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Developments in counter-rotating impeller water-jets.

 

Linear flow: Counter rotation vs stators.

The first commercial single-shaft pressure water-jet was patented in the early 1950’s.  Twin-shaft, counter rotating impeller water-jet patents soon followed.  If set-up correctly, a dual impeller counter-rotating system will produce linear flow, thus eliminating the need for a stator or diffuser section.  In what appeared to be a logical design progression, each new patent focused on the water-jet principle of creating as much pressure as possible in a nozzle section.  It made perfect sense to believe that increasing the pressure-head was the key to more efficient propulsion and greater speed.  All the hopeful high-speed water-jet inventors concentrated on this aspect, supposing that high, higher and super pressure would be attainable through the twin-shaft counter-rotating bladed impeller concept.  The notion was well established in pump technology, where multiple stage impellers rotating in the same direction were often used to produce higher pressure–head than would otherwise be achievable from a single impeller.  The potential parallel between the functioning of a propeller and an above the water line propulsor remained unrecognised and none of the counter-rotating water-jet or pump inventions were ever commercialised.

 

For several decades it appeared that the concept of counter rotation as a way of generating more efficient thrust might be non-viable, that was until the 1990’s, when brothers Barry and Richard Davies were working on the counter-rotating impeller principle in their garage.  Several prototypes were made, however they were disappointed to discover that the expected efficiency and performance gains were not forthcoming.  The project stalled until the eureka moment when it was postulated that perhaps the nozzle and associated pressure-head was the source of the loss and that reversing the thrust formulae to replicate propellers may be a better way to approach the problem.  This possible solution was not obvious, as it is counter-intuitive that slippage within an enclosed system would generate higher propulsive efficiencies, particularly at low boat speeds.  To test this theory, a completely new way of looking at the device was required.  They proposed that instead of accelerating the water in a nozzle section, it would be possible to accelerate the mass earlier from out of the intake pick-up duct and then simply “dump” the water through the outlet.  A water-jet intake pick-up duct is located upstream from the impeller and delivers water above the water-line when the craft is planing.  This was a complete departure from existing thinking as counter-rotating blades could now essentially be configured as water accelerating propellers and not pressure inducing impellers.  Aligning the theory more closely with the high mass flow, low plume velocity parameters of efficiency, as defined by the scientist Froude, looked like a good option.  The Davies’ brothers initial test results were exceptional, so they proceeded to form a company called ContraJet Ltd and patent the concept.

 

 

                                                                                  “Fly in the ointment”

 

 

A mysterious manufacturing fault was causing one of the inventors great concern.  He felt that the basic understanding of the hydrodynamics may have been flawed, leading to doubt over the efficacy of the ContraJet patents.  Unable to solve the problem and produce viable product, ContraJet Ltd was forced to shut its plant in 2000.

 

Barry Davies and brother-in-law Paul Paterson, believed that new research may provide the elusive answer.  Following the formation of a company in 2002 called PropellerJet Ltd, a series of amazing discoveries were made, which led to a completely novel understanding of the hydrodynamics and drafting of new patents.  For a conventional water-jet, the outlet size relative to the impeller diameter is all important.  The outlet is where the thrust is developed and it’s the starting point for discussions about water-jet hydrodynamics.  The cross-sectional area (assuming the optimal shaped nozzle is used) and the pressure-head, dictates how much change in velocity is produced.  Shape and reducing ratios are crucial and very small changes can make a large difference to performance.  In sharp contrast, the laws governing the Paterson-Davies invention mean the outlet cross-sectional area is treated as the end of the process.  Having been accelerated across the hybrid propellers, the water has already assumed its desired velocity and shape prior to the outlet. This phenomenon is called “continuity of mass” and the effect can be observed by running the water faucet over the kitchen sink.  The water diameter is largest as the water leaves the outlet and significantly reduces in diameter as gravity speeds up the falling flow.  Containment of water through a reducing nozzle in conjunction with pressure, is therefore not the only method for accelerating water and producing a consequential change in water shape.  If the water doubles in speed within the unit as a consequence of acceleration over the blades, the water at its highest velocity will then take up half the cross-sectional area that it did initially, without the need for constraint.  The water’s occupied area being “inversely proportional to the change in velocity”.  

 

            A change from one km per hour to two km per hour causes the cross sectional area that the water occupies to reduce by half.

 

Unlike in a water-jet, the water has accelerated across the hybrid’s propeller blades and the work has already been done as it enters the rear of the unit, consequently it is very important not to slow the water back down, by restricting the flow.  Additionally, to maintain the column of water in the intake duct, air must be prevented from entering the outlet.  This phenomenon is called ventilation.  For surface operation, a plug proportioned to fill in the space created by the acceleration, is inserted behind the downstream hybrid propeller.  The plug ensures that the hybrid propellers remain primed and that ventilation is minimised, however submerged propulsors will not require this feature.  The hydrodynamics of the system were more complex than this however and it wouldn’t be until the system was fully understood, that construction of several extremely efficient and versatile prototype water-propulsors could begin.  These advanced devices would be capable of carrying out the functions of both propellers and water-jets.  Furthermore Paterson and Davies also discovered a radical new field of hydrodynamics and named it “Super-Oscillation”.  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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