Investigation on Correlation Lines through the Analyses of Geosim Model Test Results

Abstract

The present paper, at first, presents the author's understanding of the problem caused by the use of ITTC (International Towing Tank Conference) 1957 Line in conjunction with three-dimensional analysis. Then, how the ideal friction line should be within the range of model Reynolds number is discussed. As a conclusion of this paper, the answer to the question "Should ITTC 1957 Line be revised?" is as follows. It is "Yes" in a sense that ITTC 1957 Line prepared for two-dimensional analysis should be revised, if we employ three-dimensional analysis. It is "No" in another sense that the expected gain by the revision of the friction line would be almost negligible and, on the other hand, we have to expect the setback caused by changing from the well-accustomed line to new one. 1. Preface Several papers were presented discussing what correlation line is most suitable for the separation of resistance components, on the basis of Geosim (Geometrically similar) model test results. For examples, Tanaka et.al. 1) pointed out that the values of form factor decreased for the larger model when Geosim model test results of Wigley hull form were analysed by three-dimensional analyses using Schoenherr Line, and Gometz 2) pointed out that the values of form factor obtained by the analyses of several Geosim model test results using ITTC 1957 Line increased when the larger model was used. There have been successive discussions in ITTC conferences whether ITTC 1957 Line is an appropriate line to estimate frictional resistance of ship hull forms. Even if they might not create major activities of research work, it is truly a fundamental issue for the estimation of propulsive performance of ships through model tests. Then, it is very important to try to make the reality as clear as possible. 1.1. Common Claims Most common claim seems to be that form factors analysed for test results of Geosim series models suffer scale-effect of the model size. However, because many of model test specialists know that the values of form factor varies time to time even when the resistance tests of a same model ship were repeated, it might be too hasty to conclude that ITTC 1957 Line is not appropriate only by the variation of form factor in a certain trend. Another common discussion is that flat-plate friction line should be used for the separation of resistance components. However, the author considers the basis of this discussion is quite weak by the following two reasons. (1) It is only the assumption made by William Froude that total resistance coefficient obtained by model test (C T,M) can be separated into residual resistance coefficient (C R) and frictional resistance coefficient (C F), and that the extrapolation to the full-scale total resistance coefficient can be made by the application of the obtained C R and C F calculated by the same friction line used for the separation of resistance components. (2) There are various proposals for the friction line of flat-plate. Then, even when we can suppose Froude's assumption would give definitely correct result, we cannot obtain one definite friction line for the separation of resistance components.