Abstract:
A blade for a marine propeller includes an adjustment strip located in a channel near the trailing edge of a high pressure face of the blade. The adjustment strip protrudes from the blade face, altering the hydrodynamic properties of the blade. Strips can be replaced with other strips of different heights in order to suit particular requirements for hydrodynamic properties.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a means for adjusting the hydrodynamic properties, such as the pitch, of a marine propeller. 
       BACKGROUND TO THE INVENTION 
       [0002]    Propulsion systems for marine vessels are typically calibrated to operate within narrow parameters in order to achieve efficient operation. In particular, the hydrodynamic properties of a marine propeller are generally closely matched to the speed and power of an associated motor; the weight, weight distribution and hull resistance of the vessel; and the environment, such as the water temperature, within which the vessel operates. 
         [0003]    Many engines for use within marine vessels are electronically controlled to adjust their power output depending on the ambient air and water temperatures within which the vessel is operating. This can make choice of a propeller difficult, as it is important that the propeller be designed such that a minimum speed of revolution is reached when the engine throttle is completely opened, in order to prevent overloading of the engine. 
         [0004]    The complexities of propeller design are further exacerbated by the prospect that the speed of rotation of the propeller will vary depending on the extent of sheet cavitation. The amount of cavitation varies considerably according to the speed of the vessel, the density and temperature of the water within which the propeller is working, as well as the hydrodynamic properties of the hull and shaft line. Cavitation can result in excessive vibration, wear and loss of efficiency of a propeller. 
         [0005]    Although complex, the hydrodynamic properties of propellers are sufficiently well understood that it is possible to design a propeller to match the known characteristics of a marine vessel and engine. Problems arise, however, when characteristics of a vessel are changed, for instance by the addition of new features such as a fishing tower, or by the relocation of the vessel from a cold water environment to a warm water environment. 
         [0006]    Known solutions for this problem range from the replacement of the propeller—which can be a very expensive procedure—to manual bending of the propeller blades. Bending of the blades alters the propeller&#39;s hydrodynamic properties in substantially uncontrollable ways, and also introduces stresses which can lead to fatigue cracking and ultimate mechanical failure of the blades. 
         [0007]    The present invention seeks to at least partially ameliorate these problems, and to provide a means for altering the hydrodynamic properties of a marine propeller in a controlled manner. 
       SUMMARY OF THE PRESENT INVENTION 
       [0008]    In accordance with one aspect of the present invention there is provided a blade for a marine propeller, the blade including an attachment portion arranged to receive an adjustment means, whereby engagement of an adjustment means with the attachment portion alters the hydrodynamic properties of the blade. The present invention envisages a selection of adjustment means being available, whereby a particular one of more of the adjustment means may be chosen achieved desired hydrodynamic properties. 
         [0009]    In accordance with a second aspect of the present invention there is provided an adjustment means arranged to engage with a blade of a marine propeller, the adjustment means having an engaging portion arranged to be received by an attachment portion of a blade, whereby engagement of the adjustment means with a blade alters the hydrodynamic properties of the blade. 
         [0010]    Preferably the attachment portion comprises a channel within the blade, and the adjustment means comprises a strip receivable within the channel, the strip including a portion which juts outwardly. Advantageously, the strip may be readily removed and interchanged. 
         [0011]    More preferably, the channel is located adjacent to a trailing edge of the blade, on a high pressure face. Advantageously, this allows for the use of strips to alter the effective pitch of the propeller. It is desirable that the strip be located as close as possible to the trailing edge without introducing stress concentrations within the blade. This is preferably within 50 mm of the trailing edge, and may be about            
         [0012]    The width of the strip may be less than 10 mm, perhaps about 5 mm. This provides sufficient strip strength without greatly altering blade properties. 
         [0013]    The length of the strip may be about 60% of the blade radius. Having the strip extend beyond 90% of the blade radius, and providing a concave curve at its end, allows for a useful localised increase in water pressure at this end. Have the strip commence from about 30% of the blade radius minimises losses due to water flow internally of the strip. 
