Giddings and Wermter 



DISCUSSION 



K. C. Ripley 



John J. McMullen Associates, Inc. 



Washington, B.C. 



I desire to comment on the figure of 90 percent as the reduction of roll at 

 ships resonance, that was mentioned by Mr. Giddings in connection with one of 

 the slides of his talk, namely, the slide showing Fig. 11 of the paper. 



The design of passive tanks of the particular ship to which Fig. 11 refers is 

 one with which I am familiar. Up until late 1960, I had been employed for 25 

 years with the U.S. Bureau of Ships, and during this time designed a number of 

 passive anti-rolling tanks, one of which was for the Oceanographic Research 

 Vessel, AGOR. This is the vessel that by model test in an irregular, bow sea 

 showed the reduction of roll at ships resonance of the 90 percent. 



It is my opinion that the foregoing, reported roll reduction is real, and can 

 be accepted as representative of what would have been obtained by the same or 

 similar test performed full-scale at sea. This opinion just expressed is based 

 on personal experience obtained at sea with a merchant ship. This ship tested 

 at sea was fitted with bilge keels, and was tested for the stabilizer tanks opera- 

 tive, and inoperative. The sea was a quartering sea. The reduction of roll at 

 ships resonance was found to be 85 percent. 



How is it possible for the reduction of roll at ships resonance to be as large 

 as 85 to 90 percent when the test is conducted in an irregular sea, whether the 

 sea be model-scale, or full-scale? When the reduction of roll is found for an- 

 other condition of test, namely, for a bench model type of test, the reduction of 

 roll is not as great. In this latter type of test, the roll response is that for pure, 

 steady-state, forced roll. In the former type of test, nothing closely resembling 

 steady roll is ever obtained, and what might appear to be forced roll is in real- 

 ity an interaction between the instant to instant values of stored energy of roll 

 of the ship, and the instant to instant values of input of energy of roll from the 

 sea. 



It is well known that ships at sea tend to roll at or near ships resonance 

 almost irrespective of the frequencies of excitation existing in the sea. We all 

 know that this is what actually happens in roll at sea, but then we are all prone 

 to forget what the actual situation at sea is, in order that we may treat the in- 

 stant to instant roll as representing steady-state forced roll. It is true that after 

 a ship has been well stabilized against roll, the ship will behave more like one 

 the roll of which is pure forced roll. Before the ship has been well stabilized 

 against roll, however, the ship will be rolling more often at ships resonance 

 than otherwise would be the case. It is clear that a roll reduction at ships res- 

 onance as great as 85 to 90 percent when the determination is by test at sea is 

 both reasonable, and comprehensible. A part of the roll reduction is from hav- 

 ii^ a less amount of energy tending to roll the ship at the ships natural frequency, 

 and a part of the roll reduction is from allowing less forced roll of the ship when 

 the roll can be treated as more nearly resembling pure, forced roll. 



810 



