Panel Discussion 



H. Lackenby (British Ship Research Association) said that his organization 

 had not been directly involved in work on propeller-hull interaction, but had 

 sponsored a great deal of systematic model testing over the years and, of course, 

 this had involved the determination of the usual hull interaction factors such as 

 wake and thrust deduction fractions, hull efficiency, etc. Some recent tests on a 

 very full model of an 0.85 block-coefficient tanker form had shown some very 

 interesting trends, which he thought worth reporting to the panel. These re- 

 ferred to the effects on the hull factors of systematically varying the longitudinal 

 position of the center of buoyancy (LCB) over a range of 0.5% forward of mid- 

 ships to 2.5% forward, as shown in Fig. 1. The various hull interaction factors 

 are plotted there on a base of longitudinal position of the center of buoyancy. 

 The Taylor wake fraction, the second curve from the bottom, stays remarkably 

 constant over the range; the bottom curve is the thrust deduction fraction, and 

 unlike the wake fraction it is reduced quite significantly in going from 0.5% for- 

 ward to 2.5% forward. This is somewhat unusual, because experience generally 

 shows that any wake gain is quite often offset by a corresponding disadvantage in 

 increased thrust deduction, and the hull efficiency generally remains much the 

 same. But not in this case— the wake fraction stays constant, thrust deduction 

 fraction goes down, and the effect on the hull efficiency is shown in the top 

 curve. As the LCB moves from 0.5% to 2.5% forward, the hull efficiency goes 

 from about 1.07 to about 1.22, a change of about 15%. And, of course, this is 

 reflected in the quasi-propulsive coefficient, where, in going over that LCB 

 range, there is again an increase in QPC of something like 18%. On the other 

 hand, the relative rotative efficiency remains sensibly constant. It is a very 

 simple case of some systematic experiments and there is a hull-interaction 

 gain of 18% in moving the LCB over that range. Lackenby pointed out that it is 

 not roses all the way, however, because as the LCB is moved forward of about 

 1.5% the resistance begins to go up, which begins to offset the gain in the pro- 

 pulsive effect. The overall effect of LCB, including both the resistance and this 

 hull interaction effect, is shown in Fig. 2, where the delivered horsepower co- 

 efficient is plotted, again on the same base of LCB position, and it is seen that 

 the optimum position of LCB is about 2% forward. The practical V \T for a 

 form of this kind must be around 0.56, and when the LCB gets further than 2% 

 forward the curve begins to rise again due to the increase in resistance offsetting 

 this very favorable hull interaction effect. Nevertheless, the results are very 

 striking, and if we could maintain this very favorable interaction without losing 

 out on the resistance side it would be very attractive indeed. 



Professor G. Aertssen (University of Ghent) first gave the results of the 

 correlation between the calculated and measured two-node vertical natural hull 

 frequency for a large ore- carrier, the Min Seraing, having a length of 218 m 

 (715 ft). He had made a voyage on the ship from Chile to Antwerp, in the loaded 

 condition, and in very smooth water in the Cape Verde Islands, where the ship 

 called, had been able to do an anchor -drop test in deep water. The ship was 

 instrumented with strain gages on the main deck amidships, which recorded the 

 stresses and the two-node vertical natural hull frequency. The latter was very 

 well defined, and therefore a full integral calculation was made for the two-node 

 vertical frequency. The ship length was divided into 115 parts to give a correct 

 distribution of hull weights, and the distribution of cargo was also known quite 

 accurately. The added mass of water was calculated by the Lewis-Todd method, 

 and amounted to 76,912 tons on a loaded displacement of 66,130 tons. A reduc- 

 tion factor of 0.97 was applied to the transverse moment of inertia of each 



1645 



