Screw propeller of a pod drive of a vessel and pod drive comprising said screw propeller

Abstract

The invention relates to the field of shipbuilding and more specifically to the screw propeller for a pod drive of a vessel, particularly an ice-going vessel, providing movement both ahead and astern in icy conditions where the ice is traversable, while also providing steering for the vessel, and to a pod drive comprising said screw propeller. To reduce the weight, and consequently the cost, of the pod drive, the screw propeller comprises a propeller hub, made so that it can be rigidly attached to the conical tailpiece of the propeller shaft, and screw propeller blades, each comprising a blade foil and a blade flange made in one piece, mounted on the propeller hub. The blade foil passes into the blade flange via a fillet joint forming the weakest portion for the possible destruction of the blade in the cross section of its foil, located immediately adjacent to the fillet joint. The outer surface of the blade flange has a profile, in its meridional cross section, curved inwards towards to the propeller axis, to reduce the distance from said blade foil cross section to the propeller axis.

Claims

1. A screw propeller for a pod drive of a vessel, for fitting on a propeller shaft, comprising a propeller hub, made so that it can be rigidly attached to the conical tailpiece of the propeller shaft, and screw propeller blades, each of which comprises a blade foil and a blade flange, made in one piece, mounted on the propeller hub, the blade foil passing into the blade flange via a fillet joint forming the weakest portion for the possible destruction of the blade in the cross section of its foil located immediately adjacent to the fillet joint, wherein the outer surface of the blade flange has, in its meridional cross section, a profile curved inwards towards the axis of the propeller, to reduce the distance from said cross section of the blade foil to the propeller axis, and wherein the cross section of the transition of the blade foil into the flange is located at a distance from the propeller axis which is less than a distance from the point of intersection of the propeller axis and a straight line located in the meridional cross section of the screw propeller and connecting two extreme outer radii of the propeller blade flange.

2. The screw propeller of claim 1, wherein the propeller blade flange in its meridional cross section has a hydrodynamic profile formed by smoothly combined conical portions.

3. The screw propeller of claim 1, wherein it is made in the form of a composite screw propeller with removable blades, attached to the hub using a disconnectable joint.

4. The screw propeller of claim 1, wherein all the blades of the screw propeller are made in one piece with the hub, forming an integrally cast screw propeller.

5. A pod drive for a vessel, comprising a pod drive gondola, a screw propeller according to claim 1, a spacer casing positioned between them, and a propeller casing.

Description

(1) The invention is explained with the aid of drawings, showing:

(2) in FIG. 1: a pod drive;

(3) in FIG. 2: a meridional cross section through the bush of a screw propeller;

(4) in FIG. 3: a cross section through a composite screw propeller.

(5) Identical structural components in the different figures are denoted by identical references.

(6) FIG. 1 shows a pod drive 1, attached to the hull 2 of a vessel with the aid of an attachment assembly 10 and a swivel bearing 11, the pod drive taking the form of a chamber consisting of a gondola 13 attached to a leg 12, with an electric propulsion motor 15 and a propeller shaft 20 placed inside. The propeller shaft 20 has a conical tailpiece 21, to which a driving screw propeller 22 is attached outside the gondola.

(7) Two bearings 41, 42 are positioned on the propeller shaft 20, these bearings serving to support the shaft 20 of the electric propulsion motor 15 and to transmit the thrust (traction) from the propeller 22 to the casing of the pod drive.

(8) The screw propeller 22 comprises a propeller hub 33, made so that it can be rigidly attached to the conical tailpiece 21 of the propeller shaft 20; screw propeller blades 30, each consisting of a blade foil 31 and a blade flange 32 made in one piece, mounted on the propeller hub 33; and a casing 34 of the screw propeller bush.

(9) The gondola 13 of the pod drive and the screw propeller 22 are interconnected by a spacer casing 14.

(10) FIG. 2 shows a meridional cross section through the screw propeller hub, together with a diagram of a calculation of the strength of the screw propeller as a function of the action of the blade failure load, or BFL.

(11) The outer surface of the blade flange 32 has, in its meridional cross section, a profile curved inwards towards the axis 50 of the propeller, and denoted by the reference 52.

(12) According to the shipbuilding and classification rules, the BFL 71 is applied at a distance 72 from the propeller axis, also called the radius, corresponding to R_BFL=0.8 R in the weakest direction of the blade failure load. The calculation of the bearing capacity of the blade is based on the action of the bending moment of the BFL 71 for the cross section 62 of the transition of the blade foil 31 into the flange 32, which is the weakest section outside the boundaries of the fillet transition of the blade foil 31 to the blade flange 32. This cross section is usually located in the contact area between the fillet 64 and the blade profile 61. The fillet, with another relatively axial line 51 of the screw propeller blade on the side of the blade profile 61, is denoted by the reference 65. The difference between the radius of application of the BFL and the radius of the location of the weakest cross section outside the fillet transition determines the moment arm 73 of the action of the BFL M_64, and, correspondingly, the value of the bending moment in the calculated cross section, which is the product of the BFL 71 and the moment arm 73 of the force M_64.

