Page 171 - Theory and Design of Air Cushion Craft

154 Stability

Craft bottom
/ / / / / / ( t / / / /
a s

Base line of
t - sidewall

Base line of
sidewall

(b)

Fig. 4.19 Geometry of stern seals: (a) planing stern seal; (b) twin bag skirt.

where L ss is the lift acting on the stern skirt (N) and a s the declination angle
between the lower base-line of stern seal and sidewalls (°). The calculation is also
similar for triple-bag SES stern skirts.
4. Two simplified added equations can be adopted from equation (3.12),

? ?
bi ~ bo
* W = W(t M-tM (3.12)
The wave-making drag R w can be calculated by the methods described in Chapter
3, and then the running attitude of the craft may be obtained using the foregoing
equations.

SES transverse stability on cushion in motion
It is not difficult to define the transverse stability moment and lever arm after deter-
mining the SES trim. Two conditions of the craft can be analysed as follows.

Calculation of transverse stability for the SES with flexible
bow/stern seals
In the case of an SES with flexible bow/stern seal, it can be assumed that the restor-
ing moment acting on the craft running on cushion and heeling is equal to the sum of
the heeling moment caused by the air cushion and the restoring moment due to side-
walls and both bow/stern seals. Considering that the length/beam ratio of the side-
walls is very large, normally 34-50 in fact, the dynamic lift due to the sidewalls is very
small and can be neglected, thus the restoring moment can be calculated as follows.
When (z b - O > 0 '