BACKGROUND The use of sacrificial anodes for protecting steel, brass, bronze or aluminum parts from corrosion in sea water is well established in marine engineering. It is thus well known that for example steel propeller shafts with bronze bearings are subject to corrosive electrochemical galvanic reaction in sea water, which shortens the potential lifetime of the shaft. To protect the shaft from corrosion, a replaceable sacrificial anode is placed near the shaft, said anode being made of a metal (most often zinc) which is subject to preferential corrosion relative to the shaft material when the parts are submerged in an electrolyte, such as sea water. In such a case, it is desirable to mount the sacrificial anode in a way which permits the anode to be replaced without having to detach the propeller.

Such a propeller design is described in United States Patent No. 4,077, 742, in which a nose piece, or so called spinner, apart from its hydrodynamic drag-reducing function, also serves as a sacrificial zinc anode mounted on a reusable brass propeller fastener nut. A problem with this design is, however, that the original outer peripheral contour of the spinner-which is shaped for

optimum hydrodynamic performance-gradually deteriorates and roughens as-the corrosion of the anodic material in the spinner proceeds, causing increased drag and loss of propulsion performance.

SUMMARY OF THE INVENTION The above mentioned problem is alleviated by providing a propeller spinner for a marine propeller, said propeller comprising multiple propeller blades attached to a propeller hub adapted for attachment to a propeller shaft, and said propeller spinner being adapted for attachment to the propeller shaft and comprising a sacrificial anodic material for protecting the propeller shaft and/or propeller hub from corrosion. The invention is especially characterized in that said spinner comprises: - a hollow spinner cone made of a non-anodic material, said spinner cone having at least one perforation in its outer peripheral surface, and - an anodic insert body made of said sacrificial anodic material, said insert body being substantially contained within the hollow spinner cone, the insert body having at least one radial protrusion extending at least partially through said perforation in the spinner cone.

In an advantageous embodiment of the invention, the at least one radial protrusion of the anodic insert body extends fully through a corresponding perforation in the spinner cone in such a way as to form an outer surface which is flush and conform with the outer peripheral surface of the spinner cone, in a non-corroded state of the insert body.

In one embodiment, the hollow spinner cone has multiple perforations in its outer peripheral surface and further has axially extending members located between the perforations so as to form a grid pattern which-upon rotation of the propeller-is adapted to generate a hydrodynamic rotational body identical in shape to the original outer peripheral contour of the spinner, in a

state where the protrusions 10 of the anodic insert body 9 are at least partially consumed by corrosion.

In a suitable embodiment, the perforations in the spinner cone and the corresponding radial protrusions of the insert body are longitudinally shaped in the axial direction of the propeller shaft. Further, the perforations in the spinner cone and the corresponding radial protrusions of the insert body may suitably be substantially rectangular.

In one embodiment, the perforations in the spinner cone and the corresponding radial protrusions of the insert body are arranged in multiple axially interspersed rows along the outer peripheral surface of the spinner cone. Preferably, each row includes between six to sixteen perforations and corresponding protrusions.

The anodic insert body may be either removably attached to a reusable spinner cone, or it may alternatively be permanently affixed to the spinner cone, so as to form a single replaceable unit. In the latter case, the spinner cone is preferably substantially made of plastic, and may be moulded directly onto the anodic insert body.

Although the anodic insert body may normally be made of zinc, other metals serving as sacrificial anodes may be used alternatively.

The invention further provides a marine propeller comprising multiple propeller blades attached to a propeller hub adapted for attachment to a propeller shaft, said propeller being provided with a spinner mounted on the propeller shaft and comprising a sacrificial anodic material for protecting the propeller shaft and/or propeller hub from corrosion. The invention is especially characterized in that said spinner comprises: - a hollow spinner cone made of a non-anodic material, said spinner cone having at least one perforation in its outer peripheral surface, and

- an anodic insert body made of said sacrificial anodic material, said insert body being substantially contained within the hollow spinner cone, the insert body having at least one radial protrusion extending at least partially through said perforation in the spinner cone.

Other features and advantages of the invention will be described below in the description of suitable embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in greater detail by way of example only and with reference to the attached drawings, in which fig. 1 shows a broken, partial cross-sectional side view of a propeller according to an exemplary embodiment of the invention. In the figure, the anodic insert body is new and non-corroded and thus flush with the outer peripheral surface of the hollow spinner cone; fig. 2 shows a broken, partial cross-sectional side view of a propeller according to the embodiment shown in fig. 1. In this figure, however, the anodic insert body is corroded and thus no longer flush with the outer peripheral surface of the hollow spinner cone; fig. 3 shows a perspective view of the hollow spinner cone. The figure illustrates the grid pattern of perforations in the outer peripheral surface of the spinner cone, and fig. 4 finally shows a perspective view of the anodic insert body in a non-corroded state.

DESCRIPTION OF SUITABLE EMBODIMENTS OF THE INVENTION In fig. 1, reference numeral 1 generally denotes a schematically illustrated marine propeller according to an exemplary embodiment of the invention.

The propeller 1 comprises multiple propeller blades 2 attached to a propeller hub 3, which in turn is adapted for attachment to a propeller shaft 4.

