FOLDABLE PROPELLER

Abstract: The present invention relates to a foldable propeller for a ship, comprising a hub (2) for mounting on a drive shaft of the ship, and at least two blades (1), each of which is pivotably arranged in the hub (2) between a first, essentially folded-together position and a second, essentially unfolded position, wherein each blade (1) presents a generator line (8). The invention is characterized in that each of the blades (1) has a skew distribution such that the leading edge of the inner and outer radii, respectively, are located substantially forward and aft of the generator line (8) of the blade (1), or that the mid-chord line (10) extends substantially forward and aft of the generator line (8) of the blade (1). The invention provides a foldable propeller with improved performance, in particular high reverse thrust and low noise and vibrations.

Description: TITLE : Foldable propeller FIELD OF THE INVENTION: The present invention relates to a foldable propeller according to the preamble of appended claim 1. BACKGROUND OF THE INVENTION: In the field of sailing-boats, the use of so-called foldable propellers is previously known. Normally, such a propeller is adapted to be used with a propulsion engine for propelling the boat forwardly or rearwardly. A propeller imposes a certain drag force on the sailing- boat when the propeller is not used. For this reason, the propeller can be made foldable, i.e. the blades of the propeller are pivotably arranged on a hub so that they fold together (as a result of the movement of the boat in the water) in the direction of the propeller's drive shaft, to a position in which they extend generally in the longitudi¬ nal direction of the boat. When the propeller is to be used, the blades are unfolded by means of the rotating action of the drive shaft. The propeller blades are normally designed so as to form a streamlined body in the folded-together state, which reduces the drag force. A number of foldable propelles is previously known. For example, WO 93/01972 shows a propeller which comprises at least three blades which are pivotably mounted between an unfolded position and a folded-together position. A problem which is significant for the previously known foldable propellers is that they either impose significant drag during sailing or do not deliver the required thrust in forward or reverse operation. Propellers having blades that fold together usually give a low drag force due to their streamlined bodies. However, in forward and particu- larly in reverse operation, the folding type of propeller has relatively poor propulsive performance. A particular problem regarding previously known folding propellers concerns the propulsion during reverse opera- tion. High reverse thrust at bollard or near bollard pull condition (i.e. when the propeller operates at a boat speed which is zero or close to zero) is usually achieved by adding weight to the tip of the blade, thus increasing the centrifugal moment about the blade's pivot axis. In this manner, the opening angle of the blades is increased. However, increasing the weight at the tip of the blades either involves a problem in the form of thick blade sections with poor cavitation properties or in the form of long sections which tend to reduce the propeller efficiency in forward operation of the propeller. Another problem which is common not only to sailing-boat propellers but to any propeller operating in a non-uniform velocity field, is the noise and vibrations induced by the propeller. The propeller generates pressure pulses which force the boat's hull or superstructure to vibrate fiercely and thus to generate unwanted noise. In applications where the propeller's drive shaft is connected to a high power propulsion means, the risk for high noise levels is significant for previously known folding propellers, since their blades are usually too narrow and blunt in order to avoid cavition, which is not only a cause of thrust breakdown but also a major source of noise and vibration. In the prior art, several methods for reducing noise and vibrations exist, for example increasing the number of blades. A propeller with many blades generates less fluctuating propeller forces than a propeller with fewer blades since the propeller hub acts as an integrator, i.e. the load on individual blades are superimposed by the hub and transferred via the propeller shaft to the hull of the boat. Noise and vibrations can also be reduced by reducing the pitch at either the blade's tip or its root, or both. This reduces the blade loading locally and thereby reduces the strength of the tip and hub vortices, which usually induce substantial pressure pulses on the hull. Moreover, noise and vibrations can generally be reduced by avoiding cavition or by arranging the propeller with skewed blades. Cavitation is normally avoided by giving the propeller a sufficiently large blade area. Injection of air into vapour cavities is also an effective method for eliminating their erosive behaviour and the generation of high frequency noise. A particular problem related to foldable propellers is possible thrust reduction due to cavitation at high drive shaft power. In the prior