the truncating surface 40 may be concave as illustrated in Figure 6 or convex as illustrated in Figure 7 with a radius of curvature 44. The radius of curvature is selected to reduce the thickness of the top and bottom of the nozzle 30 while preserving some of the foil profile and should therefore be selected to be large to provide as great a height reduction as possible. In particular, it has been found that a radius of curvature of greater than 10 times the diameter of the nozzle has been useful.

With reference to Figure 4, the nozzle 30 may be suspended from and supported by the hull 8 of a ship wherein the propeller 22 is located therein at the end of a propeller shaft 24. In such embodiments, the truncating surface 40 may be located at the bottom of the nozzle so as to present a substantially flat surface parallel to a bottom of the ocean, lake, river or the like. It will also be appreciated that in such embodiments, the top of the nozzle 30 may also include a truncating surface facilitating engagement with the hull 8 and permitting the nozzle and propeller to be located closer thereto. Optionally, as illustrated in Figure 5, the nozzle and propeller may be supported by a rotatable pod 50, such as by way of non-limiting example an azimuth thruster. In such embodiments, the pod 50 may be suspended from the hull 8 by a rotating member 52 which is rotatable about a vertical axis 54 with respect to the hull as are commonly known. Similar to as set out above, for such embodiments, the nozzle 30 may include either or both of a truncating surface at the top or bottom thereof.

With reference to Figure 8, according to a further embodiment the truncating surface 40 may extend across the top or bottom of the nozzle 30 at an angle relative to horizontal indicated generally at 60. Such an angle will facilitate matching a top truncating surface to the angle of the hull at that location. It will be appreciated that this will be useful in locating the truncating surface and thereby the nozzle as close to the hull as possible. Optionally, as illustrated in Figure 8, the truncating surface may be flared outwardly from the nozzle to prevent debris, for example ice, from jamming between the nozzle and the hull. As illustrated in Figure 8, the truncating surface and the outer surface 38 of the nozzle 30 may also have a curved surface 62 having a radius generally indicated at 64. The radius may be selected to be between 5 and 15 % of the nozzle outer diameter.

While specific embodiments of the invention have been described and illustrated, such embodiments should be considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims.