CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the United States national phase of International Application No. PCT/IB2013/056281 filed Jul. 31, 2013, and claims priority to South African Patent Application No. 2012/05757 filed Jul. 31, 2012, the disclosures of which are hereby incorporated in their entirety by reference. 
     BACKGROUND TO THE INVENTION 
     THIS invention relates to a propeller and more particularly but not exclusively, to a propeller for use with inboard and outboard boat engines. 
     A propeller is a device that transmits power by converting rotational motion into thrust. A pressure differential is produced between forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade, thus resulting in thrust required to drive a means of transport to which the propeller is attached. One specific type of propeller is a propeller for use as a means of propulsion in boat engines, whether outboard or inboard. 
     Many different propeller designs are known in the trade, and they all share some of the same design characteristics. A propeller comprises a plurality of blades extending radially outwardly from a central rotating hub. Each blade is shaped in the form of an airfoil having two opposite surfaces, being a blade face (which is the pressure side of the blade facing the stern), and the blade back (which is the suction side of the blade facing the bow). Each blade furthermore includes a leading edge, which is the edge of the propeller adjacent the forward end of the hub. The leading edge leads the blade into the flow when the propeller is providing forward thrust. The opposing edge is referred to as the trailing edge, and the radially outer zone extending between the leading edge and the trailing edge is referred to as the blade tip. The root of the blade is the fillet area in the region of transition between the blade surface and the hub periphery. 
     Blade surface area refers to the total surface area of the propeller blade. When a propeller rotates on a fixed axis for any period of time a centrifugal force creating a negative pressure on the blade back of each rotating blade draw water inwards, and when the oncoming blade face comes into contact with the inward flow of water the water is compressed. A positive pressure is therefore induced, and the water in this positive pressure zone then exerts a force against the adjacent body of water, resulting in thrust. Standard blade designs allow the inward flow of water to flow over the entire curvature of the blade back. This is believed to result in about 40% of the energy being wasted because on average only 60% of the negative pressure water mass is compressed by the blade face of an oncoming blade. Further energy is lost between the blade roots of each blade back, which fragments the flow of water when the positive pressure water mass collides with the negative pressure water mass. This disturbance affects the volume of water that gets displaced. It would obviously be beneficial if a way could be found to harness the wasted energy in order for the full potential energy of the water flow to be utilized. 
     It is accordingly an object of the invention to provide a propeller that will at least partially alleviate the above disadvantage. 
     It is also an object of the invention to provide a propeller having a secondary propulsion system which will aid optimizing the performance of the propeller. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a propeller including:
         a hollow hub; and   a plurality of primary blades extending substantially radially outwardly from the hub, with each primary blade including a blade face, a blade back and a root section;   characterized in that a set of secondary blades are provided inside the hub, and that an inner volume of the hub is in flow communication with a volume radially outwardly of the propeller hub.       

     The inner volume of the hub is in flow communication with a volume between two adjacent primary blades of the propeller. 
     There is provided for each primary blade to have a blade face, a blade back and a blade peripheral zone comprising a leading edge, a trialing edge and a blade tip zone extending between the leading edge and the trialing edge. 
     Each primary blade includes a root section, which is the section of the blades where the blade merges with the hub. 
     There is provided for the secondary blades to be located on a hub insert which fits inside the hub, and which is secured to the hub in order to rotate with the hub. 
     An annular volume is formed between an inner surface of the hub and an outer surface of the hub insert, with the secondary blades disposed inside the annular volume. 
     The secondary blades may be of a helical blade configuration. 
     There is further provided for tertiary blades to extend from the hub insert, with each tertiary blade located between two adjacent secondary blades. 
     The tertiary blades divide the annular volume between the hub and the insert into alternating water and exhaust gas flow passages. 
     There is provided for the annular volume to be in flow communication with the space between two adjacent propeller blades by way of an inlet opening. 
     More specifically, there is provided for each water flow passage in the annular volume to be in flow communication with the space between two adjacent propeller blades by way of an inlet opening. 
     The inlet opening is preferably located adjacent a root of a blade on the blade face side of the blade. 
     There is provided for the inlet opening to extend along substantially the entire root of the blade. 
