Page 126 - Theory and Design of Air Cushion Craft

110 Steady drag forces

Fig. 3.26 Typical dimensions for wetted surface of sidewalls.

Fig. 3.27 Correction coefficient of wetted surface area of sidewalls vs Froude Number.

where Tj, T 0 are the inner/outer drafts, hovering static (m), b s the width of the base
plate of the sidewalls (m), B s the width of sidewalls at designed outer draft (m) and /?
the deadrise angle of sidewalls (°).
A number of methods used for predicting the area of the wetted surface have been
illustrated in this section. It is important to note that one has to use these expressions
consistently with expressions by the same authors to predict the other drag compo-
nents, such as skirt drag, residual drag, etc., otherwise errors may result.
As a general rule, the methods derived from model tests and particularly photo
records from the actual design or a very similar one will be the most accurate. The dif-
ferent expressions may also be used to give an idea of the likely spread of values for
the various drag components during the early design stage.