Page 128 - Theory and Design of Air Cushion Craft

12 Steady drag forces

(3.34)
R wc + R^ w + R m
is the wave-making drag caused by the air cushion, the wave-making
where R wc R sww
drag caused by the sidewalls and R m the interference drag caused by the air cushion
and sidewalls. Therefore

^sww + ^wi = ^w ~~ ^wc (3.35)
as
2
p cBJ^g) (3.36)
R WC=C W
and
Pc=W- WJ(1 CB C)
Therefore
= g)][W- B c)f (3.37)
* wc [C w BJ(p w WJ(l c
If we substitute equations (3.36) and (3.33) into (3.35), we obtain
/-> n
R sww + K W1 = ' - R wc
c \-wjw

1
- 1 (3.38)
C, \-WJW
If R denotes the buoyancy of sidewalls and equals zero, then the whole weight of the
craft will be supported by the air cushion with an area of S c (S c = 1 CB C) and the wave-
making drag could then be written as

* Wco = [C w BJfa g)} [W/(l c B e) (3.39)
From equations (3.37) and (3.39) we have
*wc/* wco = (1 - WJW} 2 (3.40)
Upon substitition of equation (3.40) in (3.38) and using equation (3.39), then equa-
tion (3.38) can be written as
2
7? sw + R^ = * WCO [(C W/C W) (1 - WJW) - (1 - WJW} } (3.41)
The calculation results are shown in Fig. 3.28. It can be seen that the less the WJW,
the less the wave-making drag of the sidewalls (R,^ + ^ w), which is reasonable. The
greater the WJW, the more the wave-making drag of the sidewalls.
Figure 3.28 also shows that wave-making drag decreases as the WJW exceeds 0.5.
This seems unreasonable. The calculation results of [30] and [31] showed that wave-
making drag will increase significantly as WJW increases. Reference 32 also showed
that the wave-making drag of sidewalls could be neglected in the case of WJW <
15%.
The equivalent cushion beam method is therefore only suitable to apply to SES with
thinner sidewalls. It is unreasonable to use this method for SES with thick sidewalls
or for air cushion catamarans (e.g. WJW ~ 0.3-0.4) and for these craft the wave-
making drag of sidewalls has then to be considered separately.