Norrbin 



Fig. 9. Motora's equivalent added mass coefficient as defined by- 

 acceleration due to step input impact of duration t 



amplitudes , whereas the frequency is kept as low as running length 

 permits , [ 32] . A typical reduced frequency co' = co • L/V will be 

 of order 0.5, corresponding to co" = v/u" , co" = o' • u" = 0.1 in 

 Fig, 8. The derivatives so obtained may be expected to be somewhat 

 higher than the zero-frequency values. 



The theoretical zero-frequency added mass values for two- 

 dimensional LEWIS forms as well as for semi-submerged ellipsoids 

 of finite lengths indicate the main dependence on principal geo- 

 metrical characteristics. Especially, for very large length-to- 

 draught ratios the ellipsoid values tend to those of a semi- elliptic 

 cylinder, (7r/2)pT^, so that - Y'J = T/B. Moreover, it will be seen 

 from [ 21] that for LEWIS forms in general - Yj| likewise is rather 

 close to T/B for fullness coefficients corresponding to midship 

 sections . 



The ellipsoid family has a constant prismatic coefficient 

 ^ = 2/3. The correction for finite length involves a slight dependence 

 on B/T, as may be seen from Fig. 10. In a more general case 



838 



