A Survey of Ship Motion Stabilization 



design of rudders. The many Naval Architecture text books also offer practical 

 methods leading to design of directionally stable and controllable ships. 



There have been analyses of the forces and moments in yaw exerted by a 

 seaway, especially Refs. 3, 4, 5, and 6. The inherent stability on course of 

 ships is discussed in Ref. 7, and the automatic control of directionally unstable 

 ships is treated in Ref. 8. The general subject of automatic steering control is 

 treated in Refs. 9 and 10. Additional references on the subject are [11] and [12]. 



In general, yaw stabilization or course-keeping has been in the province of 

 commercial developments. The devices and methods used are largely proprie- 

 tary, and their success is evident from their widespread use. Even without 

 automatic systems, the control of a well designed ship in a seaway is well within 

 the capabilities of skilled men. 



Roll Stabilization 



General Discussion 



All ships are stable in roll in that a properly loaded intact ship will not 

 capsize, so that roll stabilization is really roll angle limitation. In contrast to 

 the yaw case, as long as rolling is a stable motion, it need not be "controllable." 

 There is an extensive background of experience on the control of roll in a sea- 

 way. The subject has fascinated inventors since steamships were invented, and 

 the general subject is dominated by inventions. A glance through the patent 

 office files on roll stabilization reveals not only the bad drafting favored by pat- 

 ent attorneys, but evidence of the highly imaginative approaches generated by 

 the problem. 



The roll stabilizers can be divided into two major groups, internal to the 

 ship and external. Each of these can be further divided into active and passive 

 types. Table 1 categorizes stabilizers from a mechanical point of view. Chad- 

 wick [13] offered a more elegant and complete categorization based on the dy- 

 namics involved. 



Bilge Keels 



The earliest deliberate roll damping devices were bilge keels, fitted to 

 steamships to make up for the roll damping lost when the sails were removed. 

 References 1 and 14 present curves of bilge keel size as a function of ship size, 

 based upon experience with ships in the past. References 15 and 16 present ex- 

 perimental results on the forces actii^ on bilge-keel-like plates oscillating in 

 water. It is rare that an occasion requiring more than rule of thumb design of 

 bilge keels will arise, but when such a case is at hand, analysis of bilge keel 

 forces can be carried out using simple concepts and data such as that cited. 



Bilge keels can be counted on to increase hull damping in roll by 50 to 100 

 percent. This will result in 25 to 50 percent reduction in roll. It must be re- 

 membered however, that the principal advantage of bilge keels is found at low 

 speeds. As ship speed is increased the hull damping increases proportionately 



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