         [0014]    In an alternative embodiment, the strip may be located on the low pressure face of the blade. It is envisaged that this will help in prevention of cavitation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0015]    It will be convenient to further describe the invention with reference to the accompanying drawings which illustrate preferred embodiments of the propeller adjustment of the present invention. Other embodiments are possible, and consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention. In the drawings: 
           [0016]      FIG. 1  is a perspective of a marine propeller blade in accordance with the present invention; 
           [0017]      FIG. 2  is a side view of the propeller blade of  FIG. 1 ; 
           [0018]      FIG. 3  is a front view of the propeller blade of  FIG. 1 ; 
           [0019]      FIG. 4  is an end view of the propeller blade of  FIG. 1 ; 
           [0020]      FIG. 5  is a cross section, through the chord A-A marked in  FIG. 4 , of the propeller blade of  FIG. 1 ; and 
           [0021]      FIG. 6  is an enlarged view of a portion of the cross section shown in  FIG. 5 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    Referring to the figures, there is shown a single blade  12  of a propeller  10 . The propeller  10  has a plurality of such blades  12  extending outwardly from a hub  14 . Typically, a propeller  10  may have five or six blades  12 , however it will be appreciated that the present invention may be applied to propellers having any desired number of blades. 
         [0023]    The propeller  10  has a low-pressure or upstream side  16  and a high pressure or downstream side  18 . 
         [0024]    The blades  12  are all substantially similar in shape and configuration. Each blade  12  has a high pressure face  20  substantially oriented towards the downstream side  18  of the propeller  10 , and a low pressure face  22  substantially oriented towards the upstream side  16  of the propeller  10 . Each blade  12  has a leading edge  24 , a trailing edge  26 , and an inner edge  30 . The inner edge  30  of each blade  12  is joined to the hub  14 . The leading edge  24  forms a convex curve extending from the inner edge  30  to an outermost part of the propeller  10 . In the embodiment of the drawings the trailing edge  26  forms a generally concave curve from the inner edge  30  to the outermost part of the propeller. The curvature of the leading edge  24  is significantly greater than that of the trailing edge  26 , thus defining a bulbous shape for the faces  20 ,  22  of the blade. 
         [0025]    In the embodiment shown in the drawings, each blade  12  curves away from the hub  14 , as best seen in  FIG. 2 . The inner edge  30  is oriented relatively along the hub  14 , making a blade angle relative to a longitudinal direction of the hub  14 . The blade angle will vary with distance from the boss and nominal design pitch. At its most curved outer portion, the leading edge  24  makes an angle of about 65° relative to a longitudinal direction of the hub  14 . 
         [0026]    It will be appreciated that all parameters of the propeller  10  as above described are substantially set during casting of the propeller. As such, they may be chosen and engineered to suit a particular application. 
         [0027]    The advantage of the present invention lies in the ability to modify the properties of the propeller without changing the engineered shape and configuration. 
         [0028]    Each blade  12  includes an attachment portion in the form of a channel  32 . In a preferred embodiment, as shown in the drawings, the channel  32  is located on the high pressure face  20  of the blade adjacent to, but slightly spaced from, the trailing edge  26 . In the embodiment of the drawings the channel extends from a first end  34 , near the inner edge  30 , to a second end  36 , near the outermost end of the trailing edge  26 . The channel  32  substantially follows the contour of the trailing edge  26 . In particular, the channel  32  has a concave curve at its outer end  36 , following the contour of the trailing edge  26  as it meets the leading edge  24 . 
         [0029]    In the preferred embodiment shown in the drawings, the first end  34  is located at a point with a radial distance about 0.3 of the propeller radius. The second end  36  is located at a point with a radial distance about 0.925 of the propeller radius. 
         [0030]    As can be best seen in  FIG. 6 , the low pressure face  22  tapers towards the high pressure face  20  of the blade  12  at the trailing edge  26 . The channel  32  is located just inside this taper, within the full blade thickness. In the embodiment shown in the drawings the channel  32  is spaced about 15 mm from the trailing edge  26 , with the channel having a thickness of about 5 mm. 
         [0031]    In a preferred embodiment, as shown in the drawings, the channel  32  is in the shape of a ‘dove-tail’, as best seen in  FIG. 6 . The dove-tail has sides  37  oriented at about 60° to the surface of the high pressure face  20 . The channel has a base  35  substantially parallel to the surface of the high pressure face  20 . In the embodiment shown in the drawings, the channel  32  has a depth of about 3.4 mm, being about half the blade thickness. 
         [0032]    The channel  32  includes an introducing region  38  at the first end, the introducing region  38  being substantially rectangular in cross section, and being wider than the remainder of the channel  32 . The introducing region  38  is tapered in depth, from the surface of the high pressure face  20  to the depth of the remainder of the channel  32 . 
         [0033]    The channel  32  is arranged to receive an adjustment means in the form of a protruding strip  40 . A suitable protruding strip  40  can be seen in cross section in  FIG. 6 . 
