船の横搖に対する摩擦抵抗について

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概要

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• The resistance to rolling is of great importance to investigate the safety of vessels in rough seas. The passive resistance due to the action of the water is mainly composed of three kinds, i.e., the frictional resistance, the eddy making resistance and the wave making resistance. Of these resistance only the wave making one was shown in the author's previous paper to follow the law of comparison for similar ships. It is therefore necessary to establish formulae for the frictional and eddy making resistancesin order to extend the results of model experiments to actual ships.In the present paper new formulae for the frictional resistance are obtained after analysing the results of experiments on the oscillations of suspended cylinders wholly immersed in the water. The frictional coefficient C_f for oscillating cylinders is found to be expressed in the form exactly same as that given by H. Blasius for laminar flow, if Reynolds number R_n be taken as 3.22r^2θ_m^2/Tv The decrement of roll per swing due to friction for a ship model without bilge keels in fresh water is given by the formula dθ_f=(2.11Sr_s^2θ_m)/(WmT^<1.5>) at 15℃ in kg, m, sec, deg, units : and the decrement for a ship model with bilge keels is written as dθ_f=(2.11Sr_s^2θ_m)/(WmT^<1.5>){1+(0.143(r_sθ_m)^<0.772>)/(T^<0.386>)} at 15℃, in which the frictional coefficient for turbulent flow is used from the following formula given by G. Hughes C_f=1.328R_n^<-0.5>+0.014R_n^<-0.114>The value of δθ_f for a ship in salt water is shown as δθ_f=(2.22Sr_s^2θ_m)/(WmT^<1.5>){1+(0.141(r_sθ_m)^<0.772>)/(T^<0.386>)} at 15℃.Though the mean radius r_s for the wetted surface may be calculated by using the formula r_s=√1/S∬r^2sin^2αdldx, it is easily and in close approximation obtained from the following formula r_s=1/π(0.877+0.145C_b)(1.7d+C_b・B)-2×<OG>^^^-}, where C_b is the block coefficient, d the mean draught, B the breadth of ship and <OG>^^^- the value of (d-KG).The ratio of δθ_f to the total decrement δθ at the mean angle of roll of 5 degrees for ordinary ship models 2m in length is ranging from about7 to 16per cent, while the ratio at the mean angle of roll of 20 degrees is from 3 to 7 per cent. This ratio for actual ships is found to be between one tenth and one fifth of that of ship models.

• 社団法人日本船舶海洋工学会の論文

著者

• 加藤 弘

  東京大学工学部

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