Spectral distribution of oscillation of a ship has a strong peak at its frequency of resonance. Then, index numbers of irregularity are as follows : (I) By Cartwright & Longuet-Higgins (Theory of maxima) ε^{( n )2}=1-ψ₀^{(2 n +2)2}/ψ₀^{(2 n )}ψ₀^{(2 n +4)} where, (-1) ⁿψ ⁽²ⁿ⁾ means r. m. s. of oscillation, and for amplitude n =0, for velocity n =1, and for acceleration n =2. Usually ε²>ε′²>ε″² so regularity increases toward higher time derivative. For narrow-banded spectrum ε²_??_ε′²_??_ε″²=0, but when it is widely distributed, ε²_??_2/3, ε′²_??_2/5, ε″ ²_??_2/7. (II) By Rice (Theory of envelope) α ⁽ⁿ⁾ = Ppⁿ √ (-1) ⁿψ₀ ^*(2n) where p =2π _fm is circular frequency of resonance and P is amplitude of resonance. (-1) ⁿψ₀ ^*(2n) causes slowly varying amplitude of oscillation and generally forms Gauss distribution above f =0. So that. ( a ″/ a ′) ²=1/3 ( a ′/ a ) ²· These results are summarized by the author's index number n ″ - n ′= n ′- n =1.