Propeller root cavitation problems can be among the most complex to solve. In this region, along with tip vortex dynamics, rigorous mathematical analysis is currently of little assistance for design or analysis purposes. In some cases, when scant attention has been paid to the propeller design environment or where ship layout difficulties have arisen and high shaft angles or a poor choice of advance angle have resulted, erosion problems can occur which are difficult to resolve.

One such case is that of a fast displacement craft where deep cavitation erosion in the blade root regions was produced after only 30 minutes running at full power.
The problem was identified as originating in the angle of attack variations at the blade roots due to the propeller shaft’s
inclination. Figure 1 shows the full-scale cavitation on the propeller blades when operating at high speed as observed
through windows in the hull.

Of interest is the cavitating structure at the trailing edge of the root cavity which was in the root erosion region.
An air injection system was devised based on previous successful experience by Lloyd’s Register’s Technical Investigations.
This proved to be moderately successful in that it allowed the vessel to enter restricted service while a more permanent solution was found.
Full-scale observation of the air injection process on the ship provided key information for a subsequent series of model tests.

The video images showed that due to the high pitch ratio of the blades, in excess of 1.7, this conventional approach did not
allow sufficient air to be held in the cavities and hence could not fully cushion the cavity collapse process, producing the erosion damage.

Model tests were used to develop an alternative air supply configuration and machined holes in the blade roots at approximately the one-third and two-third chord positions at the 0.3R radius. Through an iterative process, a suitable air-flow rate was determined such that a minimum speed loss was achieved while maintaining intact stencil ink coatings on the model blades, which were used to assess the tendency to cavitation erosion.

The machined holes in the blade roots appeared to delay the onset of erosion and increased the speed at which air injection
would need to be started. In addition, the holes provided a mechanism by which air could transfer from the pressure side to
the suction side and ensured that air was resident in the root cavities for a substantial part of each blade revolution.
The vessel is now in normal service and, after the equivalent of one full year of high-speed running, some minor erosion
damage is still present but it appears to have stabilised.

LESSON LEARNED
Cavitation problems can be solved much more readily when the problem area can be viewed either by window or boroscope.
Possible solutions include re-profiling, drilling of pressure-relief holes, flow pattern changes and/or air injection.

Lloyd’s Register EMEA’s www.lr.org