Page 100 - Theory and Design of Air Cushion Craft

3

Steady drag forces

3.1 Introduction

ACVs and SESs create drag forces as they move over the water surface. The most
important drag components are those due to friction with immersed components such
as sidewalls, skirt, propellers, rudders and other appendages; and wave-making drag
from the moving cushion pressure field and sidewalls. In addition, momentum drag
due to acceleration of the air used for the supporting air cushion, and aerodynamic
profile drag of the ACV or SES become important components at higher speeds.
In this chapter we will outline the theory behind these drag components and
describe methods for their estimation.

3.2 Classification of drag components

The method of calculating drag forces on an ACV or SES is similar to that for pre-
dicting the drag of a planing hull or a sea plane before take-off. ACVs and SES also
generate spray drag, skirt friction drag and skirt inertia drag in addition to the water
drag components associated with a normal ship. For this reason drag calculations are
more complicated than for other marine craft.
Based upon calculation methods for predicting the drag of a planing hull, the prin-
cipal author and colleagues at MARIC have developed a methodology for predicting
the drag for ACV/SES which may be summarized as follows:
• First of all we obtain the total drag from model tests in a towing tank and some
other main components of drag by means of reliable and practical methods, e.g.
according to the Reynolds analogue theory to obtain the test results in wind tun-
nels for predicting air profile drag.
• Then the residual drag of models can be determined by deducting the main
components of drag which can be calculated individually, from the total drag of
the model measurements.

According to Froude's analogue theory we can define the residual drag of full-scale
ships from that of models; consequently the total drag for a full-scale ship can be