288 abstracts: hydraulics 



HYDRAULICS. — Measurements of the suction of vessels made in the 

 experimental model basin at the Navy Yard, Washington, D. C. 

 U. S. Hydrographic Office Pilot Chart of the North Atlantic Ocean 

 for June, 1911. 



Naval Constructor D. W. Taylor has recently investigated the ques- 

 tion of the relative reactions of vessels under way and close to one 

 another. These reactions are found to be strong, and the suction clue 

 to them when vessels make ill-advised attempts to pass others too closely 

 account for many collisions especially in shallow waters. 



The models, which were of a mean immersed length of 20 feet and of 

 a displacement of 3000 pounds, were towed in pairs abreast, or at defi- 

 nite distances ahead or astern. In the abreast positions, they were towed 

 at various distances apart; for other positions, the uniform distance 

 apart of their center lines was nineteenth-hundreclths of the length of 

 the model. While this is quite close, it should be remembered that these 

 experiments were made in water many times deeper than the draught of 

 the models, and hence the suction effects under given conditions would 

 be less than if the water had been relatively shallow, as is usually the 

 case when suction phenomena are of importance in connection with 

 actual ships. Imagine one vessel overtaking another on a parallel 

 line, quite close to the right of the latter, then the sequence of phenom- 

 ena is about as follows: 



When the overtaking vessel just begins to overlap the other, there is 

 little force acting. There appears to be a repulsion at both bow and 

 stern, and curiously enough the repelling force upon the stern appears 

 to be greater than that upon the bow. The resulting tendency is for 

 the overtaking vessel to turn in toward the overtaken vessel. When 

 partially overlapping, the tendency is for the bow to be drawn in while 

 the stern is still repelled. 



As the overtaking vessel continues to pull up, the suction at the 

 bow becomes stronger and the repulsion of the stern falls off, until, as 

 they come abreast, there is a rapid change in the stern force, which 

 shifts from repulsion to strong suction. 



As the overtaking vessel draws ahead, there is a reversal of condi- 

 tions, the bow pull falling off rapidly, and soon becoming a repulsion, 

 while the stern pull becomes stronger, reaching its maximum when the 

 center of the overtaking vessel is about two-tenths its length ahead the 

 center of the overtaken vessel. It should be understood that the idea 

 of the right hand vessel overtaking the other is simply used for conven- 

 ience in description. For given relative positions, the forces upon the 

 right-hand vessel would be the same whether overtaking or overtaken. 



