Technical field

[001 ] The present invention relates to a novel propeller for a marine vessel and a method of installing the hub cap to the hub. More specifically, the present invention relates to hub caps for use in connection with both fixed pitch and controllable pitch propellers.

Background art

[002] The propulsion unit of a marine vessel comprises normally a hub and propeller blades arranged to the hub. The hub comprises drive means, which may, for instance, be a drive shaft, an angle gear, a trans- mission or an electric or hydraulic drive. The propeller may be a so called fixed pitch propeller (FPP) or a controllable pitch propeller (CPP). The former one has blades fastened to an annular hub body, and the latter one has blades arranged to means for controlling the pitch of the blades, the control means being located within the hub of the propulsion unit. Conse- quently, the hub carrying the fixed pitch propeller has a smaller radius than the hub carrying the controllable pitch propeller even when the design load for the two propellers is the same.

[003] A problem concerning the propellers is the relatively large diameter the roots of the propeller blades are arranged (as the hub has to contain at least the components listed above). The large diameter of the hub results in that the propeller hub forms a hydrodynamically inefficient object, as the flow exiting the propeller blades has to fill the open cavity downstream of the hub. In other words, the hub terminating to a substantially radial surface having a diameter corresponding to the hub diameter cre- ates, downstream of the hub, an area of reduced pressure, which, in practice, means the creation of a force directed opposite to the thrust of the propeller, i.e. reducing the net thrust of the propeller. While water fills the open cavity a so called hub vortex is created. The hub vortex is visible in its more intense form, when the vortices start cavitating, i.e. form gas bubbles in the vortex. The practical problem with the hub vortex is that it re- duces considerably the applicable thrust of the propeller. Naturally, the problem may be seen in a larger scale in the controllable pitch propellers, due to their larger hub diameter.

[004] The problem relating to the hub vortex has been attempted to be solved by arranging a hub cap to the downstream end of the hub, and fins on the hub cap, both the hub cap and the fins rotating together with the propeller. By such an arrangement the thrust of the propeller has been increased, or fuel consumption reduced, on average 2 to 3%. The hub cap may be a cylindrical extension of the hub, but normally it is a converging extension. The convergence may be either conical or curved. [005] In accordance with an exemplary prior art construction (see for instance Fig. 1 ) the hub cap is formed of the converging section and a substantially radial end surface. The radial end surface comprises a substantially round plate that closes the opening to the interior of the hub cap. The converging section of the hub cap is provided with a support member ex- tending from the end of the hub cap remote from the hub towards the hub. The hub cap is supported to the hub by means of guide pins arranged substantially at the outer circumference of the converging portion and ensuring that the surface of the converging portion is at its outer circumference flush with the outer surface of the hub, and by means of bolts attach- ing the support member to the end surface of the hub.

[006] The prior art hub cap construction is sturdy and reduces the hub vortex to such a degree that may be considered acceptable, i.e. the remaining vortex is minimal and the effect on fuel consumption negligible. However, the hub cap of prior art has a relatively complicated configura- tion and it is heavy, whereby its manufacture is expensive and the installation requires the use of cranes. The heavier the construction is the more accurate and delicate the installation has to be such as not to cause any damage to the guide pins or possible stud bolts etc.

[007] Therefore, an object of the present invention is to design a novel hub cap, which has a simple construction resulting in low costs of manu- facture.

[008] Another object of the present invention is to design a novel hub cap, which has a light construction, whereby its installation is easy.

[009] A still further object of the invention is to offer a possibility to provide existing propellers with a modern hub cap.

Disclosure of the Invention

[0010] At least one of the objects of the invention is substantially met by a propeller for a marine vessel, the propeller comprising a hub with a hub body, propeller blades, and a hub cap attached to an aft end of the hub body, the hub cap having an outer surface provided with fins, wherein the hub cap is open at its aft end facing away from the hub body.

[001 1 ] At least one of the objects of the invention is substantially met by a method of installing a hub cap on an aft end of a hub of a propeller, the propeller comprising a hub with a hub body, propeller blades, a hub cap, and optionally an aft end cover, the hub cap having and outer surface provided with fins, the inventive method comprising by the steps of

a) providing one of the aft end of the hub body and the aft end cover with threaded holes

b) threading stud bolts in the threaded holes,

c) providing the hub cap at its forward end with an inwardly extending mounting flange,

d) providing the mounting flange with holes,

e) positioning the holes in the mounting flange of the hub cap in communication with the stud bolts f) pushing the mounting flange of the hub cap against one of the aft end of the hub body and the aft end cover, g) threading nuts on the stud bolts, and

h) tightening the nuts to secure the hub cap to the hub.

