Propeller -Hull Interaction 



of analysis used by the British tanks, and wondered if the Danish tank on the 

 same set of experiments would have got the same results— he was positive that 

 they would not. At HyA, they used for model-ship correlation a combination of 

 the Hughes' method of extrapolation of the viscous resistance, together with a 

 wake scale-effect allowance. In using this combination, they got, they believed, 

 more constant C^^ values than could be obtained by any other method. With 

 respect to ship size and ship type, loaded or light condition, they could use the 

 same value of C^ for ship predictions and believed that they obtained good re- 

 sults. K this method had been used for the analysis of the experiments made 

 by the British tank, then the results probably would have looked different. This 

 is because a change of LCB position from 0.5 to 2.5% forward certainly will in- 

 fluence the form effect, the factor (1 + k), and if that is taken into account, the 

 thrust deduction figures will be completely changed and probably would not have 

 the trend shown in the figure. This is mentioned here, not as a criticism of 

 Lackenby's contribution, which obviously is very valuable and which will be 

 studied with great interest, but just to point out that we must, in our profes- 

 sion, be very careful when comparing results from other tanks, because they 

 have been analyzed and arrived at in quite different ways. And, unless the raw 

 figures are available for the actual model resistance and thrust, etc., and all 

 details about the loading of the propeller during the measurements, it is not 

 possible to compare them. One brief remark also on the figures given by 

 Professor Aertssen, who gave some results for a special case where he 

 showed the presence of wake scale effect. The Danish tank could give a very 

 great number of these from all the trial trips that have been performed, and, 

 generally, for a new ship of average size compared with a model of normal 

 size, say, 7 m, wake scale effects of the order of 0.10-0.12 are obtained. These 

 figures are largely in excess of those shown by Professor Aertssen, and they 

 are found both for loaded and for light conditions, but not necessarily the same 

 in each condition. They should be substantiated on trial trips wherever possi- 

 ble, when horsepower and revolutions are determined accurately. However, 

 one must remember that the wake scale effect on the trial trip is there on that 

 day but disappears after a period of time. It will decrease with time — it will 

 even become negative—and an original wake scale effect of say 0.12 might, 

 after five years, have changed to -0.05, a total change of 0.17 in wake. This 

 will occur with a ship which has been regularly docked and cleaned, and is only 

 due to deterioration of the hull. It is thus necessary to distinguish between the 

 wake scale effect for a new ship and for an old ship. Also, for a new ship, there 

 will, of course, be cost differences according to the paint applied. 



V. F. Bavin (Kryloff Shipbuilding Research Institute, Leningrad) commented 

 on the remarks by Dr. van Manen, in which he had made reference to the Rus- 

 sian work presented at the London ITTC Meeting, and suggested that a reduction 

 of about 50% in thrust deduction factor would be possible for a large tanker. 

 Bavin did not think that this would be so, because the influence of cavitation on 

 thrust deduction factor was due to the cavity thickness effect, and he did not be- 

 lieve that supercavitating propellers would be used on super -tankers, even in 

 the future. So the reduction, if any is possible, will be much less. The second 

 point Bavin discussed was concerned with wake-adapted nozzles. In his insti- 

 tution, also, experiments have been made with such nozzles and a reduction of 

 bending moment on the blade of the propeller of about 200% was found, i.e., it 

 became about one-third of that with the propeller operating in a conventional 



1652 



