The Three Modes of Cavitating
A HydroComp Technical Report
Consider the following boat
– 37 feet (11 m) length, twin 330 hp (250 kW) engines, and 18
inch (460 mm) 3-bladed propellers. This is representative of a
large class of “sport fishing boats” that are intended to run
at speeds in excess of 20 knots. By all accounts, they perform
well for their owners.
Yet, when evaluating the
performance, or trying to size propellers for these boats, the
numbers just do not make sense. Why? Cavitation and lots of
Propellers begin to cavitate
when there is too much thrust for the propeller to carry. Virtually
all propellers for modern boats run with some amount of cavitation
– and some run with tremendous amounts. How can a heavily cavitating
propeller do its job? To answer this, we will need to look into
the three modes of cavitating performance – sub-cavitating,
trans-cavitating, or super-cavitating.
Suction face and pressure face forces
Propellers supply thrust
to a boat by transferring lift from the blades through the hub
and up the shaft line. Propeller blade lift is similar to the
lift that is found on any foil moving through a fluid – such as
an airplane’s wing. The following graphic shows the two contributors
to thrust – lift from the suction face (negative up) and pressure
face (positive up).
– Blade pressures and cavitation region
Inception of cavitation
When the magnitude of the
suction (negative) pressure exceeds the water’s vapor pressure
– that suction which triggers the change of state from liquid
to vapor – bubbles of water vapor begin to form on the blade surface.
You can see this cavitation region illustrated in the graphic.
As thrust increases, the
magnitude of pressure on both the suction and pressure faces increase.
The increasing suction means that the cavitation region will grow
and more of the blade surface will cavitate.
If the vapor bubble cavity
is small, the water flow over the blade is unchanged, and the
suction and pressure forces are unaffected. However, once the
cavity grows to a size that water actually separates from the
blade, water flows will change and suction face lift, total thrust
and efficiency are lost.
If cavitation is so low that
it does not affect water flow and total thrust, it is sub-cavitating.
As thrust is lost due to increasing cavitation and flow changes,
it is in a trans-cavitating mode. The point where cavitation
is so extreme that the water flow has fully separated from the
suction face is called super-cavitating (or fully-cavitating).
The change in water flow
and separation from the blade also results in a reduction in the
amount of torque that is necessary to keep the propeller rotating.
In other words, the vapor cavity makes it easier to spin the propeller.
This is what allows the “overspin” that is seen in propellers
that are heavily cavitating.
It is difficult to apply
a measure of when a propeller leaves its sub-cavitating mode,
but here are some rough guidelines based on the predicted percentage
trans-cavitating (minor loss)
It is important to point
out that cavitation is not necessarily “bad” in all cases – it
is often part of the intended design performance. Many propellers
operate at such high thrust loading that extensive cavitation
is unavoidable. In these cases (outboard propellers for speedboats,
for example), the propeller relies predominately on lift from
the pressure face, and the propeller’s section shape and other
geometry is designed so that it exploits this characteristic.
Some propellers which were
designed for sub-cavitating performance have been successfully
used in trans-cavitating applications. (This is the case in the
opening example.) These propellers, however, were not originally
designed for trans-cavitating operation, so they cannot be expected
to achieve high efficiencies in these modes.
Propellers on boats operating
at high speeds tend to be able to carry more cavitation before
thrust loss begins. Propellers operating with high-thrust at low-speed,
however, show a more significant loss of thrust and efficiency.
It is very important to note that such cases are not limited to
traditional “bollard” conditions, such as for tugs or trawlers.
Planing boats can also lose thrust to cavitation when trying to
accelerate through the hump speed and get up on plane. High cavitation
during these dynamic speeds can result in significant thrust breakdown.
Performance analysis and
The analysis of propeller
performance is very well behaved for the sub-cavitating mode.
A large body of research is available for propellers in this mode,
and most propeller analysis and sizing software is based on sub-cavitating
In comparison, very little
work has been done over the years for the trans- and super-cavitating
modes. One reason for this is that propellers operating with high
cavitation are often unstable, and small changes in thrust loading
or water flow can result in large changes in RPM. Cupping a propeller
further complicates the underlying hydrodynamic analysis. For
these reasons, the prediction of propeller performance when operating
in trans- and super-cavitating modes is not always reliable.
While there are no consistent
prediction models available for highly-cavitating propellers at
this time, work is being conducted by HydroComp to develop better
numerical prediction models for these conditions. For the time
being, this simply means that it will be necessary to apply a
bit of care to the interpretation of results when the propeller
is heavily cavitating.
All HydroComp software (NavCad,
PropExpert, SwiftCraft, SwiftTrial) uses prediction formula for
the loss of thrust, torque and efficiency in these cavitation
modes – but the reliability of the prediction models is less as
cavitation increases. We have identified a simple parameter –
a power loading index (PLI) – that you can use to estimate
when you might expect to find difficulty with the prediction.
PLI is simply engine power divided by diameter squared.
In units of HP and Feet, we find these confidence ranges to be
a good guide:
In the case of our original
example, the PLI is 330 hp / (1.5 ft)2 = 147. Therefore, the confidence
in an analysis or propeller sizing for this boat would be poor.
No practical calculation
models are currently available which will allow you to analyze
or size propellers operating at extreme levels of cavitation.
The existing cavitation prediction methods are limited in their
usefulness at very high levels of cavitation, and using conventional
calculations and software for these propellers in these modes
is risky. Until we have better calculation models for these modes,
use the enclosed guidelines to help you determine when you should
– or should not – use conventional calculation methods or software
for a particular application.