Page 166 - Theory and Design of Air Cushion Craft

Static transverse stability of SES on cushion 149

h=h/B c

0.2

0.1

0.005 0.006 0.007 0.008 Q

Fig. 4.14 Variation of relative initial static transverse metacentric height with air flow rate coefficient Q.

Effect of the cushion pressure length ratio (p c /l c) on the
transverse stability
The calculated results of stability with various cushion pressure/length ratios are
shown in Fig. 4.15. It is found that the relative metacentric height h decreases from
0.133 to 0.123 when the cushion pressure/length ratio increases from 21.47 to 23.06
2
kgf/m .
Effect of the gap between the lower edge of bow/stern seals and
the base-line
To make a simple calculation of the transverse stability, at MARIC we assumed the
gaps between the base-line and the lower edge of bow/stern seals to be the same, to
calculate the transverse stability of craft type 711-3 with various inner drafts of the
sidewalls, z bs, by running the fan at different speeds.
The calculated results are shown in Fig. 4.16. Transverse stability increases with the
inner draft of the sidewalls, though the benefit is not greater than that obtained by
increasing the thickness of the sidewalls. This means that adjustment of the draft of
the sidewall is a good way to control transverse stability of a craft in operation.
This phenomenon can be traced back to the trials of craft type 717-III in 1969. At
the beginning, the craft operated quite well with satisfactory speed and transverse
stability. After some modifications, the all-up weight of the craft increased from 1.8 to
2.2 t, and the cushion pressure/length ratio increased from 19 to 24 kgf/m and it was
discovered that the transverse stability of the craft had deteriorated. The craft used to
roll slowly with a rolling angle up to 12° even in ripples. After increasing the inner
draft of the sidewall at the stern from 0.24 to 0.28 m the unstable rolling disappeared.