SHIP MOTIONS 



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• RoUschwingunqcn 

 o Stampfschwingungen 



8 12 16 20 



Begegnungsperiode T in Sec. 



24 



Fig. 5 3 Periods of rolling, T,/,, and pitching (Stampfschwin- 



gungen) Ts, as measured on ships at sea plotted versus periods of 



encounter (Begegnungsperiode) (from Mockel, 1941) 



movements.-" More precisely, the theory permits 

 abstraction of inherent characteristics of a shij) from the 

 irregnhir variations of ship motions which occur in ir- 

 regular waves. 



Thus far use of this equipment and process lias ijeen 

 made only by Cartwright (1957, 1958) and Cartwright 

 and Rydill (1957). Theirs was a pilot application and 

 empha.sis was placed on a description of the method 

 rather than on the seagoing properties of the ship. 

 Recently a series of tests using ship-borne wave recorders 

 has been conducted jointly by the Maritime Administra- 

 tion and the Da\'id Taylor Model Basin. Two modified 

 Liberty-type ships were instrumented and placed in 

 regular commercial service in the North Atlantic during 

 the winter. Information on the observed data is not 

 yet available. 



Analysis of data obtaineil on a ship at .sea po.ses a \'ery 

 difficult problem. It is clear that only a small part of 

 the collected material has been published. Writers de- 

 scribing observations on the MS San Francisco, for in- 

 stance, repeatedly mentioned their intention to publish 

 results of further analysis. However, these results 

 have not been pul)lished.-"' It is very important that 

 the method of analysis be foreseen and planned in the 

 early stages of an observational project. The type of 

 records obtained on a ship must be such that they can 

 be processed by available analyzing eriuipment. The 

 mass of data to be handled makes it a foregone conclusion 



^' The present state of the theoretical development permits this 

 only under certain limited eonilitions. 



'» Except an analysis of single wave by Schnadel (1937/38). 



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025 Q50 0.15 tOO 1,25 1.50 1.15 



Fig. 54 Reduction of ship-model motions by bow hydrofoils 

 (from Abkowitz, 1955) 



that some type of rapid electronic eciuipment will ha\e 

 to be used. 



The foregoing comments apply to recorded quantita- 

 tive data. A \aluable feature of the earlier observa- 

 tions made Ijy Kent (1924, 1927), Mockel (1953) and 

 Patterson (1955) is their vivid personal descriptions of 

 sea and shijj liehavior. This valuable feature is usually 

 lost as instrumentation becomes more elaborate. The 

 recording means are often such that it is difficult to ob- 

 tain information immediately, while the results of the 

 analysis only become available many months later. 

 The author considers it extremely important that ad- 

 vanced instrumentation be supplemented by simple 

 easily read instruments. With the help of these, ob- 

 servers on board ships should appraise and describe the 

 physical conditions of the sea and ship. 



It is clearly impossilile to formulate standards of pvr- 

 formance which a .satisfactory seakindlj' ship should 

 meet on the basis of the recorded figures alone. The 

 need is to define quantitatively the performance char- 

 acteristics which correspond to mariners' appraisals of 

 a ship's seakindliness. The subject is timely. Both 

 towing-tank technique and ship-motion theory are 

 rapidly approaching the state where it will be possible 

 in the design stage to predict a ship's responises to the 

 sea. But what is the optimmn set of many (possibly 