art, this problem is solved by giving the propeller a sufficiently large blade area, which is accomplished by using long blade sections and/or a large number of blades. However, the blade area cannot be made too large, since this decreases the propeller efficiency and also complicates the folding of the blades. A general problem related to propellers is to obtain a high forward thrust or propeller efficiency at any speed. The general solution to this problem is a large propeller diameter in combination with a low drive shaft speed. In addition, the radial load distribution of the propeller should be optimum and the blade area should be made large enough in order to avoid cavitation. Furthermore, the blades should have thin cambered sections of the air-foil type. Another problem which relates to foldable propellers concerns the folding function. Previously known folding propellers having high pitch-diameter ratio may have poor opening characteristics. The reason for this is that the blades of the propeller "shadow" each other, i.e. they cover each other more or less completely in the folded- together state. The hydrodynamic moment about the blade's pivot becomes negative and so large in magnitude when the blades are fully folded that the positive centrifugal moment never becomes large enough to start the opening of the propeller. The known solution to this "shadowing" problem is to tilt the blade sideways. However, a major drawback of tilting the blades is that they do not fold as well. This generates a higher drag force during sailing. A particular requirement relating to foldable propellers is that they should present low drag during sailing. This is generally achieved by giving the propellers a streamlined shape in the folded-together position. The usual low-drag solution is a propeller with a hub of small diameter and two straight narrow blades that fold with the flow during forward motion of the boat. A foldable propeller of this kind is previously known from GB 1416616. Another problem relating to folding propellers is that the folding mechanism malfunctions on occasion, possibly causing both personal injury and material damage. SUMMARY OF THE INVENTION: The object of the present invention is to provide a foldable propeller which solves the above-mentioned problems, in particular the problems regarding high reverse thrust of the propeller and low noise and vibrations. This object is accomplished by means of a foldable propeller according to the present invention, the features of which are defined in appended claim 1. According to a preferred embodiment, the propeller presents highly skewed blades, i.e. the blades have a generally curved shape where the leading edge of the inner and outer radii, respectively, are located substantially aft and forward of the blade's generator line. Preferably, the propeller presents a developed blade-area ratio which is greater than approximately 35%, in the case where three blades are used. Consequently, the developed blade-area can be said to be greater than approximately 10% "per blade". This gives a very effective and reliable folding function, low noise and vibration levels, less cavitation at high shaft power and a reduced moment about the blade's generator line. The term "developed" blade- area" can be defined as the area presented by one surface of a blade if it is "flattened out", i.e. the pitch angle for the blade is zero for each blade section and the resulting area is measured. The design of a propeller is always a process of finding the best compromise to a set of requirements. In this process some requirements are given more importance than others. For most previously known propellers, high effici¬ ency in forward operation and no cavitation are the two requrirements which are given the highest priority. This is also the case for previously known sailing-boat propellers. However, the requirements of the folded propeller according to the invention have been given a completely different order of priority, or weighting. Here, the highest weight has been given to high reverse thrust at bollard or near bollard pull condition. The second highest weight has been given to low noise and vibration. The third and fourth highest weight, respectively, have been given to no cavitation and high forward thrust. Consequently, the folded propeller according to the present invention is designed to provide, in particular, high reverse thrust at bollard or near bollard pull condition and low noise and vibration levels on board. Noise and vibrations can normally be avoided by using skewed blades which, contrary to conventional straight blades, gradually enter regions with disturbed flow and therefore generate a smoother blade loading history , by means of which a decrease in the amplitude of the load on the blade is achieved. However, in the prior art, no foldable propeller having a substantial skew exists. This is due to the fact that, according to the previously known technology, a skewed propeller is normally difficult to fold. It should be noted that the terms "boat" and "ship" are intended to cover different types of vessels, in the form of small boats as well as large ships, or any vehicle for use