Page 176 - Theory and Design of Air Cushion Craft

SES transverse dynamic stability 1 59

Principal dimensions and parameters of the craft:
= 3.5 m
B c
B^ = 0.13m
B swb = 0.06 m (width of the sidewall at bow)
HW = 0.42 m (height of the sidewall)
= 70° (deadrise of sidewall at bow)
a b
a = 60° (deadrise of the sidewall amidships)
S a = 14 m (frontal area of the superstructure)
a s = 9.23° (inclination between the stern plate and base-line)
x g = -0.47 m (aft amidships)
KG= 1.22m
= 0.50 m (height of thrust for water jet propulsion over base-line)
H s
Dynamic transverse righting moment of the SES
Figure 4.23 shows the transverse righting moment M g of SES type 717C hovering
statically and in motion. Figure 4.24 shows the components of dynamic righting
moments M 0 of the SES type 71 7C running on cushion. It is found that the relative
metacentric height hlB c of the craft in motion is larger than that hovering statically by
20-30%. This added righting moment is mostly provided by the stern planing plate
seal and it indicates that the craft type 7 1 1C is more stable when moving than when
static. This has been validated in practice.

Further investigation

Transverse stability of craft during take-off
As mentioned above, the transverse stability of an SES at post-hump speed can be
improved significantly with the hydrodynamic force on the stem seals of the planing
plate type. It will deteriorate in the case of craft at low speed, particularly at hump

750 -

500 -

250 -

0.5

Fig. 4.23 Transverse righting moment of SES model 717C at different forward speeds. 1: v= 26.5 kph, hi be
= 0.439; 2: v= 33.1 kph, hi be = 0.369; 3: v= 0, hlbc= 0.311.