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The Problem of Propeller Design for High Ice Class Transportation Ships А.V

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The Problem of Propeller Design for High Ice Class Transportation Ships А.V Fifth International Symposium on Marine Propulsors smp’17, Espoo, Finland, June 2017 The problem of propeller design for high ice class transportation ships А.V. Pustoshny, G.K. Darchiev, I.G. Frolova. Krylov State Research Centre, St. Petersburg, Russia ABSTRACT Some of these problems were expected to be solved by Development of large-capacity merchant vessels of so-called DAS (double acting ship) concept invented in high ice classes for Arctic has raised a range of Finland. According to this concept the vessels challenges for propeller designers. Usual propeller equipped with electric podded propulsors should sail design approaches are often not applicable for high ice- bow-first in open water and stern-first in ice. However, class merchant vessels. from the Russian experience with 3 series of heavy- tonnage merchant DAS of different types it is seen that In particular, the requirements regarding vibration the concept evolves not exactly as it was initially levels related to propeller cavitation, which are envisaged by its inventors. In deviation from the traditionally specified for merchant ships, cannot be original declarations of inventors high ice class DAS directly implemented in case of propellers designed for are built without bow bulb to enable both bow- and operation in ice because general measures to reduce stern-first operation in ice, the vessels require cavitation like skew or tip unloading prove to be either additional strengthening of stern (according to not feasible or limited by the requirement to ensure presently discussed Lloyd’s opinion stern and safe propeller operation in ice conditions. The propulsor should be one ice class higher) and special measures against cavitation are also restricted by maneuvering tactics in ice. special blade section profiles used for ice propellers. The strength issues of DAS propulsion system have Some methodological problems in recording cavitation been successfully solved (with no reported accidents), during model tests of ice propellers were also but as it was found by our analysis the specified blade identified. section thickness is significantly higher than it is The paper examines the above-mentioned aspects of required for the given ship ice class. propeller design. Still the challenges of propeller design for high ice It is shown that the above issues can be resolved only class merchant vessels are not restricted to strength when both ship designers and ship owners are fully considerations. It is seen that in recent years aware and take account of these problems. In the shipowners request and shipyards accept the same situation when the minimum level of power delivered requirements for ice propellers as for propellers to propeller is governed by safety-in-ice considerations, designed to operate in ice-free waters. First of all it it is critical to ensure correct specification of ship concerns specifications related to cavitation, in speeds as well as cavitation requirements for open particular pressure fluctuation levels on hull induced by water operation. Inappropriate choice of ship’s service propeller. Advent of Arctic cruise liners has speed, which actually sets design conditions for complicated the situation even more because in this propeller, will cause not only cavitation problems but case the noise and vibration requirements have long also economic losses because propeller designers will been the absolute priority for shipowners. have to meet the prescribed requirements by revising This study analyzes the ways how these requirements the propeller geometry. can be taken into account in the propeller design INTRODUCTION process. Large capacity merchant vessels of high ice classes are 1 CONSTRAINTS FOR IMPROVEMENT OF ICE intended to carry goods from Arctic to European and PROPELLERS Japanese ports. Such lines include hundreds of miles Traditionally, the main requirement in design of distance covered by ice and thousands of miles in open propellers for high ice class ships (earlier these were waters. It have raised a range of challenges for ship mainly logistic supply vessels, icebreakers and designers including: requirement to modify strength specialist vessels designed solely for Arctic operations) standards worked out in the ХХ century for relatively was to provide the maximum (specification) bollard small ice-going vessels; higher hydrodynamic pull to achieve the target ice going capability, as well resistance of ice-class ships in open water primarily as the requirement to assure proper blades strength in related to elimination of bow bulb; maneuverability ice in compliance with the ice class notation. These issues of large-size vessels in ice (partly resolved by criteria were met based on the rules of the appropriated implementation of azimuthing thrusters as main classification society and verification of propeller propulsors). strength in ice, in particular by finite element analysis. As a rule, hydrodynamic characteristics for transit these vessels (to be discussed in more detail below) it mode were not strictly regulated but determined only to should be reasonable to examine if the measures predict ship propulsion performance in open water. proven for fighting cavitation on open-water merchant For high ice class merchant vessels operating on routes vessels could also be applied to ice-going ships. with long open-water distances the efficiency - Cavitation mitigation by appropriate choice of blade considerations have become quite important. However area ratio is a rather limited measure in case of ice in this case we have to reckon with considerable propellers, as was noted above. constraints typical of ice propellers: - Propellers intended for ice operations are featuring - minimum thickness of ice propeller blades specified special blade section profiles designed for ice milling by classification societies is much larger than that of conditions. According to the Russian Maritime propeller blades designed for ice-free operation, while Register of Shipping, the blade thickness at 5% chord for estimation each 10% increasing of blade thickness length from leading and trailing edges is subject to inevitably reduces the efficiency in ice-free running regulation for strength reasons, which gives rise to conditions by 0.6-0.8%; additional problems in terms of blade hydrodynamic - increased water resistance of ice-going vessels results performance. Blade section outlines of ice propellers in higher hydrodynamic loads and, respectively, in can be described as “more blunt” in comparison with lower efficiency of ice propellers. conventional hydrodynamic blade sections. There is only a limited room for modification of these blade A traditional method of raising propeller efficiency by profiles for cavitation improvement because the first minimizing blade area ratio is also constrained in case priority of ice propellers is strength. of ice propellers. The lowest limit of blade ratio is determined by the requirement for bollard pull mode to - Implementation of tip unloading is also quite limited. avoid thrust break cavitation at full power These constraints in the rules of the Russian Register consumption. Such mode is usually verified are explained by the requirement to ensure some experimentally in vacuum tanks or cavitation tunnels of positive angle of attack for blade tip sections at ice large cross-section where models can be tested in milling. In case of large tip unloading there is a risk of bollard pull mode without interference of propeller- flat blade/ice interaction or negative angle of attack in induced velocities. In some cases the blade area ratio ice milling. It may result in blade tip damage or is increased based on blade strength considerations in bending. Multiple cases of blade tip bending in skewed an effort to keep relative blade section thickness within propellers reported for merchant ships in Russia reasonable limits. suggested that the blade skew should be limited for ice- class propellers (in Russian icebreakers propeller The upper limit of blade area ratio is set at 0.7-0.75, as blades were always of symmetric outlines). In recommended by ice technology experts, to make it particular, Ref. [1] recommends restriction of the skew easy for ice blocks to pass between blades as well as to angle of reversible CP propellers to 5 degrees in case minimize ice interaction with several blades of icebreakers and high ice classes Arc 7-Arc 9, and to simultaneously. 10 degrees for ships of Arc4-Arc 6 classes with Thus, the blade area ratio of ice propellers, as a rule, strength verification of higher than 5 degree cases can be varied only within a limited range of 0.6-0.75. using finite-element analysis. Also, it should be taken Let us analyse which options are available for propeller into account that skewed blades even in case of lower designers to reduce cavitation arising in open water. ice class ships are prone to damage risks, especially in reversing modes. For non-reversible propellers (CPP) Until now the issues related to cavitation inception on these skew angle constraints could be made somewhat ice propellers have not been sufficiently studied. Ice tougher for all ice classes. propellers are made of high-strength stainless steel, and in bollard pull conditions there is a prevailing tip From the analysis it seen that there are very limited vortex that does not cause erosion. Thus, erosion issues opportunities to adjust cavitation characteristics of ice were left out of consideration here. propellers by means of geometry modifications.
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