in water. Furthermore, the invention can be used on boats with or without sails. BRIEF DESCRIPTION OF THE DRAWINGS: The invention will be described in the following with reference to the annexed drawings, in which Fig. 1 shows an end view of a foldable propeller accor¬ ding to the present invention, Fig. 2 shows a side view of the propeller of Fig. 1, Fig. 3 shows a simplified view of the propeller accor¬ ding to the invention, defining the skew of the propeller, Fig. 4 is a diagram illustrating the thickness distribu¬ tion of the propeller according to the invention, Fig. 5 is a diagram illustrating the notation for desc¬ ribing the blade geometry, Fig. 6 is a diagram illustrating the skew distribution of the propeller, Fig. 7 is a diagram illustrating the pitch distribution of the propeller, Fig. 8 is a diagram illustrating the rake of a blade of the propeller, Fig. 9 shows the propeller according to the invention in a folded-together position, Fig. 10 is a diagram illustrating the pivot moments of a blade, and Fig. 11 is a diagram showing the effect of skew on the hydrodynamic pivot moment. PREFERRED EMBODIMENT: Fig. 1 shows a foldable propeller according to the in¬ vention. According to a preferred embodiment, the propeller comprises three essentially identical blades 1. The blades 1 are pivotably mounted in a hub 2 which is intended to be arranged on a drive shaft (not shown) of a boat engine of conventional type. Each blade 1 is manufactured from a relatively heavy material, for example bronze, aluminium bronze (comprising 8-10% aluminium) or steel. The hub 2 may be manufactured from a similar material or a fibre plastic composite. Each blade 1 is arranged in a recess 3 in the hub 2. The pivoting movement of the blades 1 is provided by the fact that the hub 2 presents three shafts 4 which extend through cooperating holes in the blades 1. The pivoting mechanism of each blade 1 comprises two bevel gear segments 5 per blade, i.e. the innermost end of each blade has a left and right gear segment, which segments are adapted to engage complementary gears on the adjacent blades 1 in all pivoted positions. Thus, the bevel gears 5 cooperate so that the blades 1 may fold together simulta¬ neously. For each blade 1, a leading edge 6 and a trailing edge 7 for forwards operation of the propeller, i.e. for counter¬ clockwise rotation as seen in Fig. 1, can be defined. During reverse operation, the leading edge 6 acts as a "trailing edge" and the trailing edge 7 acts as a "leading edge". It should be noted that the propeller according to the invention can be designed also for clockwise rotation when operating in the forwards direction. Fig. 3 shows, in a simplified form, the propeller according to the invention. As is apparent from the figure, the blades 1 are highly skewed, i.e. they extend along a curved line. In order to define the amount of skew of each blade 1, it is necessary to define the so-called generator line of the propeller. The generator line 8 is a reference line used for designing a propeller, which line 8 is perpendicu¬ lar to the longitudinal extension of the propeller axis 9. Furthermore, the generator line 8 extends through the centre of the propeller axis 9. Finally, the generator line 8 is perpendicular to the pivot axis about which the blade 1 can be folded. Furthermore, each blade 1 can also be said to present a mid-chord line 10, which is a line made up of a locus of points equidistant from the trailing and leading edges of the blade. An important feature of the invention is that each blade 1 has a skew distribution such that the leading edge of the inner and outer radii, respectively, are located forward and aft of the blade's generator line 8. The skew distribution can be determined by defining the skew angle a , which is the sum of a first angle β and a second angle γ . The first angle β is the angle between the generator line 8 and a straight line 11 extending perpendi- cularly from the centre 9 of the hub and through the leading edge 1 of the mid-chord line 10. The second angle γ is the angle between the generator line 8 and a straight line 12 extending perpendicularly from the centre 9 of the hub and through the end point of the mid-chord line 10, at the tip of the blade 1. The skew angle α, i.e. the sum of the first angle β and the second angle γ , is preferably between 30° and 65°, the most preferable interval being between 45° and 55°. The first angle β is preferably between 10° and 25°, the most preferable interval being between 15° and 20°, whereas the second angle γ is preferably between 20° and 40°, the most preferable being between 30° and 35°. Fig. 3 also illustrates the inner and outer radii, respec¬ tively, which can be defined for a particular blade 1. The inner radii r n and r i2 are examples of radii which extend from the propeller axis 9 to points along the blade 1 which are located inside of the point where the mid-chord line 10 intersects the generator line 8. In a corresponding way, the outer radii r o] and r o2 extend from the propeller axis 9 to points along the blade which are located outside of the point where the mid-chord line 10 intersects the generator line 8. The inner radius r,,, for