Page 161 - Theory and Design of Air Cushion Craft

144 Stability

Table 4.1 The leading particulars of two craft models
Name Units Nomenclature Model No. 1 Model No. 2
Cushion length m / 1.61 2
Cushion beam m B c 0.35 0.492
Height of sidewalls m h s 0.056 0.167
Height of hard chine m h k 0.042 0.067
Width of sidewalls m B^ 0.05 0.067
Width of sidewall keel m fi. 0.015 0.025
Sidewall deadrise angle deg a 65 75
Flare angle of sidewall above hard chine deg ft 89 90
All-up weight N W 156 406
Cushion pressure N/m : p c 290 approx. 370 approx.

2. Determining the effect of the longitudinal stability keel on the static transverse
stability if this is fitted. This can be achieved by blocking the delivery duct to the
stability keel on the model or full-scale craft to stop it inflating.

The effect of fan speed of the models on transverse stability is shown in Figs 4.8 and 4.9.
Similar experiments may be carried out for trim (longitudinal stability). In this case,
the sidewall buoyancy has greater effect trimming bow up than bow down, due to the
finer bow form.

Effect of various parameters on transverse stability

It is not practical to study the effect of all the various parameters by testing because
of the amount of work involved. Therefore the research method at MARIC has been
to gather sufficient data to verify the foregoing calculation method with aid of model
experiments. We can discuss the effect of some of the remaining parameters on the
transverse stability by using the calculation methods.
Effect of sidewall geometric configuration on the static
transverse stability
Taking the SES model 717 as an example, we calculate the static transverse stability
of the craft with a specific fan speed, duct configuration and form of the seals, and

(fl)

Fig. 4.7 Typical transverse section of two craft models with different inner sidewall geometries, (a) Model
(b) model 2.