SUB-COMMITTEE ON STABILITY AND LOAD LINES AND ON ...

Ref.: 391/09 and 520/09
Sig. wave length 113 m
Period 8.5 s
Head sea
BSU
Bundesstelle für Seeunfalluntersuchung
Federal Bureau of Maritime Casualty Investigation
Beam sea
Beam sea
Vessel
Figure 21: Sea state polar coordinate diagram, 113 m wave length and 8.5 s period
The above three polar coordinate diagrams show the calculated significant wave
height [m] necessary for the occurrence of a roll angle of 35° with a characteristic
period [s] and the significant wave length [m]. The vessel is steering to the north. The
waves are approaching from the direction indicated by the radial axis. The rings
indicate the speed of the vessel [kts] and the coloured legend shows the significant
wave height [m].
The result of all three calculations show that as long as the crew sailed the vessel in
stern seas, the wave heights required for reaching a roll angle of 35° are relatively
high (greater than 8 m). However, if the waves come from a sufficient incident angle
transverse to the longitudinal direction, larger roll angles of more than 35° would
definitely be caused by wave heights of 5 to 6 m, as was the case on the day of the
accident. The severe rolling motion is caused by the considerable roll moments
applied to the vessel; here, resonances or parametric excitation have no effect. The
slow speed on the day of the accident of about 2 kts also appears to play a role. To
illustrate that, a higher speed of up to 15 kts is considered for the period 9.5 s.
_____________________________________________________________________________________________________
Page 29 of 71