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Waterjet

The waterjet is a different type of propulsor. As an alternative for countering propeller cavitation problems for high-speed craft and special-purpose craft, the waterjet provides a jet-reactive thrust of high-velocity water expelled through a nozzle. With a speed range above 45 knots, waterjets, whose principal advantage is improvement of vehicle maneuverability over the whole speed range, are typically applied to patrol boats, surface effect ships, hydrofoils, and motor yachts.

Waterjets propel a boat or other watercraft using a water jet apparatus mounted to the hull, with the powerhead being placed inside (inboard) or outside (outboard) the hull. The drive shaft of the water jet apparatus is coupled to the output shaft of the motor. The impeller is mounted on the drive shaft and installed in a housing, the interior surface of which defines a water tunnel having a convergent nozzle. The impeller is designed such that during motor operation, the rotating impeller impels water rearward through the water tunnel and out the convergent nozzle. The reaction force propels the boat forward.

Conventional pumps include radial, axial, and mixed flow pumps. In a typical axial flow pump, the radial distance of a fluid particle from the pump centerline is constant from the pump inlet to the pump outlet. In radial and mixed flow pumps, the radial distance of a fluid particle from the pump centerline increases along the length of the pump because these types of pumps typically include a scroll or spiral type casing. Mixed flow pumps typically have a discharge that is perpendicular to the axis of impeller rotation.

A problem with conventional propulsors is that they typically do not include any flow conditioning of the fluid flow entering the pump impeller. For example, it may be desirable to condition the inlet flow to affect pump performance in some way, such as to reduce cavitation and improve acoustic performance of the propulsor, increase the head rise potential of the pump, and the like. Cavitation is generally undesired in conventional pumping systems because cavitation results in lost thrust and acoustic noise.

Waterjets have been available in small craft for both commercial and military applications for several decades. At high speeds, they offer higher efficiencies than traditional propeller and shaft arrangements. While waterjets are typically not as efficient as propellers at lower speeds, there are commercial efforts underway to remedy this. Waterjets lend themselves well to shallow water operations because they do not require projections below the bottom of the hull which are vulnerable to fouling or damage from bottom impacts. Though they have been used on small vessels, the application of waterjets to warships of frigate size and larger may create new engineering issues.

Universally, waterjet propulsion systems are mounted on the main stern transoms of surface vessels with at least a portion of the pump and the pump exit nozzle above the surface of the water. That location permits the actuators for the steering nozzle and reversing deflectors of the propulsion system to be above the water, thus simplifying the installation and maintenance of the actuators and the hydraulic lines leading to the actuators. Also, it is common to provide access ports in the pump above the waterline to permit the pump to be serviced without drydocking the vessel.

Generally, the intake opening to the water supply conduit for the waterjet pump is located on the bottom of the hull a short distance forward of the transom and just far enough below the waterline to ensure that water is taken in under most operating conditions of the vessel--i.e., absent very severe pitching of the vessel due to heavy waves when intake may be briefly interrupted by surfacing of the opening. The location of the intake opening at a minimum height below the pump improves efficiency, as compared to a deeper location, by minimizing the vertical distance that the pump has to pump the water from the intake opening to the pump rotor.

Steering is provided by a steering nozzle that is mounted for pivotal movement about a substantially vertical axis in a position immediately aft of the discharge nozzle of the waterjet pump and is pivoted, usually by one or more hydraulic piston/cylinders that are coupled between the pump assembly or the transom and the steering nozzle. Upon actuation, the cylinder(s) pivots the steering nozzle to a selected position oblique to the axial such that the nozzle deflects the water jet to a direction having a lateral vector. The water jet is reversed by a reversing deflector that is normally positioned out of the path of the water jet for ahead propulsion but is moved, usually by a hydraulic piston/cylinder actuator, into the path of the water jet. Generally, the reversing deflector pivots about a horizontal transverse axis from an inactive position above or below the steering nozzle. The reversing deflector is shaped to turn the water jet to a forward direction.

