ICGSMA Bulletin - Indian Coast Guard

ICGSMA BULLETIN (JAN - MAR 13)
A supercavitating object is a high speed
submerged object that is designed to initiate a
cavitation bubble at the nose which (either
naturally or
augmented with
internallygenerated
gas)
extends past the
aft end of the
object,
substantially
reducing the skin
friction drag that
would be present
if the sides of the
object were in
contact with the
liquid in which the
object is
submerged. A key
feature of the
supercavitating
object is the nose,
which may be
shaped as a flat
disk or cone and
COMMON PAINT DEFECTS AND REMEDIES
(DO YOU KNOW?)
BITTINESS - Bittiness is caused by dirt from the atmosphere or the
surface or from brushes that are inadequately cleaned or due to
bits of dried-up paint that gets stirred in.
SOLUTION - Use clean brushes and paint on clean surface and
strain the paint through a cloth before use.
ALGAE/FUNGUS GROWTH - Algae and fungus can grow when
the surface is continually damp and dirty. Insufficient fungicide/
algaecide in the paint can also worsen the situation.
SOLUTION - Remove algae / fungus by high-pressure washing.
Wash again to remove any residue and allow the wall to
dry before applying with recommended paint.
BRUSH MARKS - Brush marks are caused due to under-thinning
of paints or due to poor application of the final coat of paint or
due to poor quality brush.
SOLUTION - Ensure paint of the right viscosity is applied using a
good brush.
may
be
articulated, but
which likely has a
sharp edge
around the
perimeter behind which the cavitation bubble
forms. The shape of the object aft of the nose will
generally be slender in order to stay within the
limited diameter of the cavitation bubble. If the
bubble is of insufficient length to encompass the
object, especially at slower speeds, the bubble can
be enlarged and extended by injection of high
pressure gas near the object's nose.
The great speed required for
supercavitation to work can be achieved
temporarily by a projectile fired under water or by
an airborne projectile impacting the water. Rocket
propulsion can be used for sustained operation,
with the possibility of tapping high pressure gas to
route to the object's nose in order to enhance the
cavitation bubble. An example of rocket propulsion
is the Russian
Shkval
multiple nozzles.
supercavitating
torpedo. In
principle,
maneuvering may
be achieved by
various means
such as drag fins
that project
through the
bubble into the
surrounding
liquid, by tilting
the nose of the
object, by
injecting gas
asymmetrically
near the nose in
order to distort
the geometry of
the cavity, by
vectoring rocket
thrust through
gimbaling for a
single nozzle or
by differential
thrust for
APPLICATION
Supercavition can be used in a variety of
marine applications. General application consists of
propellers for high speed torpedoes, propellers for
light vessels specifically designed to carry a small
group of naval crew at high speeds. The major
limitation in implementation of supercavitation on
a large scale basis is that the technology has not
yet been refined so that larger propeller blades can
be used. Also the introduction of water jet
propulsion has provided marine industry with a
simpler and effective mode of propulsion.
An insight to trouble shooters 5