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Fig 2: Pressure and velocity distributions at different cavitation numbers, CTH


=70.


most powerful German ocean-going tugs ever built, Janus and Ursus, with a bollard pull of 220tonnes (Fig 4). The vessels were built by Mützelfeldtwerft, in Cuxhaven, and are owned by Harms Bergung Transport & Heavylift GmbH & Co KG, in Hamburg. The tugs have a length of 59.16m, a breadth of 18.50m, and a draught of 6.80m. The propulsion units are powered by


four MaK engines, rated at 3000kW and 4000kW per shaft. Schottel supplied SCP 119/4XG CP propellers. The vessels are additionally to be fitted with three Schottel STT 330 T-LK transverse thrusters, 400kW each. The propeller design parameters in the early design stage had a diameter of 4.40m, a delivered power of two 6790kW, and had 140rev/min.


Ducted propeller design The CP propeller was designed by Schottel


in cooperation with SVA Potsdam. With the propeller diameter of DP = 4.40m the following design (*) parameters were given. Advance coefficient J* [ - ] : 0.000 Torque coefficient KQ


* [ - ] : 0.0503


Total thrust coefficient


KTT Cavitation number σn


* [ - ] : 0.515 * [ - ] : 2.616


The comparison of these design


parameters with the database regarding the risk of thrust breakdown (Fig 3) showed a danger of thrust breakdown due to cavitation. Thus, the main aim of the propeller design was optimum cavitation behaviour at bollard pull condition. The propeller was designed like a free


propeller subject to inflow influenced by the duct for the available power and actual speed. The hydrodynamic model for the calculation of the inflow in the propeller plane of the ducted propeller is


Ship & Boat International September/October 2008


a lifting line method with corrections for the effect of the lifting surface. A lifting surface method was used for propeller optimisation. The design process was carried out iteratively in several loops. The blade outline and the pitch and chamber distribution were varied. The first model tests were carried out with


the propellers VP 1466 and VP 1467. After the tests, the nozzle profile was changed to a small degree and the propeller diameter was increased from 4.4m to 4.5m. The calculations and tests showed


excessively high torque (power) at the zero thrust condition of the tug. The ducted propeller was optimised again to reduce the power at this condition. The area ratio and the chamber of the propeller blades had to be reduced in this design step. The model propellers VP1499 and VP1500 are similar to the final propellers on Janus and Ursus.


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