208 



THEORY OF SEAKEEPING 



conflicting) ship characteristics? To the author's knowl- 

 edge, the only work aimed at giving the answer is 

 Mockel's (1953). This work applies to fishing trawlers. 

 It is clear that there can be no iniiversal specification 

 for good sea behavior. A separate one must be formu- 

 lated for each class of ship and linked to the states of 

 sea it is likely to meet in service. 



6 Ship Stabilization 



6.1 Rolling StabilizoHon. General reviews of the 

 many devices for controlling ship rolling ha\e been 

 made by von den Steinen (1955) and by Chadwick 

 (1955-Part 1). Only three of these devices progressed 

 beyond the initial trials and were used on a number of 

 ships. The.se were stabilizing gyros, stabilizing water 

 tanks and stabilizing fins. The use of a gyroscope for 

 stabilizing a large passenger liner was described by De 

 Santis and Russo (1936). This device has since been 

 abandoned. 



The theory and use of stabilizing tanks have been 

 treated extensively in German engineering litera- 

 ture and also have received some attention in the 

 United States. Some of the pertinent references are 

 ILsted in the Bibliography under the following names: 

 Baumann (1938), P'eld '(1937), Foppl (1934a, b, c; 

 1937), Frahm (1911a, h), Hahnkamm (1936), Horn 

 (1911, 1955, 1957), Herrmann (1934), Hort (1937), 

 Minorsky (1935, 1941, 1947) and Weinblum (1938). 

 The latest and most thorough exposition of the theory 

 of activated stabilizing tanks was presented by Chad- 

 wick and Klotter (1955 o, h). They determined the 

 criteria for the proper proportioning of the tanks and 

 tabulated data on several ships indicating satisfactory 

 and faulty designs. Their papers included a critical 

 commentarj'- on the work of others. Also they clarified 

 the apparent confusion in the existing literature regard- 

 ing the mathematical form of the cross-coupling co- 

 efficients. The reason for the differences was shown to 

 be the use of different co-ordinate axes. This question 

 was discussed previously by Klotter (1934). The 

 latter reference is pertinent to the entire problem of 

 coupled ship motions and is not in any way limited to 

 stabilization. 



Roll stabilization by activated fins was described by 

 Allan (1945), ^'olpich (1955), Wallace (1955) and Chad- 

 wick (1955-Part 2).'' Quoting from the latter work: 

 "Fins are of some interest, having a virtual monopoly 

 in the field at present. They are unexcelled where sta- 

 bilization is wanted at speed, and thus are the systems of 

 choice in a wide variety of passenger and military ap- 

 plications. The principles of operation have been proved 



'1 Only relativt4y recent references are mentioned here, and no 

 attempt is made at presenting the history of the development. 

 Reviewers of an early draft of this monograjih mentioned a patent 

 by Mr. Wilson in IS'.IS, a manually controlled design by Motora 

 in 1925, a proposal liv a, Ru.ssian naval officer submitted to Alessr.s. 

 Denny in l'J31, and a patent by Sir William Wallace in 1036. 

 Practical success was eventually achieved on introduction of the 

 "A.R.L. Continuous Control" developed by Mr. J. Bell of the 

 Admiralty Research Ijaljoratory of Teddington, England. 



in installations of all sizes ranging from cross-channel 

 steamers up to and including Queen Elizabeth. For this 

 we ha-\-e to thank, of course, the Denny-Brown Com- 

 pany." 



Both tanks and fins have pro\-ed to be very effective 

 in reducing ship nilling. The degree of reduction de- 

 pends, of course, on the capacity of the installation. 

 This is often rated by the angle of heel which a stabil- 

 izing dexdce can generate in a ship moving in smooth 

 water. A 2-degree angle is definitely too small, but, 

 nevertheless, provides a satisfactor_y stabilization if 

 conditions are not too severe. An angle of 4 deg is 

 satisfactory and a 6-deg angle is probably the largest 

 practical angle. Chadwick (1955) showed that the cost 

 and weight of an installation grow rapidly with increase 

 of the specified angle. Chadwick showed by computa- 

 tion that a device of a practical capacity will reduce an 

 uncontrolled roll of 5 deg to } [q of its value; i.e., \vill 

 practically eliminate rolling. The latest description of 

 the stabilizing-fins installatinn and the data on their 

 performance were published by Flipse (1957). A 

 20,000-ton ship, traveling at 20 knots and controlled 

 by the fins, rolled only about 2 deg (average double 

 amplitude) in a very rough beam sea in wind force 8. 

 From time to time the fins reached the limits of their 

 travel of 25 deg. Flipse emphasized that the list of a ship 

 izi side wind should be controlled by the disposition of 

 the water ballast in order to jjreserve the full fin capacity 

 for controlling rolling oscillations. 



Fins are the most effective device for controlling 

 rolling of fast passenger and naval ships. They have, 

 however, two drawbacks; namely, vulnerability to 

 damage and rapid loss of effecti\-eness with reduction in 

 speed. A ship forced to reduce its speed or to heave-to 

 in adverse weather may lose most of its stabilization.^^ 

 This is not important for ships designed according to the 

 recommendations of Fronde (1861) and Meckel (1941); 

 i.e., with small metacentric heights and long iiatural 

 periods of rolling. Howe\-er, natural periods of rolling 

 have been drastically reduced in recent years because 

 of damage-stability considerations. Modern ships, 

 therefore, may de\elop a severe rolling even in bow seas 

 under storm conditions. Such rolling is shown in 

 Fig. 48 for the SS Nissei Main. Patterson described 

 vividly the occurrence of heavy rolling in head seas. 

 This took place on a cargo ship which was loaded so as 

 to have a very short rolling jjcriod. 



Stabilizing tanks retain their effectiveness at low 

 speeds. While they appear to have been abandoned 

 on passenger ships at present, they are very useful on 

 lightships and weather-observation ships. The author 

 feels, however, that they may come into wider use agaui 

 because of their simplicity, their lack of vulnerability 

 and their effecti\-eness at reduced speeds. They appear 

 to be particularly suitable in cf)nnecti(in with ship-dam- 

 age considerations. A low metacentric height, needed 

 for naturally good rolling behavior, may be secured by 



32 It has been shown in Chapter 2 that natural damping in roll 

 also becomes smaller with decrease in speed. 



