Background of the invention

The invention relates to a rotational energy recovery appendage for a propeller, and specifically to a rotational energy recovery appendage for a propeller of a coastal vessel or a seagoing vessel, at which propeller no rudder is provided.

Prior art

In order to keep up with economical and ecological

interests in the field of watercrafts, there is a need to recover rotational force generated by propellers of vessel propulsion systems. In view of this, several solutions for enlarging energy efficiency of vessels have been proposed, in particular recovery solutions provided at rudders of vessels. Here, the main purpose of a rudder is to act as a steering device by creating a lift force in either

transverse direction. In addition, the rudder can also be used to recover some of the rotational energy in the slipstream of the propeller by adapting a rudder design accordingly .

As such a proposed energy recovery solution, it is known in the field of propulsion solutions to provide a rudder arranged behind a propeller of coastal and seagoing vessels with a fairing cap and a rudder bulb in order to streamline the flow generated by the propeller. In doing so, flow separation behind the propeller hub can be reduced

significantly, resulting in less fuel consumption and, thus, in an improvement of the vessel's energy efficiency. Such a solution is known, for example, from Wartsila' s Energopac™.

Another example of an improved rudder can be gathered from JP 2-293296 A, in which a blade device comprising several twisted blades is provided at the rudder of a vessel in order to efficiently use the propulsion force generated by a propeller arranged at a distance in front of the rudder.

The rotational energy recovery capacity of a rudder design, however, is still limited by the primary intended function, namely steering by means of the rudder. Thus, the rudder is normally just a vertical fin/wing section being provided with a gap relative to the propeller, which must be able to turn to change an angle of incoming water flow. Therefore, the rudder needs a large blade area in order to provide large enough steering force, which leads to unnecessary high resistance.

Furthermore, the above mentioned cases are specifically applicable at rudders of coastal or seagoing vessels. In a vessel with several propellers, however, not all propellers are fitted with rudders for providing steering force, or in case a vessel has other steering means than rudders, propellers can be generally provided without rudder, which is not ideal since rotational energy of the propeller will be lost. Thus, there is also a need to improve the

efficiency of propellers which are not provided in front of respective rudders.

A system for a vessel propulsion propeller provided without a rudder behind the same is known from EP 1 826 117 Al, in which a propulsion system installed under the hull of a ship is described. In particular, the propulsion system comprises a propeller pump in between conventional propellers having rudders provided in a certain distance behind them, respectively. The propeller pump includes a propeller, a nozzle-like sleeve arranged around the

propeller, an integral bulb regulator upstream of the propeller for streamlining the flow of the propeller pump, and flow uniforming ridges provided between the regulator and the sleeve upstream of the propeller. Thus, by means of the streamlining propeller pump, vibrations of the

propulsion system can be prevented and the entire

propulsion system can be made more efficient.

A further example of such an improved ship propulsion system can be found in GB 1 250 197 A, in which a back flow recovery propeller device is described. The device includes a rotatable shaft, a main screw propeller fixed on the shaft and a freely rotatable back flow recovery screw propeller of larger diameter than the main propeller mounted behind the main propeller. The recovery propeller has a particular shape, so that, in use, a twisted back flow torque of the main propeller is absorbed by a back flow colliding surface of the recovery propeller to produce a rotating force which drives the recovery propeller in the same direction as the main propeller so as to obtain an effective propulsion force. Thus, energy efficient

propulsion is generated.

Even though systems for improving the energy efficiency of protrusion propellers without rudders provided behind them already exist, there is a constant need to further reduce fuel consumption of coastal or seagoing vessels in order to improve energy efficiency of the same.

Summary of the invention It is the object of the present invention to provide a rotational energy recovery appendage for a propeller of a vessel, which can reduce fuel consumption of the vessel and which can be manufactured with low costs by simple

construction .

This object is achieved by a rotational energy recovery appendage having the features of claim 1.

In particular, a rotational energy recovery appendage according to the invention is provided at a propeller hub for recovering rotational energy of a propeller of a vessel to be driven by the propeller. In particular, the appendage is provided in a backflow area of the propeller as a counter measure against unfavorable backflow. Here, there is no rudder provided in-line with the propeller. The appendage comprises a main body mounted flush with the propeller hub, at least one strut supporting the main body at the vessel, and at least one fin fixed to and extending from the main body. The main body and the propeller hub form a streamlined shape in order to streamline the flow produced by the propeller and to prevent unfavorable hub vortex .

Preferably, the outer diameter of the main body is larger than the outer diameter of a portion of the propeller hub close to the appendage, i.e. the main part or the part with the largest outer diameter of the streamlined shape formed by the main body together with the propeller hub is

provided at the main body. Here, the main body can be torpedo-shaped. In doing so, a low hub drag is achieved, resulting in a recovery of rotational energy in the

slipstream of the propeller. Furthermore, the total number of struts and fins can be different from the number of propeller blades. In

particular, the total number of struts and fins is neither the same nor an even fraction of the number of blades of the propeller. For example, in case a 5-blade propeller is used, the appendage can comprise two struts and two fins (=4 elements in total) . In constructing the appendage accordingly, it is possible to avoid resonating impulses between the blades and the appendage. Such resonating pulses can happen, for example, in the case of in-line arranged counter rotating propellers, wherein the number of blades of each of the counter rotating propellers is equal to the other one. That same effect can occur when using an appendage according to the invention in combination with a respective propeller.