example, has a leading edge 17 (for rotation in the counter-clockwise direction) and the outer radius r 0] , for example, has a leading edge 18 (for counter-clockwise rotation). Each blade 1 has a skew distribution such that the leading edges of the inner and outer radii along the a blade are located substantially forward and aft of the generator line 8. The highly skewed blades 1 according to the invention also present a developed blade-area ratio. The blade-area ratio can be defined as the developed area of the blades divided by the total area within the circle defined by the tips of the blades. The blade-area ratio is preferably higher than 35%, preferably between 35 % and 45 %. It should be noted that these values apply to the case where the propeller comprises three blades. Consequently, this means that the developed blade-area ratio "per blade" should be higher than approximately 10%. Fig. 4 illustrates the thickness distribution of a blade. According to the invention, any cross-section along a blade has an essentially symmetrical thickness distribution, i.e. the thickness distribution is symmetrical about a plane 13 defined by the mid-chord line (see also Fig. 3). This means that the difference between the lift-drag ratio in forward operation and the lift-drag ratio in reverse operation is less than for propellers having conventional blade sec- tions. The thickness distribution is illustrated by means of the curve 15 in Fig. 4, in which the leading edge of the blade is the leftmost edge, whereas the trailing edge is the rightmost edge. It is particularly advantageous if the thickness distribution has an essentially elliptical shape. Regarding Fig. 4, it should be noted that the ellipse 15 illustrates neither the curvature nor the pitch of the blade, but merely illustrates the blade's thickness distribution. As a reference, Fig. 4 also shows, by means of the broken lines 14, a conventional wing section which presents a non- symmetrical thickness distribution. Provided that the thickness-chord ratio (i.e. the rela¬ tionship between the maximum thickness of a blade section and its length, the latter being equal to the distance between the leading edge and the trailing edge) is small, the penalty in lift-drag ratio of using elliptical sections instead of sections with pointed trailing edge is negli¬ gible. Furthermore, in reverse operation the cavitation characteristics of a section with elliptical thickness distribution is superior to sections with a pointed trailing edge. Finally, the solidity of an elliptic section area is significantly higher than for sections with a pointed trailing edge, i.e. the section area for the same thickness-chord ratio is higher. Thus, a blade with elliptical sections can be given higher weight as well as centrifugal moment about the pivot without sacrificing its lift-drag ratio or cavitation characteristics. Fig. 5 is a diagram which illustrates the notation for describing the blade's geometry. The generator line 8 extends in a direction normal to the plane of Fig. 5, i.e. towards the viewer. A section of a blade 1 is shown, with its leading edge 6 and trailing edge 7. The mid-chord line 10 is a line which bends in space so that it passes through the mid-chord point of each blade section 1. The skew of a particular blade section can be defined as a distance d, from the mid-chord line 10 to a plane perpendicular to the generator line 8. Furthermore, the rake of the blade section 1 can be defined as the axial displacement d 2 in a plane which is defined by the propeller axis and the generator line. In the case shown in Fig. 5, the rake is positive in the direction towards the aft of the propeller and zero when the chord line 16 of the blade section extends through the generator line 8. In this regard, the chord line 16 can be defined as a helix line extending through the leading edge 6 and the trailing edge 7 of the blade section 1. Finally, the pitch angle of the blade 1 can be defined as an angle P formed between the chord-line 16 of the blade section 1 and the projection of the propeller axis in the cross section in question. Fig. 6 illustrates the skew distribution of the propeller according to the invention. The diεtance d, (cf. Fig. 5) varies along the radius of the propeller, from a value which is close to zero in the propeller's hub, via a negative value along the mid portion of the propeller and to a positive value in the tip portion of the propeller. Furthermore, Fig. 6 illustrates that the inner radii of a blade are the radii which are in the interval r, from zero to the value where the mid-chord line intersects the generator line. Consequently, the outer radii of a blade are the radii which are in the interval r 0 which runs from the point where the mid-chord line intersects the generator line to the maximum radius of the blade. Fig. 7 is a diagram illustrating the pitch of the pro- peller's blades. In particular, the diagram shows a curve illustrating the pitch-diameter ratio along the radius R of the blade. As is apparent, the pitch of the blade is reduced at the blade's root and tip. The pitch-diameter ratio is reduced to approximately 75% of the pitch-diameter ratio at the point corresponding