[0012] A method of installing a hub cap on a aft end of a hub of a propeller, the propeller comprising a hub with a hub body, propeller blades, an aft end cover with its fastening bolts and a hub cap, the hub cap having and outer surface provided with fins, and each fastening bolt having a head, the method comprising the steps of

a) providing threaded blind holes to the heads of at least a part of the fastening bolts,

b) threading stud bolts in the threaded blind holes, c) providing the hub cap at its forward end with an inwardly extending mounting flange,

d) providing the mounting flange with holes,

e) positioning the holes in the mounting flange of the hub cap in communication with the stud bolts,

f) threading nuts on the stud bolts, and

g) tightening the nuts to secure the hub cap to the hub.

[0013] Other characteristic features of the present invention may be seen in the appended claims.

[0014] The present invention brings about the following advantages, among others

• simple and thereby cheap construction,

• light construction whereby the installation is easier, and

• easy installation due to easy separate installation steps.

Brief Description of Drawings

[0015] In the following, the present invention will be described with reference to the accompanying exemplary, schematic drawings, in which Figure 1 illustrates an axial cross sectional view of a hub of a prior art propulsion unit of a marine vessel,

Figure 2 illustrates an axial cross sectional view of a hub of a propeller of a marine vessel in accordance with a first preferred embodiment of the present invention,

Figure 3 illustrates an enlarged view of a second variation of the first preferred embodiment of the present invention, and

Figure 4 illustrates an axial cross sectional view of a hub of a propeller of a marine vessel in accordance with a second preferred embodiment of the present invention.

Detailed Description of Drawings

[0016] Figure 1 illustrates an axial cross sectional view of a hub 10 of a prior art controllable pitch propeller of a marine vessel. In the following description definitions 'aft' and 'forward' have been used such that 'aft' refers to an element or part of an element facing or pointing towards the aft end or stern of a vessel, and 'forward' refers to an element or part of an element facing or pointing towards the front end or bow of a vessel. The hub 10 comprises a hub body 12, an aft end cover 14 at an aft end of the hub body 12, a forward end cover 16 at a forward end of the hub body 12, a cylinder yoke 18, crank rings 20, propeller blades 22 and a hub cap 24. The forward end cover 16 at the forward end of the hub body 12 has an opening for receiving the drive shaft that extends to the central cylindrical opening 26 in the cylinder yoke 18. The drive shaft (not shown) is provided with a flange that is bolted to the threaded holes 28 at the forward end of the hub body 12 for rotating the hub body 12. The drive shaft is provided with means fastened to the opening 30 in the cylinder yoke 18 for moving the cylinder yoke 18 axially. While the cylinder yoke 18 is moved axially, it cooperates with the crank rings 20 for controlling the pitch of the propeller blades 22. The aft end cover 14 fastened to the aft end of the hub body 12 by means of bolts 32 is provided with a central opening 34 for access to the means fastened to the opening 30 in the cylinder yoke 18 for moving the cylinder yoke 18, for instance a nut at the end of a rod being used for moving the yoke 18. [0017] The hub cap 24 of the prior art hub 10 is formed, between its forward and aft axial ends, of a converging section 36 and a substantially radial end surface 38 at its aft end. The radial end surface 38 comprises a substantially round plate 40 that closes the opening 42 to the interior of the hub cap 24. The converging section 36 of the hub cap 24 is provided with a support member 44 extending from the aft end of the hub cap 24 facing away from the aft end cover 14 and the hub body 12 towards the aft end cover 14 and the hub body 12. The hub cap 24 is supported to the hub body 12 by means of guide pins 46 arranged at the forward end of the hub cap 24 substantially at the outer circumference of the converging por- tion 36 and ensuring that the surface of the converging portion 36 is at its outer circumference flush with the outer surface of the aft end cover 14, and by means of bolts 48 attaching the support member 44 to the aft end cover 14 of the hub 10. The support member 44 comprises also a radially extending planar part 50, which closes the central opening 34 in the aft end cover 14. The converging section 36 of the hub 12 is provided with fins 52 by means of which, together with the convergence of the hub cap 24, the hub vortex is, in practice, eliminated.

