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.
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
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”.