It is in the hope of stimulating further interest in the study of ventilation that 

 these remarks are made, and it is considered a significant addition to Mr. Niedermair's 

 most interesting survey because it is probably the least understood of all hydrodynamic 

 "barriers." 



E. Moberg 



Mr. Niedermair's excellent talk has brought to mind a hydrodynamic barrier 

 which recently appeared and eventually was lifted, and is the type of practical problem 

 which I believe will interest those who are not already familiar with it. A recently 

 designed ship experienced a severe hull vibration during her builders trials, and also 

 on subsequent trips. This vibration appeared at about % speed, and at top speed 

 increased to a severe level about 4 times the lower value. It was a constant frequency 

 hull mode which was being excited. This mode was of lower frequency than either 

 the shaft rotational or blade impulse frequency, which are the more common trouble- 

 some hull vibration frequencies. Considerable instrumentation and several sea trials 

 later it was agreed by all involved that the rudders appeared to be transmitting the 

 exciting forces to the hull. These ships are twin screw two-rudder ships, and the 

 rudders were originally designed so that the rudder trailing edges were toed in forming 

 an angle of 3° with the ship's centerline. As one attempt at a solution, adjustable 

 rudder links were made so that the toe-in angle of the rudders could be varied. A 

 series of runs, with toe-in angles varying from AV2 toe-in to 3° toe-out was conducted. 

 These produced a family of curves which showed very severe vibration at AV2 ° toe-in 

 and a quiet ship between W2 and 3° toe-out. Subsequently, the rudder angles were 

 set at 1 Vi ° toe-out, and no further vibration trouble has been experienced. Upon 

 drydocking of this ship after only 1800 hours sea operation, about 20 of which were 

 in the upper Va, speed range, it was seen that nearly all of the paint had been removed 

 from the suction faces of the rudders, almost as if sand-blasted, while on the pressure 

 face only small areas in the maximum thickness section had been removed. 



In closing, I will say that such problems of this nature, and their solutions, 

 should, and do, stimulate research so that practical experience can be related to 

 theoretical and empirical data, thus providing ship designers with more effective tools 

 with which to work. 



J. C. Niedermair 



The primary objective of the paper was to indicate the many uncertainties 

 which exist in the basic knowledge of hydrodynamics related to the design and per- 

 formance of ships. This objective is amply supported by those who so kindly con- 

 tributed to the discussion of the paper. Some exceptions were taken to a few of the 

 statements made in the paper. These exceptions were particularly in regard to the 

 cost of speed and the emphasis on the lack of basic theory. Fortunately, there is no 

 disagreement in principle but only with the relative degree of importance which should 

 be placed on the various "barriers". 



I am grateful for the comments contributed by Dr. R. W. L. Gawn, chairman 

 of the session at which this paper was presented, Captain H. E. Saunders, U.S.N. 

 (retd), Dr. J. F. Allan, Dr. K. S. M. Davidson, Dr. F. H. Todd, Mr. H. deLuce, 

 Mr. M. C. Eames, and Mr. E. Moberg. Their comments form an important addition 

 to the paper. Rather than a personal reply to these comments I prefer to direct 

 attention to Mr. Gawn's discussion wherein he sums up the situation regarding the 

 current status of hydrodynamics in ship design and performance. 



153 



