A New Appraisal of Strip Theory 

 DISCUSSION OF RESULTS 



Previous correlations between strip theory and experiment shown in Refs. 

 [10,14,22,23] were for the case of ahead seas only and the agreement was de- 

 scribed as very satisfactory. No direct comparison can be made however be- 

 tween the results reported in the above references and the ones described 

 herein, since the earlier correlations were based on more approximate hand 

 computations and in some cases different formulations and/or experimental 

 data for the coefficients and excitation terms were used. 



The current comparison for models of different hull shapes, shown in Figs. 

 2-57 indicates that, in directly ahead seas, reasonably good correlation is 

 achieved for heave amplitude. An exception is found in the case of Model E, 

 Cb = 0.70, B/H = 1.57 (Figs. 34 and 35), where theory fails to reveal the cor- 

 rect trends and grossly exceeds measured values. At the time Ref. [6] was pub- 

 lished, it was suspected that this was due to numerical errors which probably 

 arose for low B/H ratios in the subroutine of the computer program which cal- 

 culates damping and added mass according to Grim's theory. As discussed in a 

 later section, this suspicion was confirmed by subsequent analysis. Apart from 

 this discrepancy and for all wavelengths, except those corresponding to reso- 

 nance, agreement can be termed satisfactory. For wavelengths corresponding 

 to resonance, theory overestimates experimental heaving amplitudes by 15-20%. 

 The above deviations are due to underestimation of heave damping by the ana- 

 lytical approach which is in accord with previous findings [10,14,23]. 



Agreement in directly ahead seas is much better for pitch than for heave 

 although the trends are not the same for different models. For models A, B, C, 

 and D (Figs. 2-33) at all speeds, theoretical results are below the experimental 

 data and the effect is more pronounced as the wavelength is increased. In the 

 case of Model E (Figs. 36-37) large discrepancies are not observable as with 

 heaving motions. The best agreement in this case is found in the case of Model 

 F (Figs. 44-45). 



With respect to phase angles in directly ahead seas, it will be seen that the 

 theoretical predictions are usually higher than experiment and this is true for 

 both pitch and heave. Since phase angles are more susceptible to both compu- 

 tational and experimental errors, agreement should perhaps be interpreted as 

 satisfactory whenever deviations are less than about 15-20%. Discrepancies 

 usually occur at wavelengths equal to model length. Apparent disagreement is 

 also observable at very short wavelengths, but this is mainly due to the manner 

 in which experimental data have been presented in Ref. [18]. Pitching phase an- 

 gles as computed by theory are much closer to the experimental data than heav- 

 ing phase angles and this is particularly obvious in the case of Models F and G 

 (Figs. 44-45 and 52-53). 



In the case of directly astern seas, heaving motion is underestimated by 

 theory and the deviations increase with wavelength. For most models, agree- 

 ment of pitching motion amplitudes in directly astern seas is excellent, although 

 in the case of Models A and B (Figs. 8-9 and 16-17) theory is considerably lower 

 than experiment. No comparisons have been made of phase angles in astern 

 seas, but general indications are that theory reveals the expected trends. 



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