Page 213 - Theory and Design of Air Cushion Craft

196 Trim and water surface deformation under the cushion

fl 0 = \l(Fn^ - 1)

a = arctan [I/ (Fn^ — 1) ] (5.6)

is the Froude number at the particular water depth, v the craft speed (m/s)
where Fn H
and H the water depth (m), see Fig. 5.8 below.
The wave-making in/beyond an air cushion running over shallow water can be esti-
mated approximately by means of Fig. 5.7 and equation (5.6). This method is rough,
but it is very practical. Figure 5.7 shows the calculation and test results. It may be
noticed that theoretical prediction and test results compare well. Figure 5.7 is suit-
able for conditions where Fn > 1.38 and IIH < 10, where 1 denotes the length of air
cushion.

O BH.7 test results

—• Shallow water theory

vl-JgH

Fig. 5.7 Comparison of non dimensional wave amplitude between the calculation by shallow water wave
theory and test results on BH.7.