The propeller 1 is further provided with a generally conical propeller spinner 5 mounted on the end of the propeller shaft 5. The propeller spinner 5 comprises a hollow spinner cone 6 made of a non-anodic material, such as plastic. By the term non-anodic is meant that the material of the spinner cone 6 does not act as an anodic material in an electrochemical, galvanic reaction when submerged in sea water.

The hollow spinner cone 6 has multiple perforations 7 in its outer peripheral surface 8. The shape and number of these perforations 7 will be described in closer detail below.

In order to protect the propeller shaft 4 and/or the propeller hub 3 from corrosion, the propeller spinner 5 further comprises an anodic insert body 9 made of a sacrificial anodic material such as zinc. The anodic insert body 9 is substantially contained within the hollow spinner cone 6, and is provided with one radial protrusion 10 extending through each of the perforations 7 in the hollow spinner cone 6. As seen in fig. 1, the anodic insert body 9 is provided with a centrally extending through hole 12 which is directly abutting the propeller shaft 4 in order to galvanically protect the propeller shaft 4 from corrosion.

Each radial protrusion 10 of the anodic insert body 9 extends fully through the corresponding perforation 7 in the spinner cone 6 in such a way as to form an outer surface 11 which is flush and conform with the outer peripheral surface 8 of the spinner cone 6, in a non-corroded state of the insert body 9, as seen in fig. 1.

As is apparent from fig. 1, the spinner 5 serves as a fastening nut for the propeller 1 in the shown exemplary embodiment. To this end, the hollow spinner cone 6 is provided with a central, internally located and axially extending threaded sleeve portion 13 adapted for engagement with a corresponding threaded end section 14 of the propeller shaft 4. The sleeve portion 13 is generally conically shaped, and the anodic insert body 9 conforms fully to its shape in this embodiment.

In fig. 2, the anodic insert body 9 is shown in a corroded state, and thus the radial protrusions 10 are no longer flush with the outer peripheral surface 8 of the spinner cone 6. In this situation, the original outer peripheral contour of the spinner 5-which is shaped for optimum hydrodynamic performance-is nevertheless maintained during continued rotation of the propeller 1. This is achieved according to the invention, in that the hollow spinner cone 6 has axially extending members 15 located between the perforations 7 so as to form a grid pattern which-upon rotation of the propeller 1-is adapted to generate a hydrodynamic rotational body identical in shape to the original outer peripheral contour of the spinner 5, in a state where the protrusions 10 of the anodic insert body 9 are at least partially consumed by corrosion. This feature of the invention serves to maintain the operational performance of the propeller 1 regardless of the corrosion state of the anodic insert body 9, and also provides an indication of when the anodic insert body 9 should be replaced-the corroded surface of the anodic material being clearly visible and tangible within the perforations 7 of the spinner cone 6.

In the shown embodiment, the axially extending members 15 are arranged substantially in parallel with the propeller shaft 4. However in alternative, not shown embodiments, these members 15 may instead be arranged obliquely to the extension of the propeller shaft 4, but still in the general axial direction of the propeller shaft 4.

In fig. 3, the hollow spinner cone 6 is shown separately in perspective. The above mentioned grid pattern formed by the perforations 7 and the axially extending members 15 can here be clearly observed. In the shown exemplary embodiment, the perforations 7 are longitudinally shaped- substantially rectangularly-in the axial direction of the propeller shaft 4.

Furthermore, the perforations 7 in the spinner cone 6 are arranged in two axially interspersed rows-denoted by"A"and"B"respectively-along the outer peripheral surface 8 of the spinner cone 6. In the shown embodiment, each row A, B includes twelve perforations 7, adding up to twenty four perforations in total. In alternative, not shown embodiments, the number of perforations may suitably range from six to sixteen perforations 7 in each row A, B. However, the number of perforations 7 in each row A, B may not necessarily coincide, and the number of rows may also exceed two.

Fig. 4 shows a perspective view of the separate anodic insert body 9 is in a non-corroded state. The number, shape and arrangement of radial protrusions 10 fully corresponds to the perforations 7 in the spinner cone 6.

In one embodiment of the invention, the anodic insert body 9 is removably attached to a reusable spinner cone 6, and thus both the hollow spinner cone 6 and the anodic insert body 9 are shaped in such a way as to permit the insert body 9 to be axially inserted into-and removed from-the spinner cone 6. In this case, the reusable spinner cone may advantageously be made of a durable and substantially non-corrosive metal, such as stainless steel.

In an alternative embodiment, the anodic insert body 9 is instead permanently affixed to the spinner cone 6, so as to form a single replaceable unit. In this case, the spinner cone 6 is preferably substantially made of plastic, and may be moulded directly onto the anodic insert body 9.

It is to be understood that the invention is by no means limited to the embodiments described above, and may be varied freely within the scope of the appended claims. For example, the hollow spinner cone 6 and the anodic insert body 9 may be provided with only one perforation 7 and one radial protrusion 10 respectively. Furthermore, the radial protrusions 10 may extend only partially through the perforations 7 in the spinner cone 6. Although the anodic insert body may normally be made of zinc, other metals serving as sacrificial anodes may be used alternatively.