Page 179 - Theory and Design of Air Cushion Craft

162 Stability

Stability in waves
The SES should just be capable of surviving regular steepness limited waves (crest to
trough height - 0.14 X wavelength) with breaking (as opposed to plunging) crests, of
any individual height up to the limiting wave height, encountered beam-on while
using full available lift power and combined with:
1. A TCG equal to twice the maximum normal TCG.
2. A beam wind as specified in the design environmental conditions. Special consid-
eration of the safety margins would be required where this wind speed exceeds a
velocity (knots, at 10 m high) equal to
05
15 X £ c (in metres)
The limiting wave height shall be taken as 1.9 times the significant wave height
specified in the design environmental conditions.
An analysis of static on-cushion righting lever characteristics was conducted by
Blyth to provide a relatively simple calculation method [42], although it was found
that the minimum required properties of the curve vary substantially with hull con-
figuration, due to the dominant effects of forcing and damping characteristics. Other
acceptable methods of demonstrating compliance include mathematical simulations
and model tests.

Stability in turns
Since the behaviour of an SES in high-speed turns is very dependent on both speed
(which declines rapidly in tight turns) and the rate of turn achievable, the following
criteria should be met when the vessel is at approximately 45° change of heading, in
the test achievable turn, at a range of approach speeds within the operational range,
for each weight condition to be considered. Note that behaviour is not always most
critical at maximum speed.

1. The minimum net roll stiffness (expressed as minimum effective GMt) in the tight-
est attainable turn should always be greater than 5% of craft overall beam (Be).
This is equivalent to a percentage CG shift per degree of 0.087. This requirement
need not be met if the total roll restoring moment in the upright condition is equiv-
alent to an inward TCG greater than 2.5 times the maximum normal TCG, since
this is considered to provide a good reserve beyond the maximum roll moments
realizable in practice.
2. The net roll stiffness in a turn should not permit a greater outward heel angle than
(3 - Fn c) degrees when the maximum normal TCG is applied in an outward direc-
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tion where Fn c = cushion Froude number = VI(L c X g)° .
3. In order to avoid undesirable roll/pitch/yaw coupling effects, the hull form should
be such that when a roll moment is applied at speed, any bow-down trim angle
change should not exceed one-fifth of the heel angle.
Relatively simple calculation methods have been derived for assessment, but model
tests are also acceptable and in many cases desirable. Some full-scale trials will always
be required to demonstrate that the expected maximum rate of turn cannot be
exceeded.