to 0,7 R, i.e. a point at 70% of the blade's diameter (at which point the pitch- diameter ratio is 100%), and is reduced to approximately 70% at the tip of the blade. In this manner, a reduced strength of tip and hub vortices is obtained, which delays inception of cavitation and reduces induced pressure pulses. Thus, the noise and vibration characteristics of the propeller are greatly improved. Furthermore, as is illustrated in Fig. 8, the rake dis¬ tribution (shown by the solid line) of the propeller's blades is negative and non-linear. More specifically, the rake distribution is preferably curved. The fact that the rake distribution is negative means that the shape of the blade is slightly curved and extends forwards. In contrast to this, a conventional rake distribution is normally positive (shown by the dashed line in Fig. 8), whereby the propeller blades extend towards the aft. The advantages with the rake distribution according to the invention are an increased strength of highly skewed blades, and that cavitation at the tip of the blade (if this cannot be avoided) is stabilised and therefore less erosive and noisy. The propeller according to the invention is designed so that the propeller can be folded together in an effective and reliable manner. Fig. 9 shows the propeller in its folded-together position. The blades can be folded so that the generator line is substantially parallel to the propeller axis. In this manner, the propeller forms a streamlined body in its folded-together position. In the following, the fold principle of the blades will be described. The sign of each blade's centrifugal moment about its pivot axis is independent of the direction of rotation of the propeller, since centrifugal moments are proportional to the square of the shaft speed. However, the centrifugal moment is strongly dependent on the fold angle of the blade. In this regard, the fold angle can be defined as the angle that each blade forms with the longitudinal extension of the hub. For a given shaft speed, the centri¬ fugal moment during the opening process typically varies as shown in Fig. 10. The centrifugal moment is positive when the blades are fully folded, whereas it is close to zero when the propeller is fully opened. In the initial opening phase, and in reverse operation, the tip region of the blade is, due to the special blade skew, subject to a high angle of attack as well as a high resulting relative velocity. Thus, the lift force is high at the tip of the blade. Consequently, its contribution to the hydrodynamic pivot moment is large and positive. On the other hand, the blade sections of the inner radii are subject to a small angle of attack and a low resulting relative velocity, thus their contribution to the hydrody¬ namic pivot moment is small. In forward operation, the hydrodynamic pivot moment is also positive in the initial opening phase since the blade skew guarantees that the inner radii generate a large positive contribution to the hydrodynamic pivot moment, while the contribution from the tip is small. The above taken together, the resulting pivot moment, i.e. the sum of the centrifugal and the hydrodynamic moment, is always large and positive in the initial opening phase for the propeller according to the invention. The invention provides a blade having a geometry with very favourable resulting pivot moment characteristics in the inital opening phase. Fig. 11 shows the effect of skew on the hydrodynamic pivot moment. Once the opening process has started, the blade pivots until it either hits the end stop (forward opera- tion) or finds an equlibrium (reverse operation), which occurs when the fold angle is such that the centrifugal pivot moment is equal in magnitude but opposite in direc¬ tion to the hydrodynamic moment. In reverse operation, the blade's hydrodynamic pivot moment with the new skew distribution is less negative than for the corresponding blade with zero skew, as shown in Fig. 11. Consequently, the fold angle of equlibrium of the skewed blade is larger, that is, the propeller opens more, and the reverse thrust becomes significantly higher. Again, the special blade skew distribution is, together with the elliptic blade sections, a major reason for the improved thrust, particularly in the reverse direction, of the propeller according to the invention. It is to be noted that the blades are so arranged (see also Figs. 1 and 2) that they can pivot at least to zero degrees in the folded-together position. The invention is not limited to the above-mentioned embodiments, but may be varied within the scope of the appended claims. For example, the propeller can have two or more blades. However, a three-bladed propeller is easier to balance than a two-bladed propeller, i.e. the balance requirement for each blade can be made less strict for a given maximum propeller unbalance. Thus, the cost for balancing is lower. Finally, it should be noted that since all the blades 1 are identical, the manufacturing of the blades is greatly simplified. The bevel gears which can be effectively manufactured, provided that an advanced milling machine can be used, provide an advantage, since the blades are thus more difficult to copy.

