Page 201 - Theory and Design of Air Cushion Craft

184 Stability

4. Increase the bag-cushion pressure ratio of skirt bags and the bow skirt separately
if the bag is divided, to increase the stiffness of the bag of the bow skirt.

(B) Skirt systems
1. Install diaphragms in a D-type skirt bag at the bow to enhance the ability to pre-
vent tuck-under. Figure 4.51 demonstrates the experimental results of a skirt
carried out on a water circulating tank, which predicts in quality the effect of the
tightness of a D-type bag and its diaphragms on the plough-in resistance of the
craft. This is one of the general measures for plough-in resistance which is cur-
rently widely adopted in China.
2. Decrease the drag due to the contact of the skirt fingers with the water surface to
prevent tuck-under of the bow skirt fingers.
A large number of small air lubrication holes were fitted at the lower hem of the
bow bag of the British SR.N6, so that air leaking from the cushion through the holes
and along the outside of fingers at the water surface lubricated the contacting sur-
face of the finger with the water surface and reduced the drag dramatically at a neg-
ative pitching angle, reducing the tuck-under of the fingers as shown in Fig. 4.52.
This idea has not been developed further, as BHC redesigned the bow bag of
later skirts, moving the fingers forward and increasing the bow bag radius (the bul-
bous bow bag). This skirt geometry creates increasing stability moments as the bow
is trimmed down and so has 'safe' plough-in characteristics - see Chapter 7.
3. Careful manufacture of the skirt, to give an even bag or loop hem line and even
segment or finger tip line (tidy the skirt geometry) decreases the dynamic drag,
especially at the rear corners and reduces uneven loads within the skirt, which can
contribute to ACV instability.
4. Adopt skirt-lifting equipment to control the height of the stern skirt, and so the
trim angle of the craft.

(C) LCG Include ballast tanks, horizontal fins and duct valves, etc. to control the
trim angle of the craft, especially in the case of travelling downwind, to increase the
ability to trim bow up dynamically.

Tuck under region

Tight diaphragm
Loose diaphragm
Non D type bag diaphragm

Fig. 4.51 Influence of craft speed, immersion height of skirt fingers h war\6 the tightness of diaphragm of D-
type skirt bags on plough-in of craft.