Linearized Potential Flow Theory for ACVs in a Seaway 



with 



Q( e ,h 



rr jcosl . v 



= //p (x,y! |~k (xcos(0 + |3) + ysin(,3+0)J sec dxdy 



(6-37) 



where /3 is the angle of drift on the right-hand side. 



The side force F in drifting motion has also been obtained in Refe- 

 rence 1 



k 2 7 2 



■jfi- Fg =— ^—z\p 2 (e> /3) + Q 2 (*. )1 secS sin (0+ 0) d0 



27TpV 7 



-% 



It is not worth attempting to find a solution for a drifting ACV 

 with side hulls since the lateral motion will induce a disturbance of 

 the water which will not be consistent with the basic assumptions of 

 this linearized theory. 



VII. FORCED OSCILLATION IN CALM WATER 



Let us now consider the case of an ACV which is forced to 

 oscillate in calm water during steady translation. Obviously, we have 

 in mind the forced oscillation of an ACV model during towing tank 

 tests. We will now derive the oscillatory forces and moment acting 

 on the craft when the motion and oscillations are confined to the lon- 

 gitudinal plane. Such a motion can be deliberately imparted to the 

 model by a mechanical oscillator such as the Planar Motion Mecha- 

 nism (PMM). The added mass and damping of water can be determined 

 by experiments of this nature in calm water. 



In the case of an amphibious ACV free from water contact, the 

 discussion of the motion in the longitudinal plane can be applied direct- 

 ly to motion in the lateral plane for the disturbance of the water would 

 be comparable in both cases, the beam/length ratio of present day 

 hovercraft being of the order of unity. The discussion of surge, heave 

 and pitch in this section will then apply also to sway, heave and roll 

 in beamwise motion. 



157 



