Page 188 - Theory and Design of Air Cushion Craft
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Factors affecting ACV transverse stability 171
AM/A6>
.mpartment
20 60 100
Fig. 4.36 Influence of air cushion compartmentation on static stability.
where A# is the heeling angle (for a small value) (°), AM the restoring moment (N m),
h the initial transverse metacentric height (m), PFthe craft weight (N) and B c the cush-
ion beam (m).
K/K 0 (in Fig. 4.36) shows the effect of the depth of the stability skirt on transverse
stability. It can be seen that the stability of an ACV without cushion compartmenta-
tion will be reduced to about 60% of that with the + type of cushion compartment.
Reference 47 demonstrated experimental results such that the transverse stability
would be decreased to 59% in the case of removing the longitudinal stability skirt and
the longitudinal stability would be decreased to 68% in the case of removing the trans-
verse stability skirt.
It is clear that this investigation is very important and plays a key role in analysing
the stability of an ACV. Since transverse and longitudinal stability skirts are normally
situated inside the cushion and are very difficult to repair and maintain, it is more
convenient to remove stability skirts if at all possible. While this is now practical, it
has take some years for skirt design technology to advance to the point where
satisfactory stability can be provided (see Chapter 7).
Figure 4.36 also shows that the transverse stability for the craft with T type com-
partment will be decreased to 85% of that with + type compartment.
Effect of stability skirt clearance h e] on the transverse stability
Stability skirt clearance has a direct effect on the effectiveness of cushion compart-
ment and transverse stability. Figure 4.37 shows that the role of the cushion com-
partment will decrease as the stability skirt clearance /z eg increases to larger than 30%
of cushion depth at CG. In general, /z eg is equal to 10-20% cushion depth at CG, but
stability skirts of + type are more sensitive than those of T type.
Effect of VCG on transverse stability
VCG has a clear effect on transverse stability of craft. Figure 4.38 shows that VCG
is directly related to height of skirts, therefore one has to make a comprehensive
