Letters to the editor
Milling tests contradicted
Dear Sir,
ABB has delivered Azipod propulsion
systems for 16 vessels currently in
operation with ice class higher or equal to 1
A Super according to the Finnish Swedish
ice rules. The cumulative operation hours
since 1990 for the propulsion units for
these vessels exceeds 550,000 hours.
We were surprised to see the article
‘Optimising propulsor efficiency’ (July/
August 2007, The Naval Architect), on the
cavitation phenomena related to propeller
ice milling that was detected in the model
scale tests, as we have not been able to
verify this kind of cavitation erosion. We
were also surprised that the model scale
results were taken as a fact for the full scale
phenomena, even if there is no indication
of such cavitation.
One could say that propeller ice milling
is extremely rare, if the vessel and the
propulsion system are correctly designed.
The ship designer’s aim is to have the tip
clearance larger than the ice thickness, image from the performance ice test of Tempera.
where the vessel operates. Propeller-
ice interaction typically occurs when
operating in ridges and ice channels, but a fixed ice sheet coming continuously to 4) The stern form of the DA-vessel is made
these are not the type of ice milling that the propeller. During dedicated ice trials such that it will break the ice in a manner
was simulated in the model scale tests. of a double acting cargo ship, ice milling where there is no sheet ice coming to the
There might be several reasons why was not observed, even in over 1.5m propeller/Azipod, but there is a crack in that
reality does not imitate the results from the thick level ice in ship stern first motion. very position to reduce ice resistance and
model scale tests. I will mention only a few, Also, vessels operating already almost for loads on the propeller and the Azipod unit
as I am not too familiar with the actual test 10 years in the Caspian Sea, with small strut.
arrangements. draught and small tip clearance, have not As far as we have knowledge, there are
experienced such cavitation phenomena. no cavitation erosion marks in either the
1) Tip clearance is rather large in DA- The type of milling that is indicated in the blades nor the Azipod structure. Continuous
mode. In the case of Mastera/Tempera, tests occurs only when driven into larger ice milling is a very random case, and also
which were the tested vessels in the ice ridges, where most of the contact is therefore it could be that there is no such
cavitation tests, the tip clearance is roughly not continuous ice milling, but random cavitation erosion in real life as indicated in the
1.6m (eg in the Gulf of Finland the level contacts between the propeller blades model scale tests. See the attached photo from
ice thickness is typically max about 0.5m). and ice blocks (not level ice). Therefore double acting cargo ship Norilskiy Nickel’s
This means that level ice sheet does not the test arrangement seems unrealistic to propulsion unit after two winters of operation
actually produce propeller ice interaction actual ice conditions. in liner traffic in the Russian arctic. The
at all (see attached images of actual model, icebreaker’s propeller is intentionally designed
compared to the cavitation test image)! 3) It is very difficult to produce model quite pessimistically towards cavitation and it
Milling in the test conditions is very deep, scale ice that has the correct crushing is designed for the ballast condition, which is
which I have never experienced in a real strength. Also it is difficult to have correct worst from the cavitation point of view.
situation. inertia for the motor, propeller, and the With best regards,
propulsion system in model scale. We
2) Based on the full scale measurements assume that the tests were run with a Arto Uuskallio, sales manager, icebreaking
we have made with the existing vessels, fixed pod unit. This is not the case in real vessels
ice milling with the DA-vessels is not life, where the pod unit can turn away ABB Oy, Marine
continuous and actually it is never as the from the load, thus reducing the crushing PO Box 185, Merenkulkijankatu 1
arrangements in the test, where there is phenomena. FI-00981 Helsinki, Finland
92 The Naval Architect January 2008
NA Jan - p92+
93.indd 92 07/01/2008 12:51:50
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