Page 127 - Theory and Design of Air Cushion Craft

Sidewall wave-making drag 1 1 1

1,9 SWewall

Equivalent cushion beam method

SES with thin sidewalls create very little wave-making drag, owing to their high
length/beam ratio, which may be up to 3CMO. To simplify calculations this drag may
be included in the wave-making drag due to the air cushion and calculated altogether,
i.e. take a equivalent cushion beam B c to replace the cushion beam B c for calculating
the total wave drag. Thus equation (3.1) may be rewritten as
R w=C wp;BJ(p wg) (3.31)

where R w is the sum of wave-making drag due to the cushion and sidewalls, C w the
coefficient of wave-making drag, C w = f(Fr b \JB C) and B c the equivalent beam of air
cushion including the wave-making due to the sidewalls.
The concept of equivalent cushion beam can be explained as the buoyancy of side-
walls made equivalent to the lift by an added cushion area with an added cushion
beam. The cushion pressure can be written as

where W s is the buoyancy provided by sidewalls and W the craft weight. Then the
equivalent cushion beam can be written as
W W B
B c = — = - - - = - ^ - (3.32)
i
Pc c [(w-wy(i Bj\i c \-wjw
c
The method mentioned above has been applied widely in China by MARIC to design
SES with thinner sidewalls and high craft speed and has proven accurate. Following
the trend to wider sidewalls, some discrepancies were obtained between the calcula-
tion and experimental results. For this reason, [29] gave some discussion of alternative
approaches.
Equation (3.31) can be rewritten by substitution of (3.32) into (3.31), as

A
B r

(3.33)
C 1 - WW
Where R wc is the wave-making drag caused by the air cushion with a beam of B c and
without the consideration of wave-making drag caus_ed by sidewalls, C w the coefficient
due to the wave-making drag with respect to Fr, IJB C
= f IJB C)
C w (Fr b
and C w the coefficient due to the wave-making drag with respect to Fr, IJB C

C w = f(Fr hlJBJ
The total wave-making drag of SESs can now be written as