(13) According to the standards, the blade failure load F.sub.ex, in kN, is calculated by the formula:

(14) $F_{ex}=\frac{0.3{\mathrm{ct}}^2{\sigma }_{\mathrm{ref}}}{0.8D-2r}.Math.{10}^3,$
where
σ.sub.ref=0.6σ.sub.0.2+0.4σ.sub.u,

(15) where σ.sub.u and σ.sub.0.2 are the specific maximum values of the ultimate strength and yield point of the blade material;

(16) D, c, t, and r are, respectively, the propeller diameter, and actual length of the chord, thickness and radius of the cylindrical root section of the blade at the weakest point outside the boundaries of the fillet transition, determined by the design of the screw propeller; this section is usually located in the area of attachment of the fillet to the blade profile.

(17) As seen in the strength calculation formula, for a chosen material and a specific shape (geometry) of the blade 30, the value of the blade failure load applied at a radius of 0.8 R is mainly determined by the radius of the location of the weakest section outside the boundary of the fillet transition of the blade foil 31 into the blade flange 32, or, in other words, by the moment arm 73 of the action of the BFL.

(18) FIG. 2 also shows that the calculated section 62 of the transition of the blade foil 31 into the flange 32 is located at a distance R from the propeller axis 50 which is less than the distance R_53 from the point of intersection of the propeller axis 50 and a straight line 53 located in the meridional cross section of the screw propeller 22 and connecting the extreme outer radii R1, R2 of the propeller blade flange 32.

(19) By using an inwardly curved hub profile 52, the moment arm 73 of the BFL can be increased relative to the conventional conical generatrix 53 of the hub, and therefore relative to the shorter moment arm 74. The fillets connecting the blade foil 31 to the blade flange 32 of a conical hub are denoted by the references 66 and 67. Consequently, when an inwardly curved hub shape 52 is used, the bending moment causing the destruction of the blade foil 31 reaches its maximum calculated value at a smaller value of the blade failure load 71 than when a rectilinear conical shape 53 is used.

(20) This enables the design value of the blade failure load to be reduced, while meeting all the requirements of the Shipping Register for screw propeller strength, and thereby acting in accordance with the principle of pyramidal strength, by reducing the loads on other components of the pod drive when the screw propeller 30 is subjected to plastic bending (or damage).

(21) FIG. 3 shows the design of a composite screw propeller of a pod drive, which is commonly used for ice-going vessels. The composite screw propeller usually consists of blades 30, each formed by a blade foil 31 and a blade flange 32. Said blades 30 are joined to the propeller hub 33 by means of bolts 81 fitted in shafts 69 in the blade flange 32, the depth of these shafts being determined by a requirement to avoid the projection of the locking components of the screw propeller blade attachment bolts 81 beyond the flange 32.

(22) The diameter of the propeller shaft 20 and the length of the conical part 21 of the shaft are usually determined by a requirement to transmit the maximum torque from the electric propulsion motor to the propeller of the pod drive. The hub 33 is attached to the portion 21 of the shaft with an interference fit, and the thickness of the hub 33 is determined by a requirement to provide the necessary degree of interference fit and strength in the hub, including the threaded sockets for the blade attachment bolts 81.

(23) The minimum thickness of the blade flange 32 of the screw propeller is determined by a requirement to provide sufficient material under the heads of the bolts 81 for a reliable attachment of the blade 30 to the hub 33, and a minimum depth of the shafts 69 required to achieve the condition of ensuring that the projecting locking parts of the screw propeller blade attachment bolts 81 are sunk into the blade flange 32.

(24) When the aforementioned design constraints are met, the location of the weakest cross section, shown in FIG. 2, outside the fillet transition of the blade foil 31 into the blade flange 32 will be determined by the shape of the generatrix of the flange (bush) of the screw propeller. As seen in FIG. 2, for a given value of the radius of the leading edge of the bush R1, the use of an inwardly curved shape of the generatrix 52 enables said weakest cross section to be located at a smaller radius than in a conventional conical profile 53.

(25) This enables the design value of the blade failure load to be reduced by comparison with the conventional shape of the blade flange of a composite screw propeller, thereby acting in accordance with the principle of pyramidal strength by reducing the loads acting on other components of the pod drive when a blade of the screw propeller 30 is damaged to values which avoid damage to the components of the pod drive during operation in icy conditions.