Page 120 - Theory and Design of Air Cushion Craft

1 04 Steady drag forces

Friction drag is a large proportion (up to 3CMO% for high speed craft) of total craft
drag, as shown in Fig. 3.1. For this reason, calculation has to be made carefully. The
main difficulty is to determine the wetted surface accurately.
In general, two methods are used: one is the theoretical method as described in
Chapter 5 and the other is a model experimental method similar to that used in planing
hull design, i.e. the wetted area can be obtained by photographs. We will introduce four
different practical methods for predicting the wetted surface by experimental and sta-
tistical analysis methods.
With respect to the sidewall water friction drag, the following equation can be used:
(3.22)
f9w
where R^f is the water friction drag of sidewalls (N), C f the coefficient for the friction
of the plate, obtained by 0.455/[/ s Re]"' "' 5 (3.23), AC f the coefficient due to added rough-
3
ness of the sidewalls, between 0.3 and 0.4 X 10~ , Re the Reynolds number = v s/u, l s
/ s
the wetted length of sidewalls (m), the craft speed, (m/s"), v the kinematic viscosity
v s
2
coefficient (m /s) and S f the wetted surface of the craft running on the cushion (m ).
MARIC method [29]
The sidewall wetted surface is dependent upon craft trim. It is subsequently a function
of wave-making generated by the air cushion, the sidewalls and their interference with
each other, lift system characteristics, as well as the seal clearance over the base line
of the craft, etc. The theoretical method for prediction is therefore complicated. For
this reason, the simplest way to predict the wetted surface area may be by means of
measurement from photographs.
Figure 3.18(a) shows the outer surface of an SES model running at below hump
speed (Fr = 0.239), Fig. 3.18(b) shows that above hump speed (Fr = 2.15) and Fig.
3.19 shows the added wave-making due to the bow seal pushing water (described
in detail in the following paragraphs of this chapter). Thus, the curve showing the
relation between the wetted surface of inner/outer sidewalls and the Fr can be
obtained by the photographic method from model experiments.
Then the wetted surface of sidewalls of craft running on cushion can be written
as
= K, + (3.24)
S { S [0 K out S oul0
where K b K out are correction coefficients for the inner/outer wetted surface of side-
walls, which can also be written as

These coefficients can be determined by photographs or experimental results from
similar craft models. Also 5 io, 5 out0 are the wetted inner/outer surface breadth of side-
wall of craft hovering static (Fig. 3.20) (m) and S i} S OM the wetted inner/outer surface
area of sidewall of craft running on the cushion.