A Study ot Prevention of the Vertical Vibration at a Point Near the Stern

Abstract

In this paper, a technical investigation to prevent ship vibration by adding the other vibration is described, and the theoretical calculation made by an electronic computer is compared with the results of experimental works. An acrylite box floating on water was excited by an electromagnetic vibration generator, and this vibration was controlled by the other electromagnetic suppressor with the same frequency. The phase and the magnitude of the force originating from the suppressor and its position were changed so as to minimize the vibration at a point near the stern. An equation expressing the vertical vibration and its prevention, EI(∂^4Z)/(∂x^4)+ρA(∂^2Z)/(∂t)=δ(x-P_E)T_Ee^<iωt>+δ(x-P_S)T_Se^<i(ωl+ψ)> is presented, and its solution was theoretically obtained as follows, Z=G_e^<i(ωl+ψ)> This solution was used for the numerical calculation made by the electronic computer. The results were found to coincide with the experimental observation. In this study, the following conclusions can be summarized ; 1) When a beam model is tested as a ship for the vibration of six nodes or less, the suppressive force works satisfactorily to make the vertical vibration smaller at a point near the stern by selecting the position of the suppressor, the phase difference and the magnitude of the force generated by this suppressor. 2) When its magnitude is kept costant, the phase of the suppressive force is variable, not always π, and we can select the phase to minimize the vibration at a point near the stern. 3) Even when the phase selected by the procedure above mentioned is fixed, the magnitude of the suppressive force is not always same as that of the excited one, and we can select the magnitude of that forec to minimize the vibration on that point. 4) The suppressive force is advisably added at an edge of the stern or of the stem. When the force added at the loop of the excited vibration, it is a little less in amount than above to bring about the same effect. 5) When the number of node of the vibration is small and the frequency of the suppressive force is near or equal to the resonance frequency of the hull, the vibration can be bodily diminished. On the other hand, when the number of node is large, the vibration on the stem is increased. 6) Although the vibration of the hull is minimum at a point of the stern, it does not always small in amount at the other parts of the hull. Generally, it increases as the frequency of the vibration differs from the resonance frequency.