Feature 1 | GEARS AND PROPELLERS
The designed propellers are
characterised by a blade outline with a long chord length at the blade tip (Fig 5).
Bollard pull measurements SVA Potsdam was commissioned by MAN Ferrostaal to carry out resistance and propulsion tests, and static bollard pull measurements with the model of the AHT (M1236Z000). For these tests, the model was equipped with the designed ducted propellers from Schottel. The bollard pull measurement
was carried out for nine numbers of revolutions in the range from 70rev/ min to
160rev/min. Test results
have been extrapolated to full scale by Rn-number revaluation of
Fig 3: Estimation of the risk of bollard pull reduction due to cavitation. the
open water characteristic and a pitch correction. The bollard pull measurements showed =0.113.
a thrust deduction factor 1-FD /TT
The bollard pull was calculated as FD=221.2tonnes for PD
=2 x 6790kW and
n=140rev/min. The analysis of the test results showed
that the thrust deduction factor had to be reduced. That is why the aft ship lines were optimised. The buttock slope angle was reduced (Δϕ=-3deg) and the vertical hull-nozzle clearance was increased. The bollard pull measurements with the modified model of the AHT (M1236Z010) delivered a thrust deduction factor 1-FD
/TT =0.080. Table 1 shows a comparison of the
predicted pitch ratios, bollard pulls and thrust deduction factors for both tug variants. Minimising the thrust deduction factor results in a better inflow to the
Table 1: Bollard pull prognosis and thrust deduction factors, models M1236Z000 and M1236Z010.
Model P/D FD [-]
1-FD [t] /TT [-]
M1236Z000 1.0043 221.2 0.113 M1236Z010 1.0101 225.6 0.080
Cavitation tests The database regarding the thrust breakdown due to cavitation showed a risk for the ducted propeller system on the 220tonne tug, thus cavitation tests with the free-running ducted propeller
ducted propeller. The result is a higher pitch ratio of the propeller and a smaller interaction between the ducted propeller and the aft ship.
were carried out in SVA Potsdam’s cavitation tunnel. The cavitation number was varied at high thrust load coefficients to observe the cavitation inception, to study the cavitation behaviour and to obtain information about the forces and moments. With homogeneous inflow, tip vortex
cavitation occurs at the propeller only. Nevertheless, inception of nozzle thrust reduction could be seen at cavitation numbers smaller than 2.9. That means a risk of thrust breakdown for the full-scale propeller. The influence of cavitation on the
forces and moments of the ducted propellers and the thrust deduction factor is not included in the bollard pull prognosis on the basis of tests in the towing tank. Schottel and SVA decided to carry out additional cavitation tests with the model of the AHT (M1236Z010) in the large circulating and cavitation tunnel UT2 at Berlin Technical University. The test section of this tunnel with a
length of 11.0m, a width of 5.0m, and a depth of 3.0m allowed tests at bollard pull conditions. Measurements in the UT2 were carried out at a water velocity VS
=0,
and different numbers of revolutions at atmospheric pressure and at pressures corresponding to the model and full-size cavitation numbers. In the range of the design power there is
Fig 4: A side view of Janus. 22
an inception point for bollard pull loss due to cavitation. Analysis of the propeller and
Ship & Boat International September/October 2008
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