As to the structure of the appendage, the appendage can comprise one strut and two fins. In this case, the

appendage has the shape of an upside-down Y. As an

alternative thereto, the appendage can comprise two struts and two fins, so that the appendage has an X-shape, or, further alternatively, the appendage can comprise two struts and one fin, forming a Y-shape. In all three cases, the struts and fins can be equiangularly spaced apart from each other. In the case of the Y-shape or the upside-down Y-shape, this means that the strut/s and fin/s are

angularly spaced from each other by an angle of 120°, and in the case of the X-shape of the appendage, the struts and fins are angularly spaced apart from each other by an angle of 90°. In the case of the upside-down Y-shape, the angles can also be unequal to each other. For example, the angle between the two struts can be 45° and, thus, the angle between the fin and a respective strut can be 157,5°. With such a design, since the blades of a propeller are usually always equiangularly spaced from each other, the number of the blades of the propeller can actually be chosen to be equal to the total number of struts and fins of the

appendage, since a similar angular arrangement of the blades and the struts and fins of the appendage is unlikely in such a case.

Furthermore, in order to achieve an optimal shape in all locations, the fin/s and/or strut/s can comprise a twisted leading edge, and the respective trailing edge can extend further than the main body, so that the trailing edge projects from the main body.

Further preferably, the main body is mounted at the

propeller hub in an overlapping manner, wherein either one of the main body and the propeller hub can comprise a recess for accommodating the corresponding end of the other one of the main body and the propeller hub. Thereby, the appendage according to the invention can be adapted to every standard propeller hub.

Each fin is preferably provided on a side of the main body away from the vessel, i.e. a lower side of the appendage, and the length of each fin can be shorter than the radius of the propeller, i.e. the length of a blade of the

propeller, so that a small wetted surface area is achieved and low added resistance can be effected. Furthermore, there is the possibility to use a high aspect ratio fin as each fin in order to achieve a high lift to drag ratio of the appendage .

Furthermore, according to the invention, a propulsion system is provided, which comprises an arrangement of three juxtaposed propellers, wherein each of the two outer ones of the three propellers has a rudder being provided

downstream of the hub of the respective propeller. Here, the middle propeller has no rudder but a rotational energy recovery appendage as described before. The appendage is provided downstream of the middle propeller, preferably in the backflow area of the middle propeller in order to prevent unfavorable backflow.

Brief description of the drawings

This invention will be explained by way of preferred embodiments using the attached drawing figures, in which:

Fig. 1 is a perspective side view of a first embodiment of the appendage according to the invention;

Fig. 2 is a view from the rear of the appendage of the first embodiment as shown in Fig. 1 in the cruising

direction of a respective vessel;

Fig. 3 is a view from the rear of an appendage of a second embodiment in the cruising direction of a respective vessel ;

Fig. 4 is a view from the rear of an appendage of a third embodiment in the cruising direction of a respective vessel; and

Fig. 5 is a perspective view of a machinery concept including the first embodiment as shown in Figs. 1 and 2.

Description of embodiments

Fig. 1 is a lateral perspective view of a first embodiment of an appendage 1 according to the invention. The appendage 1 of the shown first embodiment consists of a main body 11, a strut 12 and two fins 13, which are connected with the main body 11 at one end, respectively. In this lateral view, only one fin 13 is shown.

The main body 11 has a torpedo shape extending from its front end portion 111 to its tip end portion 112, wherein the front end portion 111 of the main body 11 includes the globular-bellied or bulb-shaped end of the torpedo shape and the tip end portion 112 of the main body 11 includes the tapered or pointed end of the torpedo shape. The diameter basically decreases from the front end portion 111 to the tip end portion 112, wherein a recess 113 is

provided at a part of the front end portion 111 having the maximum outer diameter. The recess 113 of the main body 11 in this embodiment is circular-shaped, such that an annular edge portion 114 is left at the front end portion 111 of the main body 11.

The strut 12 connects the main body 11 with a hull 3 of a vessel by partly connecting a lower end of the strut 12 to an upper part of the main body 11 in this view of Fig. 1. The tip end portion 112 of the main body 11 extends into the strut 12 for about 2/3 of the entire width of the strut 12, such that the strut 12 continues after the tip end portion 112 of the main body 11. Thus, the strut 12 forms an extension of the main body 11 in an axial direction of the main body 11 opposite to the front end portion 111 of the main body 11. The strut 12 comprises a leading edge 121 at its front end in a width direction of the strut 12, i.e. at an end of the strut 12 arranged towards the front end portion 111 of the main body 11. Further, the strut 12 comprises a trailing edge 122 at its back end in a width direction of the strut 12, the back end being opposite to the before mentioned front end of the strut 12. The leading edge 121 of the strut 12 is the end of the strut 12

receiving a propulsion flow of the vessel. In order to have an optimal shape all over the surface of the strut 12, the leading edge 121 has a twisted form, as is, for example, already known from EP 2 154 064 Al .

