Page 203 - Theory and Design of Air Cushion Craft

186 Stability

80 -

Plough-in region

40 -
operating
I Safely region

20 40 60
v s(kn)

Fig. 4.53 The plough-in boundary of SR.N6.
circular motion of water particles 11 knots and speed of tide flow was about 4 knots.
It is clear that part of the fault was due to incorrect craft navigation, but at least it
teaches us a lesson that while it may seem safe to navigate a craft along the beach, as
a matter of fact it is not safe due to the high surf which could act on the craft to cap-
size it in heavy weather.
Following this accident, the UK government set up an investigation, which
included a major study of hovercraft stability including parametric model tests of
several current large craft designs to identify possible improvements, particularly to
the skirt systems [48]. This study produced much useful information and led to the
development of the bulbous bow skirt, the tapered skirt and revised cushion feed
designs. Some of these items have been introduced above, the remainder will be
described in Chapter 7.

Wave length 21.3m—|

Wind speed
60kn Wave height
4.6m

Orbital speed of
wave particles=l Ikn
(b)

Fig. 4.54 The overturning of SR.N6 in very steep beam waves.