The high-speed propeller was invented over 170 years ago and has dominated the propulsion market ever since.  Despite 60 years of development, water-jets have struggled to make significant inroads.  The Contrapel Hybrid drive is about to change that.  The system is so advanced that it carries out all the functions of both propellers and water-jets with none of the drawbacks of either.  Externally it resembles a traditional water-jet, however internally the method for developing thrust has much more in common with conventional propeller systems.  The Contrapel drive uses a pair of contra-rotating, fully enclosed hybrid propellers, capable of operating above the water-line.  The mode of operation of the hybrid propellers is to accelerate the water from out of the intake (pick-up) duct and then discharge it through the outlet, requiring only enough back-pressure downstream from the propellers to keep the system primed.  This contrasts with the traditional jet which does not accelerate the water until further downstream in the nozzle section, which requires the generation of high pressure.  The Contrapel Hybrid propellers produce lift in a similar way to open water propellers, by using slippage.  The use of hybrid propellers enables the mass component for each revolution be maximized and the plume velocity to be minimized, thus meeting the fundamental laws of Froude efficiency.  This contrasts with the low-mass per impeller revolution and high-plume velocities present in conventional high-speed water-jet designs.

 

In order to get to this point however, major design considerations still needed to be addressed.  Producing linear flow with no stator section, requires a pair of counter-rotating hybrid propellers one positioned directly in front of the other.  Both operate in tandem to produce the desired axial flow, but they also must accelerate the water across their blades and generate significant lift.  When the first prototypes were attempted, a perplexing conundrum became apparent.  In order for the propellers to generate this lift, a drop in the pressure forward of the leading blades was required.  As one propeller is located directly in front of the other, this then caused the downstream propeller to lower the pressure on the wrong side of the upstream propeller.  Counter-rotating propellers in open water do not have this problem, since water is replenished from the surrounds, as the acceleration occurs.  In an enclosed tube however, the only source of usable water is directly upstream.  Consequently the downstream propeller was scavenging against the upstream propeller causing it to stall.  Reversal of the pressure gradient on the blade was disastrous, as a huge amount of energy was being absorbed.  Once again the solution was counter-intuitive, being resolved by increasing the speed of the upstream propeller, so that the pressure correction could be made.

 

 

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CLARIFICATION OF TERMS:

 

You will have noticed that when discussing water-jet propulsion, there is a lot of interchange in usage between the words “propeller” and “impeller”. Generally, propellers are defined as devices with a revolving shaft with blades and are used for “propelling”.  The word impeller is used to describe rotating blades that are enclosed, as in a water-jet (which is also a pump).  Impellers like propellers, also have blades that revolve on a shaft.  Unfortunately there is not a generic word available that satisfactorily describes each device’s specific method for developing propulsion.  Rather the definition of the propeller is very general and that of the impeller appears to be specific to use with pumps and water jets.

 

The lack of definition may help to explain some of the continuing confusion in the industry.  They both may revolve on a shaft and have blades that ultimately induce propulsion, however the way each one works is quite different in both function and purpose.  Each one has its own specific design requirement and neither is interchangeable.  Furthermore, neither definition adequately describes the rotating blades of the Contrapel system.

 

 

Therefore some form of definition is required.  For this discussion the following definitions will be used:

 

 

• Propeller                                      Any set of blades on a rotating shaft that utilize slippage to create lift resulting in a change in velocity

                                                    of liquid as it crosses the blades, sufficient to produce usable thrust.  The liquid’s speed directly

                                                    downstream from the rotating blades must be higher than the initial velocity upstream from the device.

 

 

• Above the water-line                      Any set of enclosed blades that rotate on a shaft, resulting in entrained fluid across the impeller,

  jet impeller                                   primarily designed to minimise slippage and induce high pressure-head downstream from itself.  The

                                                    liquid’s speed directly downstream from the rotating blades need not be higher than the initial

                                                    velocity upstream from the device.  The pressure must be sufficient to create a high change in liquid

                                                    velocity through a nozzle, thereby generating thrust.

 

 

• Screw                                           Rotating blade/s on a shaft in the form of a helix, designed to move liquid axially at a constant rate.

 

 

• Contrapel                                      Any enclosed pair of counter-rotating blades with opposite pitch, that use slippage to create lift and

Hybrid Propellers                               a change in velocity of the liquid across the blades sufficient to produce usable thrust.  The water

                                                     speed directly downstream from the rotating blades must be higher than the initial velocity.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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