         [0034]    The protruding strip  40  is elongate, and of substantially constant cross-sectional shape. It comprises an engaging portion  42  and an outwardly projecting portion  44 . 
         [0035]    The engaging portion  42  is complementary in shape to the channel  32 . In the embodiment of the drawings this is a ‘dove-tail’ configuration, but it will be appreciated that other configurations may be used. 
         [0036]    The outwardly projecting portion  44  extends away from the engaging portion  42  such that, when the engaging portion  42  is engaged within the channel  32 , the outwardly projecting portion  44  juts outwardly from the high pressure face  20 . In the arrangement of the drawings the outwardly projecting portion  44  is            
         [0037]    The protruding strip  40  may be made of any suitable material. Possible materials include both nylon and polyurethane. 
         [0038]    The protruding strip  40  may be engaged with the channel  32  by sliding engagement. The strip  40  is introduced into the channel  32  through the introducing region  38 . 
         [0039]    The effect of the engagement of the protruding strip  40  into the channel  32  is to alter the hydrodynamic properties of the blade  12  and thus the propeller  10 . In particular, the engagement of strips  40  into each blade  12  has the effect of increasing the effective pitch of the propeller  10 . Rather than water flowing over the propeller from the leading edge  24  to the trailing edge  26  in a substantially laminar fashion, the flow is instead from the leading edge  24  to an upper edge  46  of the outwardly projecting portion  44 . This reduces the angle of water flow relative to the longitudinal direction of the hub  14 , effectively increasing the pitch of the propeller  10 . 
         [0040]    It will be appreciated that the degree to which the effective pitch is altered is directly relative to the height of the outwardly projecting portion  44 . 
         [0041]    Trials have suggested that the effective pitch is varied by two mechanisms, the altering of pitch due to the change in angle between the leading edge  24  and the upper edge  46  as discussed above, and also the pressure concentration along a leading face of the outwardly projecting portion  44 , causing a change in the direction of fluid flow. Testing of propellers similar to those described above and shown in the drawings has suggested that the latter effect may be represented by pitch change due to deflection (P D ) as a linear function of projecting portion height (H T ). The measured relationship in tests conducted by the applicant is P D (mm)=45+25.4(H T −1). This relationship is consistent for results for projecting portions having H T  between 0.5 mm and 4 mm. 
         [0042]    As will be appreciated, this relationship suggests that the inclusion of a small projecting portion can still alter pitch by at least 20 mm. 
         [0043]    The total change in effective pitch is equal to a superposition of the pitch caused by angular increase (P I ) and pitch change due to deflection (P D ). The effective pitch (P E (r)mm) at a radius r (mm) is thus defined by P E (r)=P D +tan(α P +α I )·2πr,          in pitch angle. The total change in effective pitch over the blade can be obtained by averaging over a range of radii. 
         [0044]    It will be understood that the length of the channel  32 , and the location of its ends  34  and  36 , will significantly affect the change in hydrodynamic properties caused by use of the strips  40 . It is considered that having the curve at the second end  36  of the channel  32  increases the deflection effect caused by water pressure. It is also considered that having the lift generated by the portion of the blade close to the hub  14  is small, and therefore the position of the first end  34  of the channel may not be as significant. 
         [0045]    In use, it is anticipated that a propeller  10  will be supplied with a plurality of sets of protruding strips  40 , each set varying from another by the height of its projecting portions  44 . In this way, the effective pitch can be chosen according to the conditions in which the propeller  10  is to operate. 
         [0046]    The procedure for constructing a propeller begins by consideration of a desired mean pitch. When this has been determined, the above equation can be implemented to design a propeller having a nominal pitch less than the desired mean, but which achieves the desired mean with use of a strip having a projecting portion of, for instance, 1.5 mm. 
         [0047]    Following casting of the propeller  10 , an appropriate channel  32  can then be machined into each propeller blade  14 . Following completion of the machining process, an initial strip  40  (with 1.5 mm height in this example) can be inserted into the channel  32 . 
         [0048]    Whilst the invention has been described with reference to the changing of pitch, it will be appreciated that suitable placement of the channel  32  may enable the invention to be used to vary other hydrodynamic properties of the blades  12 . It may be possible, for instance, to employ the invention on the low pressure face  22  to reduce or control the onset of cavitation. 
         [0049]    Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention. For instance, although          perpendicularly to the high pressure face  20 , it will be appreciated that in some applications it may be desirable for the projecting portion  44  to make an acute or obtuse angle relative to the face from which it extends.