Practical Ship Hydrodynamics

Ship manoeuvring 199
rudder tip vortex (similar to propeller tip vortices) which may be filled by
a cavitation tube. This may cause damage if it attaches to the side of the
rudder. However, conditions for this are not clear at present. If the rudder
has a sharp corner at the front lower edge, even for vanishing angles of
attack the flow cannot follow the sharp bend from the leading edge to
the base plate, causing cavitation in the front part of the rudder sole. As
a precaution the base plate is bent upward at its front end (Brix et al.
(1971)). This lowers the cavitation number below which sole cavitation
occurs (Fig. 5.24). For high ship speeds exceeding, e.g., 26 knots cavitation
has still been reported. However, it is expected that a further improvement
could be obtained by using a smoothly rounded lower face or a baffle plate
at the lower front end (Kracht (1987)). No difficulties have been reported
at the rudder top plate due to the much lower inflow velocity.
δ = P − PV r ⋅ V 2 /2
3
2
1
0
Flat rudder sole
Rounded leading edge
0° 5° 10° 15° 20° 25°
∝
Figure 5.24 Cavitation number below which rudder sole cavitation occurs as a function of
theangleofattack˛ and the rudder sole construction
3. Propeller tip vortex cavitation
Every propeller causes tip vortices. These are regions of low pressure, often
filled with cavitation tubes. Behind the propeller they form spirals which are
intersected by the rudder. Therefore, cavitation erosion frequently occurs at
the rudder at the upper and sometimes lower slipstream boundaries, mainly
(for right-turning propellers) on the upper starboard side of the rudder.
This problem is not confined to high-speed ships. The best means to reduce
these effects is to decrease gradually the propeller loading to the blade tips
by appropriately reduced pitch, and to use a high propeller skew. These
methods also reduce propeller-induced vibrations.
4. Propeller hub cavitation
Behind the propeller hub a vortex is formed which is often filled by a
cavitation tube. However, it seems to cause fewer problems at the rudder