The waterjet's efficiency falls far short of the efficiency of an underwater propeller at low boat speeds. The propulsive coefficient of a typical underwater propeller at 16 knots is about 65 percent while that of a waterjet at the same 16 knots would be only about 40 percent. Those numbers given an advantage to the underwater propeller of 38 percent at that 16 knot speed. The waterjet becomes more competitive at higher speeds where the drag of the underwater propeller's appendages including shaft, strut, rudder, etc. causes it to have a severe disadvantage. The competition to the waterjet then becomes the surface propeller that, in its normal design, operates aft of the transom or a step in the boat's bottom. Only the lower half of the surface propeller is in the water. As such the surface propeller avoids shaft, thrust, and, in some designs, rudder drag. While generally considered to be rather inefficient at low boat speeds, the surface propeller is considered the favored propulsor at very high boat speeds.

In summary, the waterjet propulsor is severely outclassed from efficiency standpoints at low to moderate, up to about 25 knot, and very high, over 60 knot, speeds. The reason for much of its efficiency shortcomings has to do with its inlet performance. A well-designed waterjet pump can have a rotor efficiency of 93 percent, flow straightening stator vane efficiency of 92 percent, and discharge nozzle efficiency of 98 percent. That comes to an overall pump efficiency of 84 percent. However, its averaged inlet pressure recovery efficiency will probably only be in the 70 percent area. Consequently, the best overall efficiency that can be expected from such a waterjet propulsor while running at its best performance at hihg boat speeds is about 59 percent. The major reason that waterjet inlet efficiency or inlet pressure recovery is so poor is because of distortion in the inlet flow. The high velocity incoming water in a typical flush with the hull waterjet inlet piles up over the lower half of the inlet duct. Due to this distorted flow, the rotor generally sees recoveries of 90 percent or more of boat freestream dynamic head over its lower half and as low as 50-60 percent over its upper half.

A disadvantage of having the waterjet pump relatively close to the water surface is the reduced hydraulic head of water at the pump inlet. The reduced suction head reduces the capability of the pump to absorb high power at slow speeds due to the onset of cavitation. The pump has to be larger than it would have to be if the suction head were greater in order to provide high power output at slow speeds without cavitation.

Another disadvantage of previously known waterjet propulsion systems is the relative complexity of the actuators for the steering nozzle and the reversing deflectors and the outboard location of the actuators. The actuators are usually hydraulic piston/cylinders and require hydraulic lines that penetrate the hull. In the event of leakage, the hydraulic fluid contaminates the environment. The outboard actuators and the lines that serve them are vulnerable to damage from impacts.

A well-known problem in ship propulsion and particularly waterjet propulsion systems, is maintaining efficiencies of a water vehicle and the propulsion system used under all speed and power requirements. One scheme used to maintain efficiency is a plurality of waterjet propulsion units in parallel. Such a parallel arrangement is desirable where a vehicle's power requirements exceed the power available from one unit. In parallel arrangements, each unit normally has its own separate intake, pump, engine, and waterjet. Another variation is to use only one inlet with downstream ducting connected to provide fluid to parallel and separately powered pumps with their separate nozzles. Partial power operation is usually accomplished by changing intake size or by reducing rotational speed and power of the pumps below full-load, design valves, obviously degrading pump and flow efficiencies. Also, another scheme used to reduce power is to shut down one or more propulsion units, and operate the remaining one or more units at full design load, values with efficiency, provided asymetric thrust does not result. But again, efficiency of the vehicle is degraded, because of the drag of the unused intake area remaining open, and the mismatch of the total in-use nozzle area to vehicle hull requirements. When operating all the propulsion units at reduced power, as before, unit-power efficiency is usually drastically reduced, particularly in gas turbine-powered units such as those often used to power many high performance marine vehicles.

An alternative method of providing variable power is to couple two or more power units to each pump-nozzle-intake system, by power-combining transmission systems. Reduced power operation is obtained by power unit shaft speed control, or by decoupling one or more power units. The latter method requires shaft clutches and controls. And, as with individual propulsion units discussed above, power unit efficiency is degraded at reduced power operation, due to shaft speed-to-power mismatch of the pump and power-unit and the losses due to the unused thrust nozzle area. The major disadvantages of the system are the cost, the complexity of the power-combining transmissions, and the restraints placed on their location and arrangement in the vehicle.




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Page last modified: 07-07-2011 12:58:07 ZULU