Page 117 - Theory and Design of Air Cushion Craft

Skirt drag 101

Fj the jet velocity from the stern cushion, / the thickness of the jet from the stern
cushion and cr t the surface tension of water.
The water friction of the stern skirt will decrease due to two-phase flow through the
gap under the segment tips or bag lower point, since the turbulent air flow creates
dense spray.

Skirt pressure drag, /? sp

This may be written as
(3.15)
As mentioned above, the various components of skirt drag, such as friction drag in
two-phase flow, the inertia drag of the skirt due to the flutter of the skirt fabric and
spray drag of skirts at both sides of the craft, are difficult to calculate. For this reason,
the total skirt drag is best estimated by experience-based formulae [25, 26], as follows:
(3.16)
+
(3.17)
(3.18)
C sk2 = {[2.8167

where R sk is the skirt total drag, /? skl the wet drag of the skirt, R sk2 the wave-making
drag due to the skirt, h the average clearance for air leakage where h = Sj/lj in static
hovering mode, where S- } denotes the area of air leakage under the skirts, /, the total
peripheral length of the skirts, including the delta area for air leakage at the tip of
finger in the case of using bag and finger type skirts, R w the wave-making drag due
to the air cushion, S c the cushion area, q w the hydrodynamic head due to craft speed
= 0.5/? w v", C skl the coefficient for hydrodynamic drag, C sk = 2.5-3.5 or [1.35 +
0.112/? c// c], and C sk2 the coefficient due to wave-making drag of the skirt, obtained by
equation (3.16) or Fig. 3.15.
These equations were obtained from model experimental data. In fact equation
(3.17) can be written as

1.0

0.5

0 8 16 24 32 40 p c/l c

Fig. 3.15 Variation of skirt wave-making drag coefficient C sk2 with cushion length/beam ratio PJL C.