Ship Maneuvering in Beep and Confined Waters 



in horizontal maneuvers, this homogeneous current will only mean 

 a steady shift of the path; alternatively, if a certain straight course 

 is required heading shall compensate for the steady drift. The 

 local finite current, on the other hand, generates varying outer 

 disturbances and shall be handled by other means. 



V. FLOW PHENOMENA AND FORCES ON A SHIP IN FREE WATER 



Ideal- fluid Concepts 



As a source of reference for further discussions this Section 

 recapitulates some of the characteristics of the flow past a ship In 

 free or open water. 



When a double -body ship form — i.e. , a body which is sym- 

 metrical about the xy-plane — moves forward in a large volume of 

 ideal-fluid water the streamlines adjust themselves according to the 

 laws of continuity* The shape of those streamlines remain the same 

 at all speeds. The increase of relative velocity past the wider part 

 of the body corresponds to a back-flow or return flow of the water 

 previously in rest. This disturbance in the potential flow pattern 

 extends far into the fluid volume — a beam-width out from the side 

 of the body the super-velocity still has a value, which is some 80 per 

 cent of that just outside the body. 



From a resistance point of view the steady forward motion 

 within this ideal homogeneous fluid may lack some realism. Accord- 

 ing to the d'Alembert's Paradox the body will experience no resultant 

 force. However, if the body is to be accelerated the kinetic energy 

 of the fluid must be increased. This energy Increase Is manifested 

 by a resistance, which for a given geometrical form Is proportional 

 to the mass of displaced fluid and the amount of acceleration. I.e. 

 to the product of an "added mass" and the acceleration component In 

 the direction considered. The resultant force Is not necessarily 

 orientated In the same direction. 



In the simple steady motion the total energy certainly will 

 remain constant, but as the body moves forward through virgin fluid 

 there takes place In each transverse section a repeated particle 

 acceleration and transformation of energy. The Impuls pressure 

 distribution thus generated will normally be unsymmetrlc, and so a 

 free moment results on the body. This moment may be expressed 

 by a combination of total-body added mass coefficients. 



In the general case of a complex motion In the Ideal homo- 

 geneous fluid all the forces and moments will then be available In 

 terms of added masses and Inertias, according to the theories 



827 



