Page 207 - Theory and Design of Air Cushion Craft

190 Trim and water surface deformation under the cushion

necessary to adjust the sidewall lines over the stern half of the vessel to ensure intakes
and propellers stay fully immersed.

Internal stability skirts

The design of longitudinal and/or transverse stability skirts inside the cushion of an
ACV strongly affects dynamic trim. The deeper these skirts, the larger the water drag,
due to skirt wetting in the complex internal cushion wave pattern. The shallower these
skirts are, the less effective they are. Determination of the optimum for a particular
craft is only practical through parametric model tests in a towing tank, or trial and
error with a prototype, which is likely to be rather more expensive.

Basic concepts for design

Misunderstanding of some basic concepts may lead to incorrect choices being made
for craft design, trials and analysis. For example:
• Does the trim characterized by outer drafts of the craft at bow and stern represent
the apparent or real trim angle of the craft?
• What is the relation between the trim angle formed by the outer water surface and
the trim angle formed by the inner water-line?
• Is the craft's trim drag defined by the trim angle at the outer or inner water-line?
• What is the relation between the trim drag and wave slope induced by moving cush-
ion pressure?
These problems are not immediately obvious without some practical experience of
ACV behaviour and in some cases have been inaccurately described in the technical
literature.
It can be seen that determination of ACV dynamic trim at different speeds is some-
what complicated and should be carefully dealt with during the design process. Based
on a clear understanding of the basic concepts, one can solve the design problem by
the method of initial predictions using theoretical analysis, later correlated with
experimental testing.
In order to understand the interaction of craft dynamic trim with the cushion inner
water surface, the water surface inside the cushion can be observed either by periscope
in a model test [52], or by direct observation via a transparent window on a craft side-
wall. This has been carried out in SES model 713. The outer water surface can be
determined by photos as shown in Fig. 5.1.

5.2 Water surface deformation in/beyond ACV air cushion
over calm water

ACV moving over deep water

When an ACV hovers statically on water, a depression will be formed between the
inner and outer water surface, the depth of which will be