Cavitation on Small Commercial Propellers
A HydroComp Technical Report
Product link: NavCad,
Cavitation occurs when pressure surrounding
the propeller dips below the water's 'vapor pressure'. This causes
the water to produce bubbles or cavities of water vapor - typically
at the face, tip, or back of the propeller.
Face cavitation usually occurs only
on propellers with uncharacteristically low P/D ratios at high vessel
speeds. A major threat to propeller corrosion, excessive face cavitation
is caused by a negative blade angle of attack. This is generally
indicated when the P/D ratio is less than , or close to, the advance
Tip cavitation is typically indicated
by excessive tip speeds. Tip cavitation generally does not affect
thrust, but can produce noise and contribute to blade corrosion.
Finally, back cavitation appears in
heavily loaded propellers, and is the principal cause of blade corrosion
and thrust loss. Back cavitation is indicated by excessive blade
pressure (too much lift) or cavitation percentage, as well as a
blade area ratio less than the recommended.
The cure for cavitation depends on
the type. For face cavitation, often the P/D ratio is too low for
the RPM and ship speed. With tip cavitation, typically the RPM is
too high. Most importantly, back cavitation generally indicates
that there is too much power being pushed through the given propeller.
So, reducing power, increasing diameter or blade area, changing
air-foil to flat-faced blade sections (low RPMs only), or reducing
the P/D ratio (possibly with cupping) may all help.
Required blade area
Blade area requirements and recommendations
are generally based on cavitation concerns. Necessary amounts of
blade area can be determined using a number of different criteria.
In fact, the multitude of different ways of determining blade area
seem to be causing no small amount of confusion. Just remember a
couple of points - most of these criteria produce more-or-less the
same recommended blade area regardless of the approach taken, and
a little difference in blade area will not make or break an application.
Theoretical performance is not greatly
affected by blade area. In general, a lower blade area produces
high efficiency (and better performance). So, the goal in selecting
a blade area ratio is to use as little blade area as possible, but
enough to control cavitation. A majority of the commercial (stock
fixed-pitch) propellers come in predefined blade area ratios, so
the choice of blade area is usually easy to make.
There are three back cavitation checks
- blade pressure, percent cavitation and recommended blade area
ratio. Keeping in mind that a little difference in blade area ratio
will not significantly degrade an installation, the user must look
at all three criteria and make an informed decision if a particular
blade area is adequate. If all three criteria are blinking madly
and the pressure is up to 20 psi, there is 50% cavitation and the
recommended blade area is twice what you have, then it is no real
stretch to assume that you need more blade area! On the other hand,
if the pressure and percentage cavitation is acceptable, and the
recommended value is 10% higher than your value, you should be in
good shape with the blade area you have chosen - particularly if
you use a propeller model that has other attributes which help to
limit cavitation, such as skew. For those situations between the
two, judge accordingly.
Some cavitation exists in virtually
every installation. Fortunately, small amounts of cavitation exhibit
no significant effect on performance.
Once a propeller reaches about 15%
cavitation, the breakdown of thrust begins. There is little loss
of the thrust-making capability of the propeller until about 30%
back cavitation. Significant thrust loss can be expected to begin
about 30% or so.
Of course, there are other detrimental
symptoms of cavitation, like vibration, erosion and noise that degrade
performance through secondary effects.