Page 204 - Theory and Design of Air Cushion Craft

5

Trim and water surface

deformation under the

cushion

5.1 Introduction

Dynamic trim is determined by equilibrium of the steady forces acting on the ACV or
SES at speed. The trim will affect drag forces acting on the craft and therefore its abil-
ity to accelerate through hump speed to the operational cruising speed. The main pur-
pose of this chapter's investigation is therefore to enable estimation of craft trim at
various speeds and identify the optimum values.
The starting point for determination of equilibrium is the centre of pressure of the
cushion for an ACV and in addition the force vector which results from the hydrody-
namic force acting on the hulls of an SES or through the skirt. This may be compared
with the forces acting on the hull of a planing boat or catamaran.
An important difference from conventional ships regarding determination of the
hydrodynamic force and moment is caused by the inner water surface under the pres-
surized cushion. Conventional ships have only an outer draft, while the ACV and SES
have both inner and outer drafts. The inner water surface profile is very difficult to
observe, though a number of model experiments have been carried out to verify ana-
lytical models developed using classical hydrodynamic theory in the 1960s and these
have largely confirmed analysis.
The SES has water propellers or water jets and other underwater appendages which
will affect its dynamic trim in a similar way to a fast planing craft. The amphibious
ACV on the other hand normally has air propellers or fans which induce pitching
moments and fins and elevators or elevens to control dynamic trim. The ACV and
SES are also both affected by the trimming moments from aerodynamic drag and lift
of the hull and superstructure above the water profile.
Considering first the dynamic equilibrium of the cushion itself, the craft dynamic
trim, including inner and outer drafts and trim angle, is influenced by a number of
design and performance parameters of the craft as follows.