Practical Ship Hydrodynamics

198 Practical Ship Hydrodynamics
Pdyn (ρ ⋅ V 2 max) / 2
2.5
2.0
1.5
1.0
0.5
0
IFS 61−TR 25
IFS 62−TR 25
IFS 58−TR 15
NACA−0024
NACA−0021
NACA−0018
HSVA−MP 71−20
NACA 64 2 −015
0.25 0.50
C Ll
NACA−0015
NACA 64 3 −018
HSVA−MP
73−20
NACA 64 4 −021
Figure 5.23 Extreme negative dynamic pressure of the suction side as a function of the local
lift coefficient CLl and the profile
If the resulting minimum pressure on the suction side is negative or slightly
positive (less than 3 kPa), the side plating of the rudder is prone to cavitation.
For a right-turning propeller (turning clockwise looking forward) the
cavitation will occur:
– on the starboard side in the upper part of the rudder relative to the
propeller axis
– on the port side in the lower part of the rudder relative to the propeller
axis
Brix (1993), pp. 91–92, gives an example for such a computation. Measures
to decrease rudder side cavitation follow from the above prediction method:
– Use profiles with small pdyn at the respective local lift coefficient. These
profiles have their maximum thickness at approximately 40% behind the
leading edge.
– Use profiles with an inclined (relative to the mean rudder plane) or curved
mean line to decrease the angle of attack (Brix et al. (1971)). For a rightturning
propeller, the rudder nose should be on the port side above the
propeller axis, on the starboard side below it.
2. Rudder sole cavitation
Due to the pressure difference between both sides of the rudder caused,
e.g., by the rotation of the propeller slipstream, a flow component around
the rudder sole from the pressure to the suction side occurs. It causes a
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