The Judging of the Stability of Ships and the Determination of the Minimum Amount of Stability – Especially Considering the Vessels Navigating Finnish Waters

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School of Engineering | G4 Monografiaväitöskirja | Defence date: 1939-05-26
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Jaakko Rahola's thesis for the degree of Doctor of Technology has been accepted on May 26th, 1939, by the Technical University of Finland, later known as Helsinki University of Technology (HUT, or TKK in Finnish), and finally merged to Aalto University, since 2010. The objective for this doctoral dissertation was to find a method of estimating the stability of ships with sufficient accuracy and for determining the minimum stability values, especially in Finnish waters. While working on his dissertation, Jaakko Rahola conducted research both in Finland and abroad, in Vienna, in Berlin and in Hamburg, and finally in London. At that time there was no internet, so in addition to some experimental research, a considerable part of this work took place by collecting details of capsizing disasters from both official and private sources, including libraries abroad. The choice of the topic of this dissertation was prompted by the subsequent disasters that had taken place in Finnish waters. The waters navigated by Finnish ships are classified as open sea, waters sheltered by the archipelago, and inland waters. Since stability theory did at that time not yet make sufficient allowance for swell, oceangoing ships were examined by means of statistics. The stability of ships that had capsized was analyzed and compared to the data with those of ships with a long accident-free history. The practical finding of this doctoral dissertation: the minimum amount of stability sufficient for a ship expressed as curves. Upper curve: the static stability curve, i.e. the righting moment as a function of the angle of heel. Lower curve: the dynamic stability curve obtained by integration from the above. Note that the values for the lever of ships of different sizes are the same, as a result of which the values of the righting moment are in direct proportion to the weight of the displacement. Rahola applied his theoretical perspective to ships in inland and coastal waters, where the swell is much smaller. He had heeling and yawing tests made using a heeling plotter of his own design. The doctoral dissertation came off the press in April 1939. After the war, Rahola’s method for judging the stability of ships soon spread around the world and the edition quickly sold out. A separate edition was made in the Soviet Union for which, in according to local custom, no royalties were paid. The strength of the dissertation lies in the fact that although its argumentation is founded on a sound command of theory, the result is presented in such a way that a ship’s officer, for example, can apply it in practice. This method, known as “The Rahola Criterion”, was the first of its kind to be widely adopted internationally. Shipbuilding engineers the world over became familiar with the name of Rahola. He himself was surprised that his method remained in widespread use for so long, that it has been so frequently quoted by theoreticians and used in practical shipbuilding and seafaring right up to the present day. Subsequent advances on it do not conflict with it; they are mere supplements and further developments. The vast improvements in the knowledge and theory of weather and swell conditions at sea have, in particular, meant that the theory for judging stability can now be applied even in extreme conditions. When the stability regulations were being honed by the predecessor of the International Maritime Organization (IMO), formerly known as the Inter-Governmental Maritime Consultative Organization (IMCO), in 1966, the name of Rahola kept cropping up, and the German engineers, among others, had told the Finnish delegate they were amazed that Rahola had arrived at such a successful criterion with the material available to him at the time. With far more extensive data at their disposal, they had tried to come up with a better criterion, but they had always arrived at one the same as Rahola’s. These IMO regulations, the wording – but not the content – of which was also influenced by a friend of Rahola’s, Professor C.W. Prohaska from Denmark, are still in force. To mark the 40th anniversary of the Rahola Criterion in 1979, the Australian branch of the prestigious British Royal Institution of Naval Architects published a 60-page article by R.J. Herd on the development of the stability criterion for ships. The name of the publication was “Rahola – 40 years on”, and at the end, Herd sums up by saying: “The world of ship design owes a great deal to the work of Jaakko Rahola. Much work had been done prior to Rahola, but I feel that if he achieved nothing else, he drew together the threads of earlier work, examined various ways of setting up criteria and concluded that the study of sufficiency of stability based on causality analysis held out most promise of success.” An article published in the RINA yearbook for 1998, and especially the discussion it provoked, still underlined the significance of the ground-breaking work done by Jaakko Rahola. The stability criterion of Rahola made him internationally renowned. Although it has not been able to prevent all disasters at sea, it has, in the space of over 70 years, saved many lives and much property.
ships, vessels, stability, minimum stability, judging methods, navigation
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