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8.0 EVALUATION OF DESIGN

8.1 BOLLARD PULLS

To be able to quantify what affect the ducting of the DJ60 waterjet has had on the propulsive

performance of the Greenliner, a standard static bollard pull was undertaken. The bollard pull was

conducted in three different configurations; the newly constructed extension with no steering attached

(Figure 46), a setup with the new stator with the new nozzle (Figure 47) and lastly with the standard

DJ60 jet configuration without the vectoring nozzle attached (Figure 48). The batteries were fully

charged and the Greenliner was entered into the survival centre. The force gauge was connected to the

vessel by ropes shown in Figure 49. The purpose of the bollard pull was to obtain the values need to

plot a graph the efficiencies with respect to power being drawn into the electric motor. This allowed

the effectiveness of the optimisation of the ducting to be quantified with respect to these static

measurements.

Figure 46: Newly constructed extension

Figure 47: New stator and nozzle

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Figure 48: Original DJ60 waterjet without vectoring nozzle

Figure 49: Bollard pull configuration

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Procedure

The procedure that was followed for testing on the new Greenliner extension is as stated:

1. Once the vessel was in the water and operational a tow line was attached.

2. Using the two bollards on the aft end of the vessel a tow line leading back to the 100kg force

gauge was secured.

3. A shorter line from the force gauge to the solid structure of the building was attached.

4. The two lines were connected to the force gauge and position in an appropriate place to read

and the zero of the gauge was adjusted.

5. An operator was placed aboard the Greenliner and tension was applied to the force gauge and

lines.

6. The RPM was increased to the maximum value and the vessel let settle before recording

results.

7. The force was recorded on the force balance.

8. The RPM of the impeller shaft was taken at the same time using a hand held tachometer.

9. Steps 6-8 were repeated for a range of speeds throughout the rev range.

10. Entire testing was repeated for the new stator and nozzle setup followed by in the original

DJ60 jet configuration.

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Results

The efficiency of the waterjet with respect to the power being consumed by the electric motor was

plotted on Figure 50. This allowed a comparison between the efficiencies of the various

configurations to be undertaken.

Figure 50: Efficiency of the waterjet configurations with reference to input power

To allow the results of testing to be compared to the impeller curves supplied by DOEN, Figure 52,

the thrust force was plotted with reference to the impeller speed in Figure 51.

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

0 500 1000 1500 2000 2500 3000

Eff

icie

ncy

Power In (W)

Complete new extension New Stator and Nozzle Original DJ 60

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Figure 51: Thrust force with respect to impeller speed

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Th

rust

Fo

rce

(k

g)

RPM

Complete new extension New Stator and Nozzle Original DJ 60

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Discussion

The calibration equation y = 0.9853x + 0.7506, was applied to all force readings from this test with

the results shown in Appendix 3. The method in finding this calibration equation is found in section

5.2. For the remainder of this discussion the forces discussed will be the calibrated values.

Figure 51 shows the graph of thrust force for the three waterjet setups tested with respect to the

impeller speed. This plot follows the same form of the graphs of the various impellers for the DJ 60

jet as shown in Figure 52. Although these graphs are for RPM’s greater than 2500 they follow a

power relationship similar to Figure 52. The DJ 60 waterjet is currently using the 16A5 impeller

which is not plotted on Figure 52 but its performance curve lies between that of the 16B5 and the

curve 16A4.

Figure 52: Impeller performance curves - DJ60 Waterjet

Figure 50 shows the graph of the waterjet efficiency with respect to the input power of the electric

motor for the three waterjet setups tested. It shows that the original DJ 60 waterjet configuration has

an efficiency advantage over the newly constructed extension. At full power this efficiency advantage

is 3%. The obvious conclusion to draw from these results would be to say that the pipe has provided

extra friction causing the loss in efficiency. The extension pipe does not account for the whole 3%

efficiency loss. This is evident when the results for the testing of the new stator and nozzle without the

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extension pipe are compared to these results. The new stator and nozzle configuration without the

extension pipe has an efficiency that lies between that of the original DJ60 waterjet and the setup with

the extension in place. The efficiency of the new stator and nozzle without the extension piece in

place is only 1% higher than with the pipe. This means that the remaining 2% difference between the

original setup and the two additional arrangements must come from additional losses.

Several issues may have caused the 2% difference in performance from the original waterjet nozzle to

that of the new extension. It is a possibility that the water lube bearing for the impeller shaft that has

been inserted into the new stator causes more friction than the greased bearing of the original design.

This is unlikely the cause of the 2% difference but to eliminate it as the cause further investigation

would be needed.

An additional possibility is the fact that in the original design, the stator is incorporated into the

nozzle of the jet and the flow is passing the stator and leaving the nozzle within 30mm. In the

extension this is not the case as the stator was incorporated with the centre bearing for ease of

construction. This raises the possibility that it may be more efficient to incorporate the stator into the

nozzle. Another possibility may be the fact that it is simply more efficient to discharge the flow as

soon as physically possible after the impeller instead of keeping the nozzle angle at the optimum 60.

Another reason considered for the slight difference in performance was the flange that connects the

nozzle to the jet. The nozzle flange when built did not have the recess placed into the flange to assist

in sealing as designed. The flange was constructed as two flat plates sealing together. This may have

caused sealing issues where the nozzle connected to the jet causing the slight difference in

performance to that of the original DJ60 waterjet.

Conclusion

From this testing it is clear that there is a difference in performance from the original DJ60 waterjet to

that of the new extension which includes the new stator and nozzle. 1% of the 3% difference in the

value can be said to be caused by the additional friction loss cause by the extension pipe. The reason

for the additional two percent decrease in efficiency could be justified by one of the issues stated

above or a combination of them all.

This testing also served the additional purpose of validating the method developed to match nozzle

diameter to jet efficiency. This testing did not aim to achieve this goal but it did prove that the testing

procedure can pick up variances in thrust to approximately 0.5% which would be needed to match

nozzle diameter.