FIELD [0001] The present invention relates to a marine propeller for propelling a vessel.

[0002] Further, the present invention relates to an arrangement comprising a vessel and a marine propeller.

BACKGROUND [0003] Different types of marine propellers for propelling a vessel such as a cruise ship, passenger ferry, tanker, container ship, etc. are known. Conventional propellers are configured to be coupled to a rotating shaft via a propeller hub comprised by the propeller. A plurality of propeller blades - for example two to eight propeller blades - extends from the propeller hub in radial direction. The propeller blades are set at a pitch angle to transform rotational power into linear thrust. The propeller blades may be e.g. further skewed and/or inclined at a rake angle.

[0004] Several attempts have been made to reduce hydrodynamic noise caused e.g. by an uneven flow or by cavitation. For example, air lubrication to the propeller tips or special ducts have been provided. Further, the rotation speed of the propeller has been reduced in order to reduce the hydrodynamic noise.

[0005] Furthermore, some vessels have propellers with a high number of blades in order to reduce noise. For example, some submarines have a propeller with seven blades radially extending from the propeller hub. By increasing the number of blades, lower pressure pulses are induced by the propeller. As the ratio between the propeller hub and the propeller disc is typically about 0.2, the possible number of blades is however limited due to the lack of space at the roots of the blades, i.e. in the region of the relatively small propeller hub. Generally, hydrodynamic noise is produced at the tips of the blades of a propeller. [0006] In view of the foregoing, it would be beneficial to provide a marine propeller which is more silent than known conventional propellers. In other words, mitigation of underwater radiated noise should be achieved. The marine propeller should be further capable of being manufactured in industrial scale.

SUMMARY OF THE INVENTION

[0007] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0008] According to a first aspect of the present invention, there is provided a marine propeller comprising a propeller hub having a first axis of rotation and configured to be coupled to a rotating shaft, a plurality of first propeller blades extending from the propeller hub in radial direction, a first ring having a second axis of rotation aligned with the first axis of rotation, wherein the first ring is connected to the plurality of first propeller blades, and a plurality of second propeller blades extending from the first ring in radial direction, wherein a number of second propeller blades is greater than a number of first propeller blades.

[0009] Various embodiments of the first aspect may comprise at least one feature from the following bulleted list:

• the number of second propeller blades is n times the number of first propeller blades, wherein n is an integer number the number of first propeller blades is in a range between two and seven

• the number of second propeller blades is in a range between eight and twenty-one

• a ratio between a radius of the first ring and a distance between the first axis of rotation and tips of the plurality of second propeller blades is in a range between 0.25-0.75

• the first ring is configured to serve as a propeller duct which guides an incoming flow into a desired direction • the first ring is connected to each tip of a propeller blade of the plurality of first propeller blades

• the first ring is connected to a section of the plurality of first propeller blades

• each propeller blade of the plurality of first propeller blades is set at a pitch angle

• each propeller blade of the plurality of first propeller blades is skewed

• each propeller blade of the plurality of first propeller blades is set at a rake angle

• each propeller blade of the plurality of second propeller blades is set at a pitch angle

• each propeller blade of the plurality of second propeller blades is skewed

• each propeller blade of the plurality of second propeller blades is set at a rake angle

• the marine propeller further comprises a second ring having a third axis of rotation aligned with the first axis of rotation, wherein the second ring is connected to the plurality of second propeller blades

• the second ring is connected to each tip of a propeller blade of the plurality of second propeller blades

• a diameter of the second ring is greater than a diameter of the first ring

• the marine propeller further comprises a plurality of third propeller blades extending from the second ring in radial direction, wherein a number of third propeller blades is greater than a number of second propeller blades

• each propeller blade of the plurality of third propeller blades is set at a pitch angle

• each propeller blade of the plurality of third propeller blades is skewed

• each propeller blade of the plurality of third propeller blades is set at a rake angle

[0010] According to a second aspect of the present invention, there is provided an arrangement comprising a vessel and a marine propeller in accordance with any one of claims 1-16.

[0011] Considerable advantages are obtained by means of certain embodiments of the present invention. An ultra-silent marine propeller is provided. The invention is based on dividing the propeller disc into two (or more) concentrical layers, wherein each layer has a different number of blades. A small number of blades is provided in the inner layer and a large number of blades is arranged in the outer layer. Noise can be reduced due to an increase in the number of provided propeller blades projecting from a ring or ring-like structure, and thus lower pressure pulses are induced by the propeller. The present invention allows increasing the number of blades twice or more compared with conventional propellers. For example, if the number of first, inner blades is four and the number of second outer blades is sixteen, then the noise can be reduced to about 1/16 of the noise of a conventional propeller having four blades.

