Page 182 - Theory and Design of Air Cushion Craft

Calculation of ACV transverse stability 165

Separated bag or cell
Figure 4.29 shows the skirt configuration of the US ACV type JEFF(A). Since the air
supply for left- and right-hand cells is separated, the pressure for the side heeling
down will be increased in the case where the craft is heeling, and the pressure at the
other side will be decreased, consequently causing a restoring moment. The French
multi-cell skirt system (called the 'jupe' skirt) possesses the same effect as the JEFF(A)
skirt system except that each jupe creates a moment independently.

Fig. 4.29 Influence of pericell type skirt on craft stability.

Skirt lifting or shifting systems

The skirt shifting system and its principle of action was developed by Hovercraft
Development Limited of the UK. The skirts might be shifted in the transverse direc-
tion to change the centre of pressure subsequently, to cause righting or heeling
moments as shown in Fig. 4.30. Such systems have been mainly applied to move skirts
side to side, particularly to allow a craft to bank into a turn. The system is convenient
to install on a loop and segment skirt with the same pressure in the loop as the cush-
ion, or with slight overpressure, 5-10%.
The British Hovercraft Corporation developed a simpler system for their bag and fin-
ger skirts whereby the segment is lifted, heeling a craft opposite to the external overturn-
ing moment. The geometry of a bag and finger skirt was found easier to deform by lifting
and the effect was similar to that of the loop and segment skirt transverse shift system.

Fig. 4.30 Skirt with transverse shifting system for improving the transverse stability.

The transverse shift of centre of cushion area

The centre of cushion area may be shifted in order to produce a restoring moment, as
shown in Fig. 4.31. When an ACV is heeling, the centre of cushion area will shift to
the side which is heeling down (from C to C' in Fig. 4.31) to offer a restoring moment.