Page 125 - Theory and Design of Air Cushion Craft

Sidewall water friction drag 109

Based on model tests in their towing tank, the following method was obtained by
NPL:
= + AS f) (1 + 5 5 smax// s) (3.29)
S t (S a
where 5 smax is the max. width of sidewalls at design water line (m), AS f the area cor-
rection to the wetted surface due to the speed change (m") and S m the area of wetted
surface of sidewalls during static hovering (m ).
This expression is suitable for the following conditions:
8</7 c // c <16 and Fr\ > 1.2

Figure 3.25 shows a plot enabling A5" f to be determined within these conditions.

B. A. Kolezaev method (USSR) [19]

B. A. Kolezaev derived the following expression for sidewall drag:
Sf = Kf S m
where S f is the area of wetted surface, hovering static (Fig. 3.26), K f the correction
coefficient for the wetted surface, related to Fr (Fig. 3.27). S m can also be written as
below (see Fig. 3.26):
sin/T
21. T (B, (3.30)
COSy?

-0.5 -

Fig. 3.25 Correction coefficient of wetted surface area of sidewall vs Froude Number. [15]