Claims: CLAIMS : 1. A foldable propeller for a ship, comprising a hub (2) for mounting on a drive shaft of the ship, and at least two blades (1), each of which is pivotably arranged in the hub (2) between a first, essentially folded-together position and a second, essentially unfolded position, wherein each blade (1) presents a generator line (8), c h a r a c t e ¬ r i z e d i n that each of the blades (1) has a skew distribution such that the leading edge of the inner and outer radii, respectively, are located substantially forward and aft of the generator line (8) of the blade (1), or that the mid-chord line (10) extends substantially forward and aft of the generator line (8) of the blade (1). 2. A foldable propeller according to claim 1, c h a r a c ¬ t e r i z e d i n that a first angle (β) , formed between the generator line (8) and a straight line extending through the leading edge of the mid-chord line (10), has a value between 10° and 25°. 3. A foldable propeller according to any of the preceding claims, c h a r a c t e r i z e d i n that a second angle (y), formed between the generator line (8) and a straight line passing through the tip of the mid-chord line (10), has a value between 20° and 40°. 4. A foldable propeller according to any one of claim 2 or 3, wherein the skew angle ( ) of the blade (1) is defined as the sum of the first angle (β) and the second angle (γ) , c h a r a c t e r i z e d i n that the skew angle (a) iε larger than 25°. 5. A foldable propeller according to any of the preceding claims, c h a r a c t e r i z e d i n that the developed blade-area ratio, is greater than 10% as counted per blade. 6. A foldable propeller according to any one of the preceding claims, c h a r a c t e r i z e d i n that the blades (1) are pivotably arranged for pivoting at least to zero degrees, with reference to the propeller axis, in the folded-together position. 7. A foldable propeller according to any of the preceding claims, c h a r a c t e r i z e d i n that the blade (1) sections have an essentially symmetrical thickness dis- tribution along the radius. 8. A foldable propeller according to claim 7, c h a ¬ r a c t e r i z e d i n that said thickness distribution has an essentially elliptical form. 9. A foldable propeller according to any one the preceding claims, c h a r a c t e r i z e d i n that the pitch at the tip and the root of the blade (1) is reduced by at least 10% in relation to the pitch-diameter ratio at 0,7 radius. 10. A foldable propeller according to any of the preceding claims, c h a r a c t e r i z e d i n that the rake distribution is negative and shaped essentially as a circular arc. 11. A foldable propeller according to any of the preceding claims, c h a r a c t e r i z e d i n that the innermost end of each blade (1) has at least one gear section (5) which is adapted to engage complementary gears on the adjacent blades (1) in all pivoted positions. 12. A foldable propeller according to claim 11, c h a ¬ r a c t e r i z e d i n that each blade (1) is provided with at least one bevel gear (5).

Inventors: GUSTAVSSON BROR, JONSSON CHRISTER, GUSTAVSSON, BROR, JONSSON, CHRISTER
Applicants: GUSTAVSSON BROR, JONSSON CHRISTER, VOLVO PENTA AB, AB VOLVO PENTA, GUSTAVSSON, BROR, JONSSON, CHRISTER
Application year: 1996
Main classification: B63H1
IPC codes: B63B1, B63H1