Pg 3 Basic Water-jet Theory. (Fluid
expelled above the water-line)
Axial-flow impeller water-jets use an Archimedes screw principle to force water into
a reducing nozzle. The water is “squirted” out of this nozzle above the water-line
enabling high speed propulsion. The process produces a “change in velocity” at the
nozzle resulting in an equal and opposite reaction, sufficient to drive the boat
forward. There are several different impeller and pump designs, but they all require
the pressure downstream of the impeller to be raised as high as possible. As a consequence,
the water velocity ‘directly downstream’ of the impeller is low. It is the reducing
area at the outlet-nozzle still farther downstream that provides the “change in velocity”.
It is not uncommon for the cross-section of a jet outlet-nozzle to reduce in area
by about four or five times relative to the impeller cross-sectional area. The principle
is similar to a doctor’s syringe, the plunger serving the same purpose as the water-jet
impeller. Both require that the fluid is pressurized and “forced” through a small
opening, causing the liquid to accelerate. It is the change in velocity at the outlet
(Newton’s equal and opposite forces law) that drives the boat along. The great advantage
of the water-jet compared to propeller systems, is that the impellers are fully enclosed
and protected, thus allowing the craft to enter extremely shallow waterways with
no fear of impeller damage.
Essentially the water-jet impeller is operating as an Archimedes screw, entraining
or partitioning water between its blades in order to generate high pressure (Pressure-head)
downstream from itself. This is the opposite principle to a propeller which requires
slippage and a drop in pressure immediately downstream from the blades. Slippage
in a water-jet impeller is undesirable as essentially it reduces an impellers ability
to develop high pressure-head. Unfortunately for water-jets, the underpinning principles
are their basic weakness. To have an effective rotating impeller, sufficiently sealed
to produce pressure in the nozzle, they require tight tolerances, fine-pitched, overlapped
impeller blades, which work on a relatively small mass of water per revolution. It’s
the relatively small mass of water being processed during each revolution of the
impeller that precludes them from being true competitors with the propeller. In
order that a high-speed propulsor produce efficient thrust at low and mid speeds,
the mass component passing through the device during each impeller revolution must
be as high as possible, while the wash velocity should be minimised. The work to
establish these tenets was carried out by Rankin and Froude who developed the foundation
principles of resistance and efficient propulsion. Because thrust is derived by
multiplying mass flow rate by its change in velocity, equivalent thrust can be achieved
by accelerating either high mass to relatively low velocity (propeller) or conversely,
accelerate low mass to a high change in velocity (water-jet). The two systems, propellers
and water-jets, are in many ways polar-opposites.
Fundamentally, the pressure requirement for all high speed water jets dictates a
need for fine pitched impellers, leading to a lower water mass component being processed
per impeller revolution. This makes them unsuitable as efficient low and mid speed
propulsors, where high speed is also a requirement. Propellers on the other hand,
can process a greater mass flow per revolution and can therefore effectively produce
efficient thrust at low, medium and high speeds from the same device.
In addition, there is a further compounding factor present in all high-speed water-jets
and that’s a structure called a diffuser or stator set. These are essentially water
straightening vanes and are necessary for removing the radial energy imparted to
the water by the rotating impeller. They are placed downstream from the impeller,
in the area of the pump prior to the water exiting the nozzle. Without them, the
water entering the atmosphere would dissipate perpendicular to the desired direction
of flow and little thrust would result. These structures are characterised by a
very large wetted surface area and are the source of a great deal of energy loss
when the water-speed through them is raised. Efforts by some water-jet manufacturers
to increase the mass component simply resulted in slow jets. Increasing the nozzle
cross-sectional area in relation to the cross-section of the impeller, causes higher
water velocity in the part of the jet pump which contains the wetted stator, which
in turn induces high frictional losses. The rate of energy absorption is exponential.
Along with lower top speed, there is also a greater susceptibility to impeller skid
which is sometimes called cavitation.
Effective high speed water propulsion is basically a choice between one of two systems,
a water-jet or a propeller. Within these, there are several different sub-categories,
each designed to optimise performance for a given task. Basically however, at slow
speed the propeller will always be more efficient than a high-speed water-jet and
at high speed (30 knots plus) the differences are very little. The decision as to
which system to use, becomes an operational one. Most boat applications require
efficient thrust at all speeds, so the propeller is by far the desired choice. Some
operators must enter shallow or turbid waters where the depth is marginal or unknown,
so the only viable option is the water-jet. If high speed and thrills are all that
is required, then water-jets are a good choice. This explains why water-jet usage
has remained marginal, primarily being used for military, high speed ferries, personal
water craft and tourist operations based on shallow waterways.
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