Page 115 - Theory and Design of Air Cushion Craft

Skirt drag 99

of the skirt immersed in the water. The overall equilibrium between these forces
within the whole skirt and R sp, is the component along the ^--direction L s called skirt
pressure drag.
Skirt drag not only relates to the immersion depth of skirts and spray making, but
also the density of the skirt material. Therefore, it is very difficult to predict this
drag by theoretical calculation; it can only be estimated with accuracy by full scale
experimental methods or scaling from model test results.

Total skirt drag, /? sk

With respect to an ACV, skirt friction with the water surface is a large component of
total drag, owing to the high density of water, 800 times larger than that of air.
Most of the skirt makes only slight contact with the water, while at the stern and
the two stern corners of a skirt, segmented or fingered skirts may cause a large
amount of scooping drag at lower speeds (particularly below hump speed, or
Fr < 0.75). This can cause a particular problem for transiting hump speed if the skirt
geometry is unfavourable. In addition the craft trim can strongly affect scooping.
Above hump speed the skirt in the rear third of the craft is the most important for
determining skirt drag in a steady condition. Normally the rear skirt lower tip is
raised to minimize skirt drag.
Sometimes skirt drag will increase severely because of poor running trim, when
either the bow skirt contacts the water surface (LCG too far forward) or the corners
of the stern skirt scoop water (LCG too far back).
The skirt drag of a model tested in a towing tank will generally be less than that
experienced at full scale, around 35% of total drag, as the running attitude of the craft
can be regulated to be optimal. In contrast, the skirt drag for full scale craft will
increase to about 55% or more of the total drag in the case of unfavourable craft trim.
This drag level is generally not reproducible in the towing tank, so powering estimates
for craft need to account for this difference.
A short description of each component is now given below.

Skirt friction drag, /? sf
We take the bag-finger type bow skirt or open loop type as an example to analyse the
force exerted on the skirts and assume that the skirt fabric is perfectly flexible. That
means the skirt fabric will be flattened and in close proximity to the water surface as
the skirts make contact with the water surface as shown in Fig. 3.14; the deformation
and applied force on the skirts can be expressed as in [24].

L, + L 2 = [d + R(\ - cos #]/sin 9 (3.11)
where L\ is the arc length of the part of the skirt in contact with the water surface (m),
the flattening part of the skirt in close proximity to the water surface (m), R the
L 2
radius of curvature due to the bending part of the skirt (m), d the immersion depth of
the skirt (m) and 0 the declination angle of the skirts, (°).
Meanwhile the water friction of the skirt balances with the tensions in the fabric,
which can be written as