Abstract:
Apparatus for reducing cavitation erosion is disclosed which includes ducts and openings (30, 32, 34, 40) for discharging a stream of gas positioned upstream of and adjacent to a propeller (11), in a direction perpendicular to the oncoming flow and at a lateral position relative to the propeller rotation axis.

Description:
This invention relates to apparatus for reducing cavitation erosion. 
     The undesirable effect of cavitation erosion upon propeller blades has long been recognised. Proposals have been made for limiting the damage which such erosion can cause. One such proposal is to reduce the effect of cavitation by injecting air into the water flow over the propeller, for example as disclosed in GB 2 067 709B. 
     It is an object of the invention to provide an improved apparatus for reducing root and hub erosion of propeller blades. 
     According to the invention in a first aspect there is provided apparatus for reducing cavitation erosion comprising means for discharging a stream of gas, into the liquid supporting the boat, upstream of and adjacent to the propeller, with the gas stream being directed transversely to the oncoming flow of liquid over the propeller, and with the discharge location being arranged at an angle position about the axis of rotation of the propeller which is within a predetermined range of angle positions of each given propeller blade passing the discharge location, such that a substantial proportion of the gas is entrained into the flow over the propeller at a range of blade angle positions at which blade root erosion occurs. 
     Preferably the discharge position is greater than 60° and less than 180° from the uppermost blade position, in the direction of rotation of the propeller. 
     According to the invention in a second aspect, there is provided apparatus for reducing cavitation erosion comprising means for discharging a stream of gas from a position upstream of and adjacent to a propeller and in a direction substantially normal to the oncoming flow direction. 
     According to the invention in a third aspect, there is provided apparatus for reducing cavitation erosion comprising means for discharging a stream of gas, from a position upstream of and adjacent to a propeller, in a direction transverse to the direction of flow of the liquid over the propeller, the discharging means comprising a passage formed in a support for a shaft for a said propeller and the passage having an opening formed in a side wall of the support over which side wall water flows towards the propeller. 
     An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a propeller assembly; 
     FIG. 2 is a fragmentary side elevational view of the propeller shaft bracket of FIG. 1 and illustrates an arrangement utilizing, according to one embodiment of the invention, a partly external and partly internal passage for the gas through the shaft bracket; 
     FIGS. 2A and 2B are views similar to FIG. 2 and illustrate, respectively, arrangements utilizing, according to other embodiments of the invention, an entirely internal and an entirely external gas passage through the shaft bracket; 
     FIG. 3 is a rear elevational view of the structure shown in FIG. 2, the view being taken in the direction of the arrow A of FIG. 2; 
     FIG. 4 is a schematic drawing showing the propeller air supply system; and 
     FIG. 5 is a graph illustrating the variation of propeller blade angle of attack and blade root erosion with rotation angle. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIGS. 1, 2 and 3, a propeller assembly is shown, generally designated 10, connected to the underside of a hull 12 of a water borne vessel adjacent the stern. The propeller assembly 10 comprises a propeller 11 having a propeller hub 14 upon which a plurality of propeller blades, for example, five propeller blades, are connected at the respective blade roots, of which only one blade, labelled 16, is shown. The propeller hub 14 is connected via a propeller shaft 18, to a prime mover and gearbox (not shown) for rotation of the propeller 11 about propeller axis 19. The propeller axis 19 is inclined by an angle φ to the flow or to the adjacent hull contour 12, φ being in the range of 5° to 20°. 
     The propeller shaft 18 is supported adjacent to the propeller by a shaft support, which comprises a shaft bracket 20 connected to a shaft bracket barrel 22 in which the shaft 18 is journalled. 
     The shaft bracket and shaft bracket barrel include means for introducing a stream of gas into the water flow over the propeller, as is more clearly shown in FIGS. 2 and 3. Air, or another gas or gas mixture, for example exhaust gas, may be used for this purpose. The gas introducing means comprises a bore 30 drilled through the barrel 22 which terminates at one end in a discharge opening 40 and which at its other end connects via a passage 31 with a channel or passage 32 machined out of the shaft bracket 20 at its exterior, which channel, in turn, communicates with a further internal drilled duct or passage 34 connected, via a shut off valve 42, to a gas supply (not shown in FIG. 2). The channel 32 is covered with a wrapped plate 33 which is welded in place. 