[0018] However, as discussed earlier, the relatively complicated structure of the hub cap makes its production expensive and the construction heavy. Thus, for making the manufacture cheaper and the hub cap easier to handle the present invention proposes the following two embodiments.

[0019] Figure 2 illustrates an axial cross sectional view of a hub 60 of a propeller 74 (here a controllable pitch propeller) of a marine vessel in accordance with a first preferred embodiment of the present invention. For a significant part of Fig. 2 the construction of the hub 60 is the same as the hub 10 of Fig. 1 . In other words, the hub 60 comprises a hub body 12, an aft end cover 14 at the aft end of the hub body 12, a forward end cover 16 at the forward end of the hub body 12, a cylinder yoke 18, crank rings 20, propeller blades 22 and a hub cap 62 arranged to the aft end of the hub. The forward end cover 16 at the forward end of the hub body 12 has an opening for receiving the drive shaft that extends to the central cylindrical opening 26 in the cylinder yoke 18. The drive shaft (not shown) is provided with a flange that is bolted to the threaded holes 28 at the forward end of the hub body 12 for rotating the hub body 12. The drive shaft is provided with means fastened to the opening 30 in the cylinder yoke 18 for moving the cylinder yoke 18 axially. While the cylinder yoke 18 is moved axially, it cooperates with the crank rings 20 for controlling the pitch of the propeller blades 22.

[0020] The hub cap 62 of the hub 60 of the present invention is formed, between its forward and aft axial ends, of a hollow shell 64 provided with an inwardly extending mounting flange 66 at the forward end of the hub cap 62 facing the aft end cover 14 of the hub 60. The outer surface of the hollow shell 64 is provided with fins 68. The hub cap 62 is fastened at its forward axial end to or on the aft end cover 14, preferably, but not necessarily, by means of stud bolts 70.The aft end cover 14 is bolted to the aft end of the hub body 12 by means of bolts 32. For fastening the hub cap 62 to the aft end cover 14 the aft end cover 14 is provided with threaded holes. The stud bolts 70 are first screwed to the threaded holes in the aft end cover 14. In accordance with a first variation of this embodiment the inwardly extending mounting flange 66 is at the forward end of the hub cap 62 provided with holes for the stud bolts 70 such that the stud bolts 70 act as guide pins when installing the hub cap 62 in communication with the stud bolts 70 on the aft end cover 14. After the mounting flange 66 of the hub cap 62 lies against the surface of the aft end cover 14, nuts (preceded by locking washers, if desired) are screwed and tightened onto the stud bolts 70 to secure the hub cap 62 in place. The locking washers may be replaced with a locking wire. The central opening 34 in the aft end cover 14 is closed by means of a separate cover 72.

[0021 ] The hub cap 62 of the first embodiment of the present invention is open at its aft end 74 facing away from the aft end cover 14. Performed experiments have shown that the hydrodynamic properties of the hub cap have not changed when moving from the prior art closed hub cap 24 of Figure 1 to the open end hub cap 62 of the present invention. The only thing that has changed is the weight of the hub cap that has reduced by more than 50% with the invention. One reason for the weight reduction is the shift of the cover for closing the opening 34 in the aft end cover 14 from the hub cap to a separate cover 72, which may be moved manually, if desired.

[0022] The shell 64 of the hub cap 62 preferably converges from its forward axial end facing the aft end cover 14 towards its aft axial end 74 facing away from the aft end cover 14 of the hub 60. The shell 64 may have the shape of a truncated cone, a truncated paraboloid or a truncated ellipsoid, just to name a few potential options. In other words, the shell has a generally rotationally symmetrical, truncated converging shape.

