Practical Ship Hydrodynamics

92 Practical Ship Hydrodynamics
conditions differ strongly. Viscosity and free surface effects, including spray
and overturning waves, play significant roles making both experimental and
numerical predictions difficult.
Valid predictions from tank tests for the resistance of the full-scale ship
in unrestricted water are only possible if the tank is sufficiently large as
compared to the model to allow similarity in flow. Blockage, i.e. the ratio
of the submerged cross-section of the model to the tank cross-section, will
generally be very low for models of high-speed ships. However, shallowwater
effects depend mainly on the model speed and the tank water depