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2.0 SCOPE OF WORK

This project was undertaken to overcome the issue that the jet outlet would be blocked by the

proposed stern extension. The aim of the project was to maintain the current waterjet efficiency while

extending the waterjet to clear the stern extension. As part of this investigation it was decided that

several areas would be examined. The first main area was looking at having the waterjet discharge

whilst submerged in water which is the opposite of current industry best practice. The second main

area was looking into the possibility that diameter matching the nozzle outlet for a certain speed

would help improve the efficiency. The final area of interest was the location of the discharge relative

to the new stern section and whether by strategically locating this discharge location a high pressure

region could be created pushing the hull forward. These areas of interest lead to the project title:

“Ducting optimisation for slow speed waterjets.”

As this project was being undertaken on the jet section, Oliver Thornalley (2009) was examining the

issue of the control relocation and maintaining the current efficiency using vectoring vanes. It was

necessary to define interfaces between each project to clarify objectives and guidelines. The interfaces

that were defined were compiled into the following statement:

“The issue of improved control and reversing at equivalent efficiency was to be explored by Oliver

Thornalley and the waterjet ducting was to be included into Jacob Gerke’s Research Project.”

To assist in defining this project, the thesis was broken down into hypotheses that incorporate the

main aims of the project. The hypotheses are defined below with explanation of what has been

explored and how it was achieved. For the overall aim of the research project to be deemed a success

one or all of the hypotheses need to be proven.

2.1 DIAMETER MATCHING NOZZLE TO JET EFFICIENCY

Tapered discharge jet pipe can improve propulsive efficiency by optimising jet to vessel speed

ratio, for a given speed.

The DJ60 waterjet was initially designed for high speed, not for consistent low velocity. It was

thought that by investigating the actual current jet velocity at a certain speed and relating this to the

speed of the vessel, the flow rate of the waterjet outlet could be optimised to provide the greatest

thrust from the power delivered. Current theory on how best to match the jet water speed to the

vessel’s speed was reviewed. Steps to undertake optimisation of an existing waterjet installation were

not clear as all literature found was for a new installation. An attempt was then made to develop a

method to calculate the optimum jet nozzle. During this development it became apparent that the mass

flow rate of the jet needed to be obtained. It was decided that although the current waterjet mass flow

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rate was estimated by DOEN, it was necessary to experimentally measure the flow rate in the initial

configuration accurately. The mass flow rate was obtained by using a pitot tube to measure the

velocity of the water across the jet. This flow rate was then used for a calculation of the optimum jet

velocity ratio (JVR).

2.2 ABOVE SURFACE VERSUS SUBMERGED JET DISCHARGE

Submerged waterjet can match the efficiency of surface discharge

This hypothesis was created because the Greenliner jet in its current location discharges into the water

whilst submerged. Current industry practice is to discharge the waterjet into the air pocket created by

the vessel once it is moving. As the Greenliner, in its current configuration, cannot achieve the speed

required to create this air pocket, it discharges whilst submerged in water.

Initially all relevant theory was reviewed, the review found no commonly know reasons for the

surface discharge of the waterjet into air other than the extra drag caused by its submergence. Leading

from this review a test was developed to attempt to evaluate the differences in performance of the two

discharge locations. The testing was undertaken using the Greenliner with the installed DJ60 waterjet

and an extension that allowed for the discharge locations to be easily achieved. These results were

then analysed and a conclusion was drawn regarding the best vertical location for the jet discharge.

2.3 OPTIMUM JET DISCHARGE LOCATION TO REDUCE VESSEL DRAG

It is more efficient to extend the waterjet past the exit of the stern than to discharge at the stern

exit.

In the preliminary discussion on which parameters affect the performance of the waterjet, the point

was raised that the waterjet creates a low pressure regions around the nozzle when operating. The low

pressures can act upon the hull of the vessel and cause excess drag upon the hull. It was decided to

investigate whether the effects of these low pressure regions could be eliminated. Also it was

investigated whether a high pressure region could be created to produce a stern wave. It was proposed

that this stern wave could provide thrust in the forward direction much like a wave in a following sea

does. After initially considering where pressure regions should form, an investigation was undertaken

into the drag caused by discharging the waterjet at various locations with reference to the new stern

extension. This was undertaken using Computation Fluid Dynamics (CFD) with the help of Nicolas

Heysen from Ensieta University from France. Jet lengths ranging from 800mm to 1400mm from the

existing transom were modelled and the drag estimated. The results from this were analysed and a

theoretical optimum jet discharge location with reference to the stern extension was found.

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2.4 DESIGN DEVELOPMENT

The outcomes of these hypotheses were used in the design and development of the final DJ 60

waterjet extension. As well as the above inputs various operational and practicality confinements that

exist with the Greenliner being a trailable reconfigurable hull form were considered during the design.

The design development section of this project justifies and states why the design has evolved to the

final built version.

2.5 EVALUATION OF DESIGN

To evaluate the new stern extension a test was developed. The test involved performing a static

bollard pull of the original DJ60 waterjet and the newly constructed jet extension. From this test an

efficiency curve was developed with respect to input power into the electric motor. From these curves

an evaluation was performed on the effectiveness of the extension.