Page 119 - Theory and Design of Air Cushion Craft

Skirt drag 103

Rin-lchi Marao method [28]
Model experiments were carried out on an SES with thin plates as sidewalls and with
= 2 approximately, by Rin-lchi and Marao. The air profile drag was then deducted
l c/B c
and the aerodynamic momentum drag and wave-making drag obtained from analysis
of the measured wave profile. These components were then deducted from the total
drag of models to obtain the residual drag. The form drag of sidewalls can be
neglected due to the thin plate sidewalls used, so that the residual drag can be approx-
imated directly to the seal drag and written as

= (3.20)
# sk C sk B c h c q w
is the seal drag of an SES, C sk the coefficient for skirt (seal) drag, as shown
where R sk
in Fig. 3.17 and q w the hydrodynamic head of the craft at speed.
B. A. Kolezaev method
From reference 19, expressions for skirt drag can be written as
= (a + bFr d)B cPcv (3.21)
R sk
is the Froude number due to the volumetric displacement,
where Fr d
17 u ri 0.33x0.5
Fr d = v/(g D
3
and D is the volumetric displacement of the craft (m ), a the experimental coefficient,
0.00225 s= a ss 0.021, b the experimental or experience coefficient, 0.0015 ^ b ^ 0.0087
and v the velocity (m/s). Coefficients a and b are dependent upon the material config-
uration and aerodynamic performance of the seals.

0.24

0.16

0.08

0.04

0.4 0.8 1.2 1.6

Fig. 3.17 Seal drag coefficient vs Froude Number at optimum trim angle.