4.5 Alternative procedures

  4.5.1.1 In calm water the model was initially inclined up to heel angles larger than 25° and released with zero roll angular velocity. Four tests with different initial angles were conducted. As an example, the relation of φ_m (mean roll angle) and δφ (decrement of roll angle per half cycle) for a test is shown in figure 4.2.

  4.5.1.2 The measured nonlinear roll damping coefficient, N, as a function of roll amplitude is shown in table 4.1. The dependence of N coefficient on roll amplitude is small since the linear component (wave making damping) is small for this ship.

 To measure the effective wave slope coefficient r, two options are described in paragraph 2.6.1.2 of the Guidelines. Here the results of roll motion tests, mentioned in paragraph 4.4 above, have been used for s = 1/60 and the value r = 0.759 was obtained.

 By iterative calculation as described in 2.6.1.2 of the Guidelines (Third step), φ _1r was determined as 27.9° and thus φ ₁ = 0.7φ _1r = 19.5°, which is very close to the estimation of direct measurement procedure (section 4.4 above). One reason for this agreement is the almost linear characteristics shown by the GZ curve up to 20° (see figure 2.2).