     There is also provided for a guide vane for guiding flow into the inlet opening to be located adjacent the inlet opening. 
     There is provided for the exhaust gas flow passage in use to be in flow communication with an exhaust gas outlet of an engine to which the propeller is secured. 
     The hub insert may be releasably securable to hub. 
     The hub insert may have a first end which is securable to the front end of the hub, and a second end protruding beyond the second, rear end of the hub. 
     There is provided for the second end of the hub insert to form an expansion nozzle of reduced area in order in use to provide a high velocity water outlet jet, and accordingly thrust, when water is expelled therethrough. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the invention is described by way of a non-limiting example, and with reference to the accompanying drawings in which: 
         FIG. 1  is a front perspective view of the propeller incorporating a secondary propulsion system in accordance with one embodiment of the invention; 
         FIG. 2  is a side view of the propeller of  FIG. 2 ; 
         FIG. 3  is bottom plan view of the propeller of  FIG. 1 ; 
         FIG. 4  is a bottom perspective view of the propeller of  FIG. 1 ; 
         FIG. 5  is an exploded perspective view of the propeller of  FIG. 1 ; and 
         FIG. 6  is an exploded perspective view of another embodiment of the propeller. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring to the drawings, in which like numerals indicate like features, a non-limiting example of propeller in accordance with the invention is generally indicated by reference numeral  10 . 
     The propeller  10  comprises a hub  20 , which may be of many different configurations. In one embodiment the propeller includes a flow guide as described in the applicant&#39;s co-pending application entitled “Propeller including a blade back flow guide”, the contents of which is incorporated herein by reference. In one embodiment the propeller also includes the provision of edge members on the blades of the propeller entitled “Propeller including a discrete edge member”, the contents of which is also incorporated herein by reference. 
     A plurality of primary blades  30  extend radially outwardly from the hub  20 , with each blade defining an airfoil extending from the hub  20  at a root section  33  thereof, and terminating in a peripheral tip zone  34 . The blade  30  includes a blade face  31  and a blade back  32 . The periphery of the blade  30  comprises a leading edge  35 , a trailing edge  36 , and an outer tip zone  34  extending between the leading edge  35  and the trailing edge  36 . 
     The hub  20  of the propeller has a hollow interior  21 , and includes receiving formations for receiving and securing a hub insert  60  as is described in more detail below. A hub inlet opening  23  is provided in a sidewall of the hub  20 , and provides flow communication between the hollow interior  21  of the hub  20 , and a space between adjacent blades  30  of the propeller. More particularly, the inlet opening  23  is of an elongate configuration, and is located adjacent a root  33  of a blade  30 , on the blade face side  31  of the blade  30 . The inlet opening  23  extends along substantially the entire root  33  of the blade  30 . An inlet guide vane  24  may be located adjacent the inlet opening  23  on the side of the opening opposite the blade  30 , and aids in directing water into the inlet opening  23 , and thus into the interior volume  21  of the hub. 
     The hub insert  60  is in the form of a tubular body having a front end  61  and a rear end  63 . The hub insert  60  fits inside the hub  20 , with the rear end  63  of the hub insert  60  protruding from the hub  20 . The front end  61  of the hub insert  60  is secured to the hub  20  by way of securing means that engages the receiving formations  22  provided on the hub  20 . It should be noted that the manner in which the hub insert  60  is secured to the hub  20  may vary, and that the way in which the hub insert  60  is secured to the hub  20  is therefore not of a limiting nature. Various ways of securing the hub insert  60  to the hub  20  can easily be conceived by a person skilled in the art. 
     The hub insert  60  is centrally located inside the hub  20 , and an annular volume  80  is therefore formed between the inner surface of the hub  20  and the outer surface of the hub insert  60 . 
     In the embodiment of  FIG. 5 , secondary blades  65  extend from the hub insert  60 , and are disposed inside the annular volume in order to displace water that enters the annular volume through the inlet opening  23  of the hub  20 . In this example the blades are in the form of helical blades that extend substantially to the rear end  63  of the hub insert  60 . The rear end  63  of the hub insert terminates in an outlet nozzle  64 , which is of a smaller diameter than the rest of the hub insert  60 . This causes the water displaced by the blades  65  to be accelerated, and hence in an increase in thrust. However, the inventor has found that the performance of this arrangement is not optimal, due to the fact that the annular volume is also the conduit through which exhaust gasses are discharged from an engine to which the propeller is secured. The high-pressure exhaust gasses interferes with the intake of water through the inlet openings  23 , and an improvement of the design (although not an absolute limitation) is shown in  FIG. 6 . 
