Panel Discussion 



with a pumpjet or ducted propeller might be very important for delaying the 

 inception of cavitation. 



Dr. M. Kinoshita (Hitachi Shipbuilding and Engineering Co. Ltd.) had some 

 remarks to make on H. Lackenby's statement about the effect of LCB position on 

 ship power. He understood from Lackenby's report that the position of the longi- 

 tudinal center of buoyancy must be chosen as far forward from the midship sec- 

 tion as possible as far as propeller interaction factors and vibration problems 

 are concerned. He felt quite agreeable to this opinion so long as the ships' sizes 

 and speeds are moderate. But as regards very large tankers, with a deadweight 

 of more than 200,000 tons, the conditions change, and he hesitated to agree with 

 this conclusion, and would like to recommend to designers of such large tankers 

 selection of a longitudinal position of center of buoyancy not so far forward. The 

 reason for such hesitation is that recently there have been occasions, on the 

 speed trials of large oil tankers with a deadweight of more than 200,000 tons, 

 when the measured speed has fallen short of the value predicted from the model 

 experiments. Since the beginning of this year his company had started a very 

 productive study to solve the probable difficulties to be encountered in the 

 course of designing and building a large tanker with a deadweight of 400,000 

 tons. In these studies, the problem of discrepancies between the results of tank 

 experiments and the results of speed trials is included as one of the important 

 items, and has been carried out under Dr. Kinoshita' s supervision. As to the 

 cause of these discrepancies, we must examine both sides— model-experiment 

 and sea-trial. In his personal opinion, however, the latter must be examined 

 more carefully, and he considered that there were three main causes for these 

 discrepancies. One of the three is concerned with the matter which Lackenby 

 pointed out. In consequence of the expansion of the ship size, the length in- 

 creases but the speed of tankers has kept nearly constant, so that the optimum 

 longitudinal position of the center of buoyancy has a tendency to be chosen more 

 and more ahead of midships, as he recommended. Furthermore, the value of 

 the L/B ratio has become smaller and smaller and finally has reached a value 

 less than 6.0. All of these above-mentioned factors lead to the so-called 

 Kempf phenomenon, not only on a straight course during a service voyage but 

 also at the important time of the speed trials. Dr. Kempf s phenomenon is the 

 small yawing, long-period, snake action of the ship under straight-course sail- 

 ing, which leads to increased resistance. It also happens sometimes— not always, 

 but sometimes— that the ship loses its course stability slightly, and even on the 

 maiden voyage of such newly built large tankers the expected speed cannot 

 always be obtained for such ships. The ship designers also have a tendency to 

 make the stern aperture and cut-up as large as possible for such tankers with 

 large values of block coefficient to avoid vibration problems and to get a smaller 

 value of thrust deduction, and this further reduces the course stability. In con- 

 clusion, Dr. Kinoshita emphasized that for all these very, very large vessels the 

 location of the longitudinal position of center of buoyancy must be chosen care- 

 fully, not only from the point of view of the tank tests, but also to keep good 

 course stability features. 



Professor C. W. Prohaska (Hydro- og Aerodynamisk Laboratorium, Denmark) 

 said that H. Lackenby had shown a most interesting diagram, from which it ap- 

 peared that the position of the LCB had an enormous, and in some respects 

 rather unexpected, influence on some of the propulsion factors. Professor Pro- 

 haska pointed out that this result necessarily must be a function of the method 



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