     The bore 30 is disposed so that it faces to starboard for a right-handed propeller and to port for a left-handed propeller. A separate shaft bracket may be used for each propeller, either right-hand or left-hand, with the bore 30 so disposed as before. Alternatively, in order to allow a single shaft bracket to be used for both right and left-hand propellers, a further bore 36 preferably symmetrical with the bore 30 and also branching off the passage 31 and terminating in a respective discharge opening 40&#39; (see FIG. 3), is drilled in the shaft bracket. In use, one bore 30 or 36 is blocked off with a steel plug 38 welded in place. The shaft bracket shown in FIG. 3 is arranged for use with a right-handed propeller, the bore 36 being blocked off by the steel plug 38, and the reverse would be done in the case of a left-handed propeller. 
     The bore 30 is arranged to discharge gas into the water flow around the shaft barrel 22 from a position and in a direction to enhance the gas/water mix and distribution and enable gas to be injected into the flow adjacent the most critical blade angle position, so as to enter and mix with any cavities, for reduction of erosion. 
     With reference to FIG. 5, this shows a graph illustrating the angle of attack α of a propeller blade against angular position, from the uppermost angular position of the propeller blade reference line (θ=0°) to the lowermost position (θ=180°), in the direction of rotation of the propeller. It can be seen that the angle of attack α peaks at the midway (90°) position, and it has been found that this position marks approximately the earliest point at which the onset of blade root erosion occurs (illustrated by area E). Root and hub erosion can occur throughout the 90°-180° quadrant but dies away after 180° due to subsequent reduction in angle of attack. Thus, injection of gas into the flow, to minimise the cavitation damage, must be such that gas is entrained into the flow in the 90°-1/2° region. A slight lead angle for entrainment can be advantageous and gas injection in the angle range of 60°&lt;θ&lt;180°, more preferably in the angle range of 80°&lt;θ&lt;150°, has been found to be effective, the most preferable position being 90° as shown in the drawings. 
     It has been found that air or gas bubbles can displace displaced by the vapour filled cavities (formed in the low pressure regions) on the propeller blades. In order to improve the mixing process, the gas is introduced into the localised flow at opening 40 in contact with the shaft bracket barrel side wall. This allows the gas to remain in contact with the surface of the bracket barrel and thus to follow the flow on to the propeller boss and to mix with or enter into the cavities on the blade root and hub surface more easily. 
     The gas is also directed by bore 30 into the flow in a direction substantially normal to the oncoming flow over the surface of barrel 22. This has been found to improve the gas flow distribution. 
     FIG. 4 illustrates a propeller air supply system for a two propeller vessel. The propellers are disposed about the longitudinal centre line of the vessel (the propeller supports being labelled port (P) and starboard (S)). The air supply system is connected via shut off valves 36, bleed valves 50 and control valves 52, to an air compressor, 54, via a throttle 56. 
     The actual air flow rate which is required for each propeller depends upon numerous factors, for example, shaft angle, ship and shaft speed, type or shape of blade section and the number of blades. The air flow rate may be determined, for example, for a given selection of the factors mentioned above, by calculation, estimation, scale model tests or in actual use, as would be apparent to those skilled in the art. 
     Although the discharging means has been described primarily as a passage formed in the propeller shaft support partly externally and partly internally thereof, this is not to be construed as limitative and the passage may be otherwise constituted. For example, as shown in FIG. 2A, the partly external/partly internal combined passage 32/34 could be replaced by a single passage 35 located entirely internally of the shaft bracket 20 and communicating at its upper and lower ends with the passages (not shown) in the boat leading to the gas supply and in the shaft bracket barrel 22 leading to the opening 40 or to the openings 40 and 40&#39;, or, as shown in FIG. 2B, the partly external/partly internal combined passage 32/34 could be replaced by a pipe 37 located entirely externally of the shaft bracket 20 and communicating at its upper and lower ends with the passages (not shown) in the boat leading to the gas supply and in the shaft bracket barrel 22 leading to the opening 40 or the openings 40 and 40&#39;. 
     The discharge may also be aft of the shaft barrel, in front of the propeller. 
     While only a single hole or discharge opening located in a position at 90° from the uppermost propeller blade position (see FIG. 1) has been shown, a plurality of such holes or discharge openings located at angle positions in the range of 60° to 180° from the uppermost propeller blade position may be used.