Page 104 - Theory and Design of Air Cushion Craft

Steady drag forces

channel width greater than
10/ c and infinite depth

8 /B c
l c
Fig. 3.3 Cushion wave-making drag coefficient for a rectangular air cushion over calm water against L CIB C.

predicting the other components of drag also developed by the same authors, e.g.
when one uses the equation (3.4) for estimating the wave-making drag, it is better to
use this together with the other formulae offered by ref. 19 for estimating the seal drag,
sidewall water friction and the residual drag of sidewalls, otherwise the user may find
inconsistencies in calculation of the total resistance of the ACV.
Owing to the easy application and accuracy of Newman's method, MARIC often
uses Newman and Poole's data for estimating the wave-making resistance of craft. It
is evident from this work that the bow wave strongly interacts with the stern wave. The
lower the cushion beam ratio, the stronger the disturbance between the two compo-
nents. This causes a series of peaks and troughs on the resistance curve. With respect
to water with infinite depth, the last peak appears at Fr — I/A/TT = 0.56.
The theory mentioned above was validated by the experimental results carried out
by Everest and Hogben [20]. The theoretical prediction agreed quite well with exper-
imental results except at low speed. In this latter case, only two pairs of troughs and
peaks appeared in the test results rather than that in the calculation results. This can
be interpreted as follows:
• Hogben proposed that the wave steepness (hiA) at lower Fr predicted by linear
theoretical calculation exceeded the theoretical limit value of 1/7 between the
troughs and peaks, so that the surface geometry would be unstable, similar to a