[0023] Figure 3 illustrates an enlarged view of a second variation of the first embodiment of the present invention. The illustrated way of fastening the hub cap 62 to the aft end cover 14' of the hub 12 is especially preferred in such a case that the hub cap 62 is installed on an existing aft end cover 14'. In other words, in such a case that the hub cap 62 is needed for improving the properties of such a propeller hub that has been in use without any hub cap. Such hubs have aft end covers 14', which, most often, have convex aft end surfaces and thereby the only planar dimensionally accurate surfaces in connection with the end covers 14' may be found on the heads of the bolts 32 used for fastening the aft end cover 14' to the hub 12. Thus, it is practical to use the bolts 32 for fastening the hub cap 62 on the aft end cover 14', too. Thereby there is no need to machine the aft end cover 14' but drilling central blind holes along the axis of the bolts 32 and providing such with threads is sufficient. When fastening the hub cap 62 on the aft end cover 14' stud bolts 70 are first threaded in the threaded holes at the ends of the fastening bolts 32, next distance members, i.e. sleeves 72 are inserted on the stud bolts 70 to arrange an appropriate distance between the ends of the fastening bolts 32 and the inwardly extending mounting flange 66 of the hub cap 62. By means of the sleeves 72 both the convexity of the aft end cover 14' and the depth of the recesses machined into the aft end cover 14' for the fastening bolts 32 are taken into account. Then, the openings in the inwardly extending flange 66 of the hub cap 62 are brought into communication with the stud bolts 70 and the hub cap 62 is slid in its place. And finally, when the mounting flange 66 of the hub cap 62 rests against the sleeves 72 the nuts 74 are threaded on the stud bolts 70 and the hub cap 62 is secured on the aft end cover 14. For proper alignment of the hub cap 62, i.e. for preventing any local contacts between the hub cap 62 and the aft end cover 14' a gap 76 is left between the aft end cover 14' and the hub cap 62. Naturally, if the convexity of the aft end cover and/or the depth of the machining is/are such that there is no mechanical contact between the aft end cover and the hub cap without the sleeves, the use of the sleeves is not needed. [0024] Figure 4 illustrates an axial cross sectional view of a hub of a propeller (here a fixed pitch propeller) of a marine vessel in accordance with a second preferred embodiment of the present invention. In this embodiment the hub 80 of the propeller is formed of a hub body 82, and a hub cap 62 arranged at the aft end of the hub body 82. The hub body 82 has propeller blades 86 and an interior opening 88 to which the drive shaft (now shown) is to be installed.

[0025] The hub cap 62 of the hub 60 of the second preferred embodiment of the present invention is, in practice identical with that of the first preferred embodiment, i.e. it is formed of a hollow shell 64 provided with an inwardly extending mounting flange 66 at a forward end of the hub cap 62 facing the aft end of the hub body 82 of the hub 80. In other words, the hub cap 62 of the second embodiment of the present invention is open at its aft end 74 facing away from the hub body 82.

[0026] The outer surface of the hollow shell 64 is provided with fins 68. The hub cap 62 is fastened at its forward end to the hub body 82, prefer- ably, but not necessarily, by means of stud bolts 70. For fastening the hub cap 24 to the hub body 82 the aft axial end of the hub body 82 is provided with threaded holes. The stud bolts 70 are first screwed to the threaded holes in the aft axial end of the hub body 82. The inwardly extending mounting flange 66 at the forward axial end of the hub cap 62 is provided with holes for the stud bolts 70 such that the stud bolts 70 act as guide pins when installing the hub cap 62 in communication with the stud bolts on the hub body 82. After the mounting flange 66 of the hub cap 62 lies against the end surface of the hub body 82, nuts (preceded by locking washers, if desired) are screwed and tightened onto the stud bolts 70 to secure the hub cap 62 in place. The locking washers may be replaced with a locking wire.

[0027] Performed experiments have shown that the hydrodynamic properties of the hub cap have not changed when moving from the prior art closed hub cap 24 of Figure 1 to the open hub cap 62 of the present in- vention. The only thing that has changed is the weight of the hub cap that has reduced by more than 50% with the invention.

[0028] As to the various optional constructions it should be understood that the stud bolts 70 used for fastening the hub cap 62 to the aft end cover 14 or to the hub body 82 may also be replaced with ordinary hexagonal head bolts, whereby the use of one or more separate guide pins for facilitating the installation of the hub cap on the aft end cover or the hub body is preferred. Naturally the guide pins may be used in connection with the use of the stud bolts, too.

[0029] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the invention, as defined in the appended claims. The details mentioned in connection with any embodiment above may be used in connection with another embodiment when such combination is technically feasible.