Page 250 - Introduction to Naval Architecture

PROPULSION 255

correctly scaled pressure as is done in a special depressurised tomng tank
facility at MARIN in the Netherlands. The tank is 240 m long, 18m wide
with a water depth of 8 m. The pressure in the air above the water can
be reduced to 0.03 bar. The more usual approach is to use a cavitation
tunnel

Cavitation number
2
The value (p 0 - p v)/pV or (p 0 - p^)/\pV^ is called the cavitation
number. Water contains dissolved air and at low pressures this air will
come out of solution and below a certain pressure, the vapour pressure
of water, water vapour forms. Hence, as the pressure on the propeller
blade drops, bubbles form. This phenomenon is called cavitation and
will occur at a cavitation number given by:

cavitation number, <j = (p a - e)/\pV*

where e is water vapour pressure.
The actual velocity experienced, and the value of p 0, vary with
position on the blade. For a standard, a representative velocity is taken
as speed of advance of the propeller through the water and p 0 is taken
at the centre of the propeller hub. For a local cavitation number the
actual velocity at the point concerned, including rotational velocity and
any wake effects, and the corresponding p 0 for the depth of the point
at the time must be taken. Blade elements experience different
cavitation numbers as the propeller rotates and cavitation can come
and go.

Occurrence and effects of cavitation
Since cavitation number reduces with increasing velocity cavitation is
most likely to occur towards the blade tips where the rotational
component of velocity is highest. It can also occur near the roots, where
the blade joins the hub, as the angle of incidence can be high there.
The greatest pressure reduction on the back of the blade occurs
between the mid-chord and the leading edge so bubbles are likely to
form there first. They will then be swept towards the trailing edge and
as they enter a region of higher pressure they will collapse. The collapse
of the bubbles generates very high local forces and these can damage
the blade material causing it to be 'eaten away'. This phenomenon is
called erosion.
Water temperature, dissolved air or other gases, and the presence of
nuclei to provide an initiation point for bubbles, all affect the pressure at
which cavitation first occurs. Face cavitation usually appears first near the
leading edge of the section. It results from an effective negative angle of
incidence where the wake velocity is low. This face cavitation disappears