Feature 1 | GEARS AND PROPELLERS
nozzle thrust measurements showed that the cavitation only had a small influence on the propeller thrust. The reason for bollard pull reduction is mainly the thrust breakdown of the nozzle. In addition, there was an influence of
cavitation on the thrust deduction factor. Tip vortex and suction side cavitation occurred on the propeller blade. The maximum cavitation extent occurred in the angle range where the nozzle was partly integrated in the hull. The thrust loading of the propeller was especially high in this region due to the bad inflow condition (risk of propeller hull vortex cavitation).
Bollard pull prognosis and sea trials An accurate estimation of the Reynolds number effect on the performance of ducted propellers is very important for extrapolation of the model results to full-scale. Schottel and SVA used a Reynolds number correction for ducted propellers. The Reynolds number correction
for the model of the tug with the final propellers delivered an increase of the bollard pull by 4.8% for the tests without cavitation, and 4.2% for the tests at cavitation identity. The prognosis of the available bollard pull for the AHT is influenced by the cavitation. Bollard pull tests with Janus were
carried out in Stavanger, Norway, in October 2007. The measurements by the engineering office Nolte & Szczesnowski showed a bollard pull of 219tonnes.
Fig 6 presents the bollard pull prognosis
and the model test results without and with cavitation compared with the full-scale measurements with Janus. The pitch ratio of the propeller was predicted for the MCR point. It can be seen that there is a good
agreement between the prognosis based on tests with cavitation identity and the full-scale measurement. The prognosis based on tests without cavitation identity delivers excessively high bollard pull values.
Conclusion The design of ducted propellers for tugs delivering maximum bollard pull requires much more than determining the propeller pitch for specified engine power and propeller revolutions per minute. Schottel’s experience and the study on the propeller design for the 220tonne AHT tugs has enabled the company to gain more insight into the physics of the viscous flow inside a duct and the interaction between propeller and nozzle. For Kaplan propellers and propellers
with diminishing chord length at the blade tip, a design engineer is now able to estimate the risk of thrust breakdown at a very early state of a project by means of
the cavitation
number σn 10KQ
, and the torque coefficient . In general, the results show that
propellers with Kaplan blade shape and outline are more advantageous for high blade loads, which are increasingly necessary because of draught of tugs.
Fig 5: Propeller VP 1466 in nozzle and 3D model of full-scale propeller (Dp=4.40m).
The final bollard pull result for the
220tonne Janus confirmed the preliminary estimation of the thrust breakdown by means of the correlation σn versus 10KQ
. It the restricted
also confirmed the implemented procedure with calculations and measurements at cavitation identity, in order to be able to prepare a bollard pull prognosis with an accuracy in the region of 1%. SBI
The 220tonne bp Janus during sea trials. 24
Fig 6: Janus comparison of bollard pull prognosis and measurement. Ship & Boat International September/October 2008
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68