Page 149 - Theory and Design of Air Cushion Craft

132 Steady drag forces
MARIC experience and ref. 39, it is suggested that the LCG of an SES (also an ACV)
should normally be in the range
- « -(0 ~ 0.5)% [aft of CO.]
(X g X C)/L C
is the LCG, measured from the stern, the longitudinal centre of buoyancy,
where X g X c
measured from the stern and L c the cushion length.

Effect of cushion pressure-length ratio

Wave-making drag increases in proportion to the square of cushion pressure. Cush-
ion pressure also affects the craft outer draft, therefore the cushion pressure seriously
affects the craft drag. The effect of cushion pressure on hump drag will be further
increased in the case of poorly designed skirt/seals. Figure 3.47 shows the effect of
cushion pressure of an SES model on drag, while Fig. 3.48 shows the effect of vari-
ous cushion pressure / length ratios on the drag of craft tested in a towing tank at dif-
ferent times.

Effect of inner draft

Hump drag will decrease in the case of increasing the inner draft of an SES during
take-off, because air leakage from the cushion is decreased amidships, decreasing the
cushion pushing/scooping drag of the bow/stern seals. The drag of SES will also
increase at the post hump speed in the case of increasing the inner draft, because this
will lead to an enlarged wetted surface of the sidewalls. Figure 3.49 shows the effect
of inner draft change on drag of an SES model running on calm water.

1 2 3 4 v,,,(m/s)

Fig. 3.47 Influence of cushion length/beam ratio and pj( on total drag of craft models. 1, 2, 3: see Fig. 3.44.