Primary resonant response of a folding rudder with gaps was investigated. The vibration equations of a folding rudder with gaps both in up-down and pitch directions were established. Harmonic external excitation forces with same frequency were considered in these two directions of motion. The primary resonant periodic solution of the system was analyzed by the average method. Combining with numerical solution

the amplitude-frequency response curves were presented

and the influence of gap

external excitation force

preload on primary resonance of rudder were discussed. Influences of gap

external excitation force and preload on the primary resonance of folding rudder were investigated. The results show that

as the gap increases

the overall main resonance response curve shifts leftwards; non-linear characteristic is more prominent

amplitude jump and hysteresis amplitude appear. With the increase of external excitation force

the proportion of response in the gap range is reducing in line with the nonlinear effects

the vibration characteristic of the system is nearly linear; with increase of fixed preload

the resonance shape changes greatly.