[0012] According to certain embodiments, the first ring or ring-like structure may serve as a propeller duct which guides an incoming flow into a desired direction and/or increases or decreases speed of the flow. Adequately oriented profile sections of the propeller duct provide continuity of hydrodynamic circulation from the propeller roots to the propeller tips. Thus, an efficiency improvement may be obtained for the plurality of first propeller blades. The propeller duct may also be designed to influence the direction of the incoming fluid flow at the roots of the blades of the plurality of second propeller blades, thus further improving efficiency.

[0013] The marine propeller in accordance with the present invention may be used in different applications where noise reduction is required. For example, noise reduction may be beneficial for comfortably travelling with a passenger ship. Other applications lie in navy ships, for instance. The shape and the size of the marine propeller can be selected based on its intended use and in accordance with predefined requirements. Variable parameters are, for example, size and shape of hub, span of propeller blades of plurality of first propeller blades, radius of first ring, shape of first ring, pitch angle of propeller blades of the plurality of first blades, skew of propeller blades of the plurality of first propeller blades, rake of propeller blades of the plurality of first propeller blades, chord length of the propeller blades of the plurality of first propeller blades, nose radius of the propeller blades of the plurality of first propeller blades, span of propeller blades of plurality of second propeller blades, pitch angle of propeller blades of the plurality of second propeller blades, skew of propeller blades of the plurality of second propeller blades, rake of propeller blades of the plurality of second propeller blades, chord length of the propeller blades of the plurality of second propeller blades, nose radius of the propeller blades of the plurality of second propeller blades, radius of second ring, shape of second ring, etc. The design of the marine propeller can be optimized and verified using CFD (computer aided fluid design) simulation tools or utilizing model tests.

[0014] Manufacturing of the marine propeller can further take place in a conventional way, for example utilizing a casting mould. BRIEF DESCRIPTION OF THE DRAWING

[0015] FIGURE 1 illustrates a schematic perspective view of a marine propeller in accordance with at least some embodiments of the present invention.

EMBODIMENTS

[0016] In FIGURE 1 a schematic perspective view of a marine propeller 1 in accordance with at least some embodiments of the present invention is illustrated. The marine propeller 1 comprises a propeller hub 2 having a first axis of rotation and configured to be coupled to a rotating shaft. The first axis of rotation is typically arranged in the center of the propeller hub 2. A plurality of first propeller blades 3 extends or projects radially from the propeller hub 2. The blades of the plurality of first propeller blades 3 are evenly distributed in circumferential direction around the propeller hub 2. The marine propeller 1 further comprises a first ring 4 or ring like element having a second axis of rotation aligned with the first axis of rotation. The first ring 4 or ring-like element may be symmetrical or of any other suitable annular form. The first ring 4 is connected to the plurality of first propeller blades 3. Additionally, the marine propeller 1 comprises a plurality of second propeller blades 5 extending or projecting radially from the first ring 4. The blades of the plurality of second propeller blades 5 are evenly distributed in circumferential direction around the first ring 4. As can be seen, a number of second propeller blades 5 outside of the first ring 4 is greater than a number of first propeller blades 3 inside of the first ring 4.

[0017] In FIGURE 1, the number of first propeller blades is four and the number of second propeller blades is sixteen. However, also other combinations are possible. For example, the number of first propeller blades 3 may be three and the number of second propeller blades 5 may be fifteen. Typically, the number of second propeller blades 5 is n times the number of first propeller blades 3, wherein n is an integer number between two and eight. However, the number of second propeller blades 5 is not necessarily n times the number of first propeller blades 3. The number of first propeller blades is typically in a range between two and seven. The number of second propeller blades is typically in a range between eight and twenty-one. Each of the blades of the plurality of first propeller blades 3 and each of the blades of the plurality of second propeller blades 5 is set at a pitch angle to transform rotational power into linear thrust. Each of the blades of the plurality of first propeller blades 3 and/or each of the blades of the plurality of second 5 propeller blades may be further skewed, for instance. Further, each of the blades of the plurality of first propeller blades 3 and/or each of the blades of the plurality of second propeller blades 5 may be inclined at a rake angle, for instance.