The fins 13 of this embodiment are high aspect ratio fins and are connected to the main body 11 of the appendage 1 at one end, respectively, in a similar way to the strut 12, such that the tip end portion 112 of the main body 11 extends into each fin 13 for about 2/3 of the entire width of each fin 13. In doing so, each fin 13 continues after the tip end portion 112 of the main body 11 and, thereby, forms an extension of the main body 11 in an axial

direction of the main body 11 opposite to the front end portion 111 of the main body 11. The fins 13 of the first embodiment have a similar outer shape as described before for the strut 12.

As can be seen in Fig. 2, which is a view from the rear of the appendage according to the first embodiment of the invention together with a propeller 2 and the hull 3 of the vessel, the fins 13 and the strut 12 are arranged in an angularly spaced manner, namely at a same angular or equiangular distance of 120° spaced apart from each other. Thus, the fins 13 are provided below a horizontal center axis of the main body 11, connecting with the strut 12 in the longitudinal axis of the main body 11. In this

embodiment, the propeller 2 comprises four propeller blades 22. Thus, the total number of strut 12 and fins 13 is three, which is neither the same nor an even fraction of the number of the four propeller blades 22. It can also be gathered from Fig. 2 that the fins 13 are shorter than each of the propeller blades 22, resulting in a small wetted surface area and, thus, resulting in low added flow

resistance .

In Fig. 1, a propeller hub 21 of the propeller 2 is

attached to a propeller shaft 23 driven by a motor (not shown) which is provided inside the hull 3 of the vessel, wherein the propeller blades 22 are attached to a front end portion 211 of the propeller hub 21. A back end portion 212 of the propeller hub 21 comprises a circular projection 213 with an annular recess 214 provided around the same and is adapted to be positioned partially inside the recess 113 and the edge portion 114 of the front end portion 111 of the main body 11 when used. The outer diameter of the propeller hub 21 increases towards the recess 214. By such an arrangement, the propeller hub 21 and the main body 11 form an almost gapless streamlined shape in order to streamline the flow generated by the propeller 2.

Furthermore, the strut 12 and the fins 13 serve for

recovering rotational energy of the propeller 2 by

transferring the received rotational energy into lift energy for the vessel.

Fig. 3 shows a second embodiment of the appendage 1 according to the invention. In this embodiment, the

propeller 2 comprises five propeller blades 22.

Accordingly, since the total number of struts 12 and fins 13 should be neither the same nor an even fraction of the number of propeller blades 22, two struts 12 and two fins 13 are provided. Here, the struts 12 and the fins 13 are arranged with random angles in between, wherein the two struts 12 are basically arranged in a V-shape. However, the struts 12 and fins 13 can be arranged equiangularly, i.e. with an angle of 90° in between each other. The remaining construction is similar to the construction as described in the first embodiment shown in Figs. 1 and 2. A construction having two struts 13 arranged substantially in a V-shape as shown in Fig. 3 instead of a construction with only one strut 13 as shown in Figs. 1 and 2 increases the structural integrity of the appendage 1 significantly.

A third embodiment of the appendage 1 according to the invention is shown in Fig. 4, wherein the propeller 2 comprises four propeller blades 22, and, accordingly, the appendage comprises a total number of struts 12 and a fin 13 of three. In particular, the struts 12 are basically arranged in a V-shape, and the single fin 13 is arranged in a vertical direction downwards of the main body 11. The remaining construction is similar to the construction as described in the second embodiment as shown in Fig. 3. Here again, a construction having two struts 13 arranged

substantially in a V-shape as shown in Fig. 3 instead of a construction with only one strut 13 as shown in Figs. 1 and 2 increases the structural integrity of the appendage 1 significantly.

A machinery concept is shown in Fig. 5, consisting of three propeller 2 rotatably connected to a motor-gear mechanism- assembly 4 via a propeller shaft 23, respectively. At the two outer propeller 2A and 2C, a respective rudder 5 is arranged downstream of each propeller 2A, 2C in-line, which rudder 5 is known from, for example, Wartsila' s Energopac™. Furthermore, the middle propeller 2B has an appendage 1 as described in the first embodiment of the invention arranged downstream for recovering rotational energy of the

propeller 2B. With such a machinery concept, not only rotational energy can be recovered by means of the rudders at the outer propellers 2A and 2C but also at the middle propeller 2B. Furthermore, the appendage 1 at the middle propeller 2A is optimized for efficiency, providing more efficient propulsion than the outer propellers 2A and 2C. Also, a fixed appendage construction as described above is cheaper than a rudder structure. As an alternative, the outer propellers 2A and 2C can also be replaced by

thrusters or pods as propulsion generating devices. In this case, the middle propeller 2B would still have an appendage 1 as described above.