Manoeuvrability and Propulsion of Very Large Tankers 



In some analysis work we have been carrying out recently, however, involv- 

 ing more pronounced ram bows, there is a fairly clear indication that the bow 

 modification is affecting the propulsive efficiency, and one has to be very care- 

 ful about considering the two ends of the ship separately and then assuming the 

 effects to be additive. 



Perhaps the authors would comment on this question of bow and stern inter- 

 action. 



DISCUSSION 



J. Strom-Tejsen 



David Taylor Model Basin 



(after March 31, 1967 renamed the Naval 



Ship Research and Development Center) 



Washington, D.C. 



It is certainly refreshing to read a paper like this in which the authors have 

 studied the application of an unconventional control system to very large tank- 

 ers. It would have been more interesting had the proposed jet control system 

 shown definite merits as an alternative to the traditional rudder configuration. 

 Unfortunately the jet system alone according to the paper would require exces- 

 sive power in order to produce forces comparable to those obtained by a con- 

 ventional rudder. Also the authors' alternate proposal, combining the jet system 

 and the small tail piece as a movable rudder, is questionable in its present con- 

 figuration as it is unlikely that the small tail piece tested would suffice in area 

 to produce the desired maneuvering forces. It would be of interest to know if 

 the authors actually have explored this alternative further. 



The extreme cigar -shaped afterbody having a shrouded propeller is appar- 

 ently an attractive solution from a resistance and propulsion point of view since 

 critical problems due to vibration and cavitation are reduced. But let us for a 

 moment consider the merits of this form with respect to some of the problems 

 encountered in the field of controllability of very large tankers. These ships, 

 for instance, are known to have very poor inherent course stability qualities. 

 As deadwood has been reduced and a big conventional rudder eliminated in the 

 proposed stern arrangement, it is possible that the dynamic stability problem 

 would become more critical in this case, and the proposal thus would be unreal- 

 istic unless additional fin area is added. I think many of us would appreciate 

 the authors commenting on this problem and possibly adding to the paper the 

 results from a course stability study. 



The stopping and backing are other examples of maneuvers which by many 

 are considered critical for the very large tankers. As shrouded propellers are 

 known to be inferior to conventional propellers with respect to the retarding 

 and backing force which can be produced in a stopping maneuver, it is necessary 



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