Page 187 - Theory and Design of Air Cushion Craft

170 Stability

Transverse stability of an ACV without the cushion
compartmentation

An ACV without the cushion compartmentation can also provide a positive restoring
moment during heeling of the craft. This is due to the skirt finger with so small an
inclination angle as to change the centre of cushion area and provide a positive restor-
ing moment, as the skirt contacts the supporting surface during heeling of craft.
Figure 4.35 shows a typical static transverse stability curve of an ACV without com-
partmentation.
With respect to ordinary ACVs, the restoring moment is due to the follow-
ing factors:
1. The restoring moment is due to the cushion compartmentation inducing different
pressure in different cushions. This is called compartment stability.
2. The restoring moment is due to the change of cushion area, either arising from
the inclination angle of skirt fingers, or from the deformation of the skirt in
the horizontal direction. This is called area stability or stability due to ground
contact of the type of cushion compartment on static transverse stability of an
ACV
Figure 4.36 shows the effect of three types of cushion compartment on static
transverse stability, in which / g denotes the length of longitudinal stability skirt
(measured from stern); therefore, // c = 0 denotes that without cushion compart-
/ g
ment; / g// c = 0.6 denotes compartmentation of T type, and / g// c =1.0 denotes +
type compartmentation.
The criterion for transverse stability can be written as

where 5" R is the relative shifting distance of centre of pressure per unit heeling angle.
This criterion is equivalent to the relative height of initial stability:
h = h/B c = [AM/A0 X 57.29] / [WB C] = S R X 57.29 (4.27)

M(x9.8N«m)
0.8
On rigid surface

3 6>(°)

Fig. 4.35 Static transverse stability of ACV models with no air cushion compartmentation.