B. INVESTIGATION OF FORCE CANCELLATION MEANS 



The most effective way to solve the problem of hull vibration would be the cancella- 

 tion of the cyclic forces acting upon the hull. 



Even the reduction of the steady-state hydrodynamic forces induced by the propeller 

 is difficult. Emperical concepts have to be developed concerning pertinent design features 

 such as: position of the propeller in relation to the hull and rudder, clearance around the 

 propeller, modification of the propeller rake, the skew, and the inclination of the propeller 

 to the hull. In the future, vibratory motions resulting from the pulsating pressure fields at 

 the stern area may well be expected to increase. This may either require a change in stern 

 configuration or the development of force cancellation devices. 



To cancel cyclic forces acting upon the hull, forces equal and opposite to the exter- 

 nal forces must be considered in form of dynamic vibration absorbers, adjustable rotating 

 eccentrics, and shaft synchronization devices on multiple-screw ships. In this area we are 

 interested in receiving results of studies designed to absorb or cancel the oscillating forces 

 in the vicinity of the propeller by replacement of plates with other flexible material. 



Counter-rotating propellers may show advantages in reducing vibration. Several in- 

 stallations of this type are presently under study. 



Practical design consideration as well as variation in operating conditions preclude 

 the possibility of cancelling the exciting forces. These forces can be reduced but probably 

 involve higher ship construction costs or lesser design efficiency. 



IV. EXPERIMENTAL STUDIES 



A. VERIFICATION OF DEVELOPED THEORIES 



One of the main theories to be verified by experiments concerns the beam theory with 

 its last advances, i.e., effect of sprung masses, appendages, and superstructures on frequen- 

 cies and mode shapes. Experimentally this requires transducers and test arrangements which 

 allow a clear definition of mode shapes and frequencies. This is accomplished by the addi- 

 tion and subtraction of outputs of various gages. A still more complex task is the verifica- 

 tion of response at low hull vibration levels where the magnitude of vibration displacement 

 may be of the order of microinches instead of millinches. 



Case studies were made on USS FARRAGUT for the rudder-hull system (Flutter), 

 NS SAVANNAH for the sprung-mass effect of the reactor compartment, on several submarines 

 for the hull-propulsion system as a sprung mass, and on USS LONG BEACH (CG(N) 9) for 

 the effect of tall superstructures. 



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