67 



pattern. ■'■■'■'* This discrepancy in the behavior of t introduces an element of 

 uncertainty in our reasoning which should be eliminated as far as possible by 

 reliable experiments. Another disturbing fact is the bad quantitative agree- 

 ment between the values of the suction force derived from the difference be- 

 tween thrust and resistance and those evaluated from pressure measurements; 

 this discrepancy is not due primarily to wave phenomena. 



The fact that the wave formation around a hull is only slightly in- 

 fluenced by the propeller action gives a valuable support to our present 

 technique of model towing phenomena; a model test without screws discloses a 

 basic property of the hull which normally remains unchanged in a self- 

 propelled condition. Additional suggestions are made by theory: 



a . To locate the propeller in a region of a high wave wake . 



b. To change the wave pattern of the hull itself in such a way that a 

 high wake may be created at the propeller disc . This viewpoint has not hither- 

 to been applied in model research; it is even doubtful if it would lead to 

 practical results. 



Dickmann has extended his analysis to actual flow. He has explained why non- 

 uniformity of the wake increases the thrust deduction.* 



In the light of this knowledge and our remarks on stern bulbs an 

 idea due to E. Hogner and G. Kempf^^® may be reconsidered. These authors pro- 

 posed to create an axially symmetrical wake by giving an appropriate shape to 

 the run. Unfortunately, according to van Lammeren, gains in propulsive ef- 

 ficiency so reached are counterbalanced by increased resistance. 



However, it is worth while investigating whether with such a run a 

 stern bulb effect can be obtained which combined with a bow bulb may reduce 

 the wave resistance. The remark applies to high-speed single-screw ships as 

 v/eli as for triple- screw vessels. 



Finally the possible influence of propeller suction on separation 

 over full sterns has been often mentioned. Unfortunately this idea cannot be 

 dealt with in a reliable way until adequate research is carried out into the 

 flow patterns at the stern. 



8. RESISTANCE IN RESTRICTED WATER 



Two types of problems are met with: Motion in shallow water, in 

 which only the depth is limited, and motion in canals. 



*van Lammeren has deduced this statement from experiments. 



