Page 205 - Theory and Design of Air Cushion Craft

1SS Trim and water surface deformation under the cushion

Trim
Trim is influenced by many cushion characteristic parameters, for example:

• position of cushion LCP;
• bow/stern skirt clearance (air gap) over the base line;
• cushion pressure ratio of air supply from lift fans and thus skirt stiffness;
• position of LCG based on distribution of craft mass, payload and ballast;
• position of the thrust line and thus dynamic trimming moments.

Bow and stern seal interaction

The inner water surface at bow and stern seals will influence skirt drag and trimming
moment, particularly in the case of craft take-off through hump speed.

Wetted surfaces

The geometry of inner/outer water surfaces will directly influence wetted surface and
friction drag of sidewalls (SES) and side skirts (ACV).

Location of SES inlets and appendages

The design and location of water-jet propulsion inlets, cooling water inlets, propellers,
rudders and stabilizer fins, are all influenced by the shape of the inner/outer water sur-
face. At the same time all of these items introduce thrust or drag forces affecting the
craft's dynamic trim.
Some early SES projects at MARIC suffered a lot from imperfect selection of
water-jet inlet locations. The water-jet propulsion inlet of SES model 717 and water-
cooling pump of SES model 713 were not ideally positioned when first built. Due to
lack of knowledge about the inner/outer water surface shape, MARIC located the
inlet of the water-cooling pump of SES 713 inside the air cushion and the inlet of
water-jet propulsion of SES 111 at the outer wall of the sidewalls. Air was ingested
into the inlet of both these systems in the course of take-off through hump speed.
On SES model 713, the air ingesting into the cooling water pump led to air block-
age of the system and interrupted the circulation of the cooling water. Thus the tem-
perature of cooling water rose rapidly, sometimes up to 95°C, which was very
dangerous for the engines. As for SES model 717 with water-jet propulsion, the craft
sometimes did not pass though hump speed due to air ingestion into the water-jet
pump, decreasing thrust. Both these problems almost became stumbling blocks for
SES development in their early phase of research in China, arising from lack of
knowledge concerning the dynamic trim of ACV/SES.
Figure 5.1 shows a picture of the inner and outer draft of model sidewalls taken
from a towing tank model. Figure 5.2 shows the deformed water surface inside the
craft cushion, obtained by the theoretical calculation. It may be noticed that a large
hollow in the water surface at the rear and centre parts of the cushion occurs at
Froude numbers close to 'hump speed', the transition between displacement mode
and planing mode of operation. It is this that caused the air ingestion which happened