船舶横揺安全度示数の上下両限界に就て

抄録

We defined the stability creterions for ship's rolling as [numerical formula] where Sd: dynamical stability arm, δ_r: stability range, m: metacentric height. θ0: resonance amplitude of rolling. u0: wind velocity. λu^2: statical couple lever due to wind pressure, θ0 can be calculated by the following formula. θ0≒1.86√δ, β_s=gT_S/2πu0 with the aid of Sverdrup-Munks' δ-β curve for ocean waves and the ship's natural period of rolling T_S. In the surrounding districts of region of storm, there occur the fully grown-up regular swells by the constant prevailling wind, which may cause the ship' perfectly resonamced rolling. However, in the centre of the storm waves are confused and very irregular only gales blowing violently. Then the above cited two creterions must be applicable to the outer and inside region of storm respectively. From the another point of view, ship is always more or less drifting by the various weather conditions. Then the character of resonance wave may be changed compared with the case when the ship's position is stationary, namely u=u0(1-ε0), [numerical formula] Where u is the relalive speed of wind to the ship and uo is absolute wind velocity on the sea surface, then the drifting speed of the ship is u0ε0. Here appear three resonance waves. The other β=β_s/2[-1-√<1+4ε/β_S>] is omitted for the reason of negative valued. The former corresponds to C_1, and the latter to C_2, which tend to each other when wind velocity is low or high respectively, but the former and the next vanished beyond u0≧gT_s/8πε0. Of course the three creterions which is calculated from above equations lie always between C_1 and C_2 for every wind speed. Thns we have proved that the C_1 and C_2 is the lower and upper limit of creterion of a ship for every weather conditions.