However, the ship experiences another kind of horizontal force which 

 is independent of time. Such a force is usually called the steady drift 

 force by the wave. When the ship is travelling among ambient waves, the 

 steady drift force results in an increase of resistance to the forward 

 motion of the ship. This is called the added resistance in waves. In some 

 cases, the added resistance attains an amount even larger than the re- 

 sistance when the ship travels in calm water. It is generally regarded 

 that the added resistance should occupy a major part of the sea margin 

 which should be considered when the power of ships in the service condition 

 is predicted from model test results. This problem looks to be more 

 complicated when compared with the problem of resistance in calm water or 

 periodical hydrodynamic forces in oscillations, because the steady drift 

 force originates from the periodical action of waves. The model test in 

 waves is difficult also in respect to achieving accurate measurements. It 

 needs much time and labor. A reliable method of prediction by theoretical 

 formula has been expected for a long time. From the theoretical side, this 

 problem looks difficult because the steady drift force belongs to the 

 second order forces. However, the problem of added resistance is now 

 regarded as one of the most successful applications of hydrodynamic theories 

 in the practical field of ship building. 



Since the drift force is a kind of second order phenomena, one may 

 think of the necessity of the second order solution of the boundary value 

 problem, but it is not the case. The steady force can be calculated from 

 the pressure integral on the hull surface. In doing so, however, we have 

 to pick up terms of the second order without exception. This is quite a 

 difficult task, because we need solutions up to the second order complete- 

 ly. Fortunately we have another method which does not need the second 

 order solutions. That is the momentum analysis by assuming a large surface 

 surrounding the ship at a great distance as a momentum control surface. 

 The steady horizontal force is evaluated by the time average of the 

 momentum flux across the control surface. Its principle can be easily 

 understood by considering a two-dimensional case. Here, we consider a cy- 

 lindrical body of uniform cross section which is floating on the surface of 



85 



