Page 202 - Theory and Design of Air Cushion Craft

Overturning in waves 185

no air lubrication
air lubrication
@ yaw angle=0

-1.0 -2.0 -3.0 -4.0 -5.0 -6.0 -7.0 trim angle

Fig. 4.52 Relation between the lift-drag ratio and trim angle of ACV.

(D) Driver technique Improve the drivers' technique for preventing the plough-in of
craft and understanding the operational boundary curve for avoiding plough-in. Figure
4.53 shows the operational boundary for preventing plough-in on British ACV model
SR.N6. Drivers have to take care during operation to avoid large drifting angles for
overturning.

(E) Hull design It is not possible to completely avoid the possibility of plough-in
on an ACV or SES. The designer should therefore investigate the attitute of the lower
hull in the case of a bow-on or side-on collapse of the leading skirt. The hull lower
plating, or the configuration of the deflated skirt drawn back over the hull structure,
should form a planing surface with between 5 and 10° trim to the water surface. The
moment arm of the resulting drag and lift forces from the planing surface should pro-
vide a righting moment sufficient to stabilize the craft at the roll or pitch angle from
the plough-in.

4.7 Overturning in waves

Accidents of this type have occurred to ACVs as shown in Table 1.4. SR.N6-012, run-
ning on a passenger route between Portsmouth and Ryde, Isle of Wight, in England,
overturned in March 1972, a result of the combined action of wind and waves. The
craft flooded, which led to capsizing due to the waves and the deaths of five passengers.
This is the biggest tragedy in ACV/SES transport history to date. Figure 4.54 [51]
describes the situation of the ACV at that time. Owing to the large winds and waves,
the driver decided to navigate the craft along the beach. However, the surf close to the
beach was very steep and acted with the wind and tide on the craft which had to run
in beam seas and this caused the craft to roll severely, the side skirt to tuck under at
the trough of a wave and then the craft capsized.
At that time the wind speed was about 60 knots, wave height 4.6 m, the speed of