Some Hydrodynamic Aspects of Ship Maneuverability 



brilliant synthesis of the work already done, but concerning the work which is 

 now to be undertaken, he has formulated numerous pertinent personal views 

 which, in my opinion, will be very fruitful. For those reasons, I think that Dr. 

 Newman fully deserves to be very warmly congratulated. 



SUMMARY OF ORAL DISCUSSION 



Nils H. Norrbin 



Swedish State Shipbuilding Experimental Tank 



Goteborg, Sweden 



This paper offers interesting reading — still, I suppose, that will apply even 

 more to the next paper by Dr. Newman on this subject. 



It should be somewhat embarrassing to many of us that the mathematician 

 was the one to bring with him so many instructive photographs illustrating basic 

 events around a hull in oblique flow. These flow studies will help to judge the 

 merits of the two simple analogies usually applied to explain the viscous lift on 

 the submerged body, or the surface ship at low speeds where hull and image 

 move together, i.e., the slender body of revolution and the low-aspect-ratio flat 

 wing. 



We remember the concepts of Nonweiler for the viscous flow around a body 

 of revolution, touched upon by the author: the crossflow is decelerated in the 

 boundary layer along the body, and at some distance aft of the bow the up-wash 

 is reversed, on the leeward side of the afterbody the radial displacement flow 

 will change into an in-flow, and only more near to the stern the roUing-up of 

 two trailing vortices will be completed. This physical picture suggests that 

 viscous ring vortices at the after part of the hull should be added to the ideal 

 flow model, characterized by a double-vortex line along the axis in the parallel 

 middle-body connecting ring vortices bound at bow and stern. These added ring 

 vortices are shed away in trailing line vortices from positions rather far aft; 

 there effect is to reduce the negative lift on the afterbody by say 50 or 60 per- 

 cent. In our experiments with the divided double-body ship model, referred to 

 by the author, this reduction amounts to about 85 percent, bringing the result 

 quite close to that of the wing analogy. 



The double-vortex line along the axis of the body of revolution induces an 

 up-wash angle, which close to the hull is equal to the angle of attack. If we now 

 look back to Fig. 2 or 3 of Dr. Newman's paper we do not find the additional up- 

 wash associated with the ideal flow around the slender body, but we might note a 

 local separation along the bilges. 



If the low-aspect-ratio wing analogy does apply, then I think the normal 

 force on the forebody will be very close to the force on the total hull, as indicated 



235 



