Page 172 - Theory and Design of Air Cushion Craft

SES transverse dynamic stability 155

Bcl2
AM = p c\y tan 0 - (z b - t^]
J (_- b - / bl)cot 0
3
3
= 1/3 p cc sca b tan 0 [B J8 - (z b - t b[) 3 cot 8]
- Q.5 Pcc Ka b(z b - - (z b - cotfl] (4.18)
where AM is the transverse restoring moment due to bow/stern skirts (N m) and y the
abscissa of the craft (m) (Fig. 4.20).
The block diagram for predicting the craft trim in motion is as shown in Fig. 4.21.

Calculation of the transverse stability of SES with rigid stern seal
The foregoing calculation procedure cannot be used in the case of the rigid stern seal,
because the lift acting on the planing plate is so much larger than that on the flexible
skirts at same heeling angles, and leads to a trim moment to change the running atti-
tude, cushion pressure and other parameters, etc. The changing running attitude may
be obtained by means of an iteration method, from which the stern plate lift and
restoring moment on the craft can then be determined.
Since the end of the planing plate is close to the craft sidewall when heeling it can
be considered as a two-dimensional planing plate and the other end of the plate can
be considered as a three-dimensional planing plate. The lift of whole plate can also be
considered as the arithmetic mean of both two- and three-dimensional planing plates.
The transverse restoring moment due to the stern plate can be written as

^BJ2 2
AM = (0.5 p wv na s)[y tan 0 - (z s - t s[)]c sca,ydy
J (r s - / si)cot 6
3
3
2
= 1/3 (0.5 /? wv 7ra s)c sca s tan 0 [£ c /8 - (z b - f bl) 3 cot 0]
- 0.5 (0.5 Pw (4.19)
where AM is the restoring moment due to the stern plate of the craft at heeling (N m)
and 9 the heeling angle (°).

Lower edge of stern seal, or
bow skirt seal

Heeled
water line

Fig. 4.20 Calculation for righting moment of bow/stern seal during heeling of craft.