Page 118 - Theory and Design of Air Cushion Craft

102 Steady drag forces

(3.18a)
where PFis the weight of the craft (N).

Drag of bow/stern seals of an SES, fl sb, R s

This is also estimated based on test data. Two methods for predicting this drag are as
follows.

MARIC method [27]
Based upon a series of model tests, a statistical analysis was carried out at MARIC.
Putting the skirt (bow/stern) drag, the drag due to the differential momentum from
the bow/stern seals, the wave-making drag of sidewalls, and the interference drag due
to sidewalls, all into a single term 'residual drag' and then processing this by regres-
sion analysis, the following equation is obtained:

R r = C r' (3.19)
where R r is the residual drag of an SES (N), C/ the residual drag coefficient and h c the
water surface depression induced by air cushion pressure.

C/ may be obtained from Fig. 3.16. An intermediate value might be chosen. The lower
value relates to the better performing seals (making the flow of air leakage under the
stern seal significantly larger than that under the bow seal), and the better running
attitude of the craft. In contrast, the larger value relates to a skirt with no designed rear
gap at level trim, or the craft trim being more bow up, for example due to a service
requirement for open sea conditions rather than coastal or protected waters.
This experimental expression is based upon the following conditions:
= 3.5-4.0 = 15-18.5 kgf/m 3 0.7 < /r, < 1.2
IJB C p cll c

0.08

0.06

0.04

0.02

0.6 0.8 1.0 1.2

Fig. 3.16 Variation of seal drag coefficient C, with Froude Number.