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Watorfot Propulsion For Fast Craft

There are many challenges associated with the correct selection of waterjet propulsors for a vessel designed to perform a given mission.

When the primary purpose of the proposed new vessel has been defined — in terms of payload, range, speed, endurance, sea state and other environmental factors — the ship design proceeds as an iterative process to define the optimum hull parameters, the optimum propulsor type and size, the internal arrangements, auxiliary equipment and many other features. The goal: arrive at the ship design which best meets the requirements of the mission with the minimum financial outlay and maximum economic return.

The application of waterjet propulsion to fast commercial craft is still relatively new. The last 15 years have seen a tremendous growth in the waterjet propulsion industry, both in terms of total numbers of units supplied and in the size and power of the units themselves.

Consequently, there has been a parallel growth in the number and displacement of waterjet- propelled vessels, and in the number of hull types to which waterjet propulsion may be applied. Due to the newness of the technology, the tendency has been to fit available waterjet propulsors into a ship design rather than to optimize the ship and propulsor combination. The consequence has been that the true overall economic potential of a waterjet application is often not fully realized.

As with propellers, the waterjet with the highest propulsive efficiency is not usually the best choice for a given ship design. The size and weight of the propulsors have a significant impact on the hull design and on the ultimate vessel displacement. While it may be satisfying to boast of a high propulsive efficiency, the boast is empty if the ship requires more fuel and costs more to build and operate than a slightly smaller ship that performs the same mission with propulsor that operate with a lower propulsive efficiency.

Identifying candidate-optimal designs can be done quickly by using a whole-ship design integration tool, often referred to as a ship design synthesis model. Obviously, the validity of the results will depend on the accuracy of the representation of the elements which comprise the computer model of the ship. Such elements include the hull design and structure, prime movers, propulsors, auxiliary systems, fuel, payload, weapons systems (if any) and many other items.

Growth In Popularity The reasons for the growth of waterjet propulsion in recent years are many. Initially, the reasons were the advantages of waterjets compared with propellers for certain applications. These advantages included shallow draft, absence of underwater appendages such as rudders, shafts and brackets, relative independence of thrust and torque from variations in ship speed, whereby propellers tend to overload engines, and reduced vibration and noise.

As the database for waterjet-propelled ships increased, it became evident that, for fast craft at least, waterjet propulsion was more efficient than conventional marine screw propulsion, largely because of the absence of appendage drag, and the development of large waterjet propulsors with high mass flow rates and jet velocities suited to ships having speeds in the 30 to 45-knot region, whereas earlier waterjets were more suited to speeds above 50 knots.

Examples of such applications include hydrofoil and surface effect ships (SES).

Waterjets enjoy considerable popularity for high-speed car and passenger ferries which operate at speeds up to 45 knots with various hull forms, including SES, catamarans, semiplaning, monohulls, and SWATH. In addition to car and passenger ferries, very large waterjets (30,000 kW and above) are being designed in conjunction with high-speed cargo vessels for trans-oceanic use with speeds up to 50 knots. At present, the largest ships would require four or more of the largest existing waterjets, but this will change as very much larger units, which are presently on the drawing board, become available.

Pump Types Pumps may be of radial flow, mixed flow or axial flow. Early radial flow pumps had centrifugal impellers. Recent radial flow pumps employ impellers similar to those of mixed flow pumps, but with a predominantly radial discharge. Mixed flow pumps which include KaMeWa and MJP products have the highest pump efficiency of currently available propulsors. There are many designs of axial pumps, such as Hamilton, and others in smaller sizes. Some axial designers have a small degree of mixed flow geometry. Inducer pumps are a special case of axial pump design, although inducers have been used followed by a mixed flow stage. Inducers use cavitating blades. At design point, a thin cavity covers the back of the impeller blades. This cavity collapses harmlessly before the flow enters the next stage, usually the stator. Inducers can operate at much higher suction specific speeds than other types of impellers. Because inducer pumps are of axial design, the outside diameter of the pump casing is smaller than that of mixed flow pumps, so the pump is more compact for a given power and thrust, and is easier to install, particularly in SES, catamarans, SWATH and high speed monohulls. The consequent structural weight advantage more than offsets the slightly lower pump efficiency, currently 88 percent versus 91 percent, for mixed flow pumps. Waterjet Selection During the design synthesis, a waterjet sizing routine is called. This routine will retain, for the known performance requirements of the ship at the stage where the design synthesis is, an optimum waterjet size. In order to do so, the routine will calculate the performance of a whole range of waterjet sizes, along with a range of nozzle sizes for each waterjet size, for the same required thrust. Jets too small will probably exceed their breakdown limit, or at least the limit for cavitation free operation. Also, the smaller jets will probably have poor performance efficiency as the jet velocity will be very high to provide the required thrust.

On the other hand, waterjets that are too large will offer better propulsive efficiency and will undoubtedly operate well within the cavitation free zone. However, the ship will be penalized by the heavy weight of the jet and attendant structural weight increase.

Also, for certain hullforms, a larger jet will force the hull to be wider and result in more drag. The larger waterjets rotate at a relatively low speed, and as a result, a higher reduction ratio may be required, adding the weight penalty of a heavier gearbox.

An optimum jet size lies in between the very small and very large waterjet. In order to determine such an optimum, a criterion that relates to the overall ship impact needs to be established. […]




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