Practical Ship Hydrodynamics

V a /V
0.4
0.3
0.2
0.1
V a /V
2.0
1.6
1.2
0.8
0.4
0
5.0 7.0
0.02 0.04 0.06 0.08 0.10
X /C
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
1.5
X /C
3.0
Leading edge: nose
radius r (% of c )
as parameter
Ship manoeuvring 187
Figure 5.16 Relative change of velocity Va/V on the surface due to the angle of attack ˛ for
CL1 ³ 1
in the Fig. 5.16 have to be multiplied by the actual local lift coefficient:
lift per length
CLl D D CL Ð
c Ð q
4 �
� �2 z
Ð 1
b/2
where CL⊲3⊳ is estimated by the usual formula for the force coefficients
as given at the beginning of this chapter.
The dynamic pressure is then:
� � � �
2
vt va
pdyn D 1 š Ð CLl Ð q
V V
Due to the quadratic relationship in this equation, the pressure distribution will
generate the given CLl only approximately. For better accuracy, the resulting
local lift coefficient should be integrated from the pressure difference between
both sides of the profile. If it differs substantially from the given value, the
pressure distribution is corrected by superimposing the va/V distribution in the
above formula for pdyn with a factor different from CLl such that the correct
CLl is attained by the integration of the pressure difference.
The dynamic pressure is negative over most of the profile length, for
moderate lift coefficients even on the pressure side of the rudder. This is
illustrated in Fig. 5.17 for an NACA0021 profile. The curve for CLl D 0
corresponds to the component due to the profile thickness alone. For other
CLl values, the upper and lower curves refer to the pressure and suction