[0018] The propeller disc is divided into two concentrical layers, wherein each layer has a different number of blades. The layers are connected by the first ring 4 which is supported by the first blades 3 and which provides support to the external layer of second blades 5. A ratio between a radius of the first ring 4 and a distance between the first axis of rotation and tips of the plurality of second propeller blades 5 is typically in a range between 0.25-0.75, for example 0.3 or 0.4.

[0019] According to the present invention, the first ring provides enough space for arrangement of a substantial number of second blades 5, and thus the amplitude of force fluctuations can be reduced proportionally to the number of blades. Referring to the marine propeller shown in FIGURE 1, replacement of a four-bladed propeller by a sixteen-bladed propeller leads to reduction of the fluctuating forces on the blade by about four times. The overall fluctuation of forces of all blades is further reduced four times more due to the phase shift of the forces. A similar reduction of pressure fluctuations, i.e. sixteen times, occurs in the vicinity of the propeller.

[0020] According to certain embodiments of the present invention, some of the blades of the plurality of second propeller blades 5 may be designed such that they form a hydrodynamic extension or continuation of the blades of the plurality of first propeller blades 3. In other words, each of the blades of the plurality of first propeller blades 3 forms a unit 10 with one specific blade of the plurality of second propeller blades 5, wherein each unit 10 is interrupted by a part of the first ring 4. Thus, the propeller efficiency can be improved for each unit 10. The remaining blades of the plurality of second propeller blades 5 are identical in shape and size with each part of said units 10 arranged outside of the first ring 4. The first ring 4 provides sufficient stability to said units 10 in order to be able to arrive at a high ratio between a length or span of the unified blade and a maximum chord length of the unified blade. Additionally, the first ring 4 provides sufficient space for arrangement of the remaining blades of the plurality of second propeller blades 5, i.e. for blades of the plurality of second propeller blades which do not form a unit 10 with one specific blade of the plurality of first propeller blades 3. However, continuity of shape between the inner first propeller blades 3 and some specific outer second propeller blades 5, thus forming a unit 10, is not a necessity. Instead, there may be a shift in position circumferentially around the first ring 4 between the inner first propeller blades 3 and some specific outer second propeller blades 5. In such a case, typically none of the blades of the plurality of second propeller blades 5 forms a hydrodynamic extension of the blades of the plurality of first propeller blades 3.

[0021] According to certain embodiments, the first ring 4 is configured to serve as a propeller duct which guides an incoming flow into a desired direction and/or increases or decreases the speed of the flow in the propeller disc. The propeller duct may also be designed to influence the direction of the incoming fluid flow at the roots of the propeller blades of the plurality of second propeller blades. If the first ring 4 works as an accelerating duct, the speed of the flow is increased inside of the first ring 4 and decreased outside of the first ring 4. In case that the first ring works as a decelerating duct, the speed of the flow is decreased inside of the first ring 4 and increased outside of the first ring 4.

[0022] According to certain embodiments, the marine propeller 1 further comprises a second ring (not shown) having a third axis of rotation aligned with the first axis of rotation. The second ring is connected to the plurality of second propeller blades 5, typically to each tip 9 of a propeller blade of the plurality of second propeller blades 5. The second ring provides further stability to the marine propeller 1. Further, also the second ring may be configured to serve as an additional propeller duct which guides an incoming flow into a desired direction and/or increases the speed of the flow in the propeller disc.

[0023] According to certain embodiments, the marine propeller 1 furthermore comprises a plurality of third propeller blades (not shown) extending from the second ring in radial direction. In such a case, the propeller disc is divided into three concentrical layers, wherein each layer has a different number of blades. The number of blades of the plurality of third propeller blades may be greater than a number of blades of the plurality of second propeller blades 5. Each propeller blade of the plurality of third propeller blades is set at a pitch angle. Each of the blades of the plurality of third propeller blades may be further skewed and/or each of the blades of the plurality of third propeller blades may be inclined at a rake angle. [0024] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0025] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0026] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0027] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0028] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below. [0029] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.

INDUSTRIAL APPLICABILITY

[0030] At least some embodiments of the present invention find industrial application in propulsion of all types of ships, for example in passenger ferries or navy applications.

REFERENCE SIGNS LIST

1 marine propeller

2 hub

3 first propeller blade 4 first ring

5 second propeller blade

6 root of first propeller blade

7 tip of first propeller blade

8 root of second propeller blade