1. Longitudinal 



Although the major source of excitation of longitudinal vibration is the propeller, the 

 response of the hull is a function of the driving forces, which may be seriously magnified by 

 the presence, or near presence, of resonances in the machinery system. Early in World War 11, 

 very serious longitudinal vibration of the main machinery system of the latest carriers and bat- 

 tleships was observed. The basic solution to the machinery problem was well documented in 

 TMB Report 1088, "Longitudinal Vibrations of Marine Propulsion Shafting Systems," by Kane 

 and McGoldrick. This paper, which later appeared in Transactions of SNAME, did not com- 

 pletely satisfy the problem of hull vibration. Tests conducted on the MIDWAY-Class car- 

 riers, from 1947 through 1949, clearly demonstrated that the best that could be obtained was 

 a compromise between the needs of the hull and the requirements of the machinery system. 

 The use of the optimum propellers with regard to hull vibration would result in damaging ma- 

 chinery vibration. Conversely, the ideal propeller for the machinery system leaves much to 

 be desired from the point of view of hull vibration. 



Longitudinal vibration of the machinery system, has also been a problem in some clas- 

 ses of submarines. As a result, renewed emphasis is being placed on the understanding and 

 control of longitudinal vibration of machinery systems and its influence on the response of 

 the ship structure. 



2. Rotational 



a. Torsional. Torsional vibration has generally been considered as strictly a machin- 

 ery problem. This is far from the truth when you encounter torsional vibration in propulsion 

 systems with large exciting forces, as is the case with diesel engine drives. When the excit- 

 ing frequencies of the machinery system coincide with hull natural frequencies, structural vi- 

 bration may occur. Cases have also been encountered where engine-excited torsional criti- 

 cals in the propulsion system result in high alternating thrust components as a result of the 

 torsional alternating load exerted on the propeller. Some of these cases have been sufficient- 

 ly serious as to cause failures of the main thrust bearing and serious vibration of the hull. 

 These vibrations were of engine frequency and not propeller frequency. 



The Model Basin has evaluated the torsional characteristics of proposed propulsion 

 systems in a number of cases and contributed to the general understanding of the value of 

 the "nodal" drive in the case of the geared turbine system. Our analog computer has been 

 developed to handle such problems and in the future, we expect to delve more deeply into 

 the subject. 