     In the embodiment of  FIG. 6 , secondary blades  65  also extend from the hub insert  60 , and are disposed inside the annular volume in order to displace water that enters the annular volume through the inlet opening  23  of the hub  20 . In this example the blades are in the form of helical blades that extend substantially to the rear end  63  of the hub insert  60 . The rear end  63  of the hub insert terminates in an outlet nozzle  64 , which is of a smaller diameter than the rest of the hub insert  60 . This causes the water displaced by the blades  65  to be accelerated, and hence in an increase in thrust. Tertiary blades  66  are provided in addition to the secondary blades  65 , and are disposed between adjacent secondary blades  65 . The configuration of secondary blades  65  and tertiary blades  66  define a plurality of distinct and alternating water (A) and exhaust gas (B) passages. The passages are isolated from one another, and exhaust gas will not enter the water passages and vice versa. The water passages (A) are in flow communication with the hub openings  23 , whereas the exhaust gas passages (B) are not. In use, exhaust gasses will enter the exhaust gas passages (B) from an open end of the passages at the front end  61  of the hub insert, and will leave the passages at a rear end  63  of the hub insert. Water will enter the water passages (A) via the hub openings  23 , and will exit the water passages (A) at a rear end  63  of the hub insert. It should be noted that an operatively forward end of a water passage  68  is a closed end, because the water enters the passage in a radial direction, as opposed to the axial inlet direction of the exhaust gas passage (B). 
     Eddie stabilizers may be provided to stabilize the vortex of water being created by the intake of water through the intake openings and pushes out a unidirectional jet of water. 
     In use, the propeller creates secondary propulsion by water being drawn into the annulus between the hub and the hub insert via the inlet openings, with the water then being accelerated by the secondary blades, and forced through the reduced area throat of an outlet nozzle. Due to the provision of tertiary blades, this happens in isolation to the flow of exhaust gasses through the same annulus. 
     The secondary blade design and efficiency is intricately linked to the design of other components that make up the secondary propulsion i.e., the intake openings, the volumetric area, the laminar transition of the intake openings, the positioning and configuration of eddy stabilisers&#39; positioning (including angle of trajectory), venturi rate of compression, venturi “bowl” area and the outlet nozzle. 
     The design of the intake openings is of particular importance and there are a multitude of factors that determine its length and size. For instance, a larger hub and greater blade displacement will influence the intake opening design with a widening of the intake gradually and altering the degree of angle of each secondary blade. This maximizes the amount of water available for acceleration. It has been found that an intake opening extending substantially the entire length of the root of a blade will result in superior performance. There is also provided for the width of the intake opening to be approximately between 10% and 35% of the distance between two adjacent blade roots. Preferably the width of the intake will be between 15 and 25% of the distance between two adjacent blade roots. 
     The propeller, and in particular the hub insert of the propeller, will be secured to a drive shaft of an engine by way of a spline, which is a methodology and configuration which is well known in the art. 
     The propeller  10  body is made from magnesium or a magnesium alloy, which is made in a moulding process known in the art. The inlet guide vanes  24  are integrally formed with the propeller blades and body. In addition, the inlet openings  23  are also formed a part of the moulding process, and are in particular not machined afterwards. 
     It will be appreciated that the above is only one embodiment of the invention and that there may be many variations without departing from the spirit and/or the scope of the invention.
THIS invention relates to a propeller and more particularly, but not exclusively, to a propeller for use with inboard and outboard boat engines. The propeller includes a hollow hub and a plurality of primary blades extending substantially radially outwardly from the hub, with each primary blade including a blade face, a blade back and a root section. The propeller is characterized in that a set of secondary blades are provided inside the hub, and that an inner volume of the hub is in flow communication with a volume radially outwardly of the propeller hub.