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Acceleration turn in Seakeeping and Manoeuvring Basin (MORE project)


Five decades of research into manoeuvrability


In the quest for supertankers 50 years ago, the CRS investigated the effects of ship size on the controllability, and in particular how adequate controllability could be achieved. Today, the ship types and the requirements for manoeuvrability have changed, but the CRS applied research still holds. Frans Quadvlieg, f.quadvlieg@marin.nl


T


he unique composition of CRS working groups means that the research finds its way into practical


applicability. Developed tools and knowledge ensure a competitive advantage in the day-to-day business of shipyards, class societies and shipowners.


Why was the manoeuvrability of large tankers so different from smaller ships? In the 70s, economy of scale pushed to achieve larger and larger ships, allowing the installed power per tonne displacement to reduce. From a manoeuvrability point of view, this had two consequences. Due to


the larger ships, the ship reacted much slower and therefore, the stopping distances increased. When the helmsmen gave rudder, the reaction time was longer. The second consequence was related to the Froude number. Due to the increasing length and the same speed, the design Froude number reduced. This means that it was interesting to design ships with larger block coefficients. However, to achieve acceptable added resistance in waves, the bow section became sharp. This has led to fuller aft ships. These fuller aft ships result in less flow over the rudder and hence, less control. Moreover, it resulted into different


lift characteristics for the hull and therefore, different directional stability.


Awareness about the impact on directional stability led to investigations concerning the effect of the hull form. In particular, the increasing fullness of the aftbody on the course-keeping ability was a growing concern. Investigations were carried out systematically, both by captive tests in rotating arm basins and free running model tests. This showed which aft body shapes lead to problems. CRS has also merged this knowledge into practical software tools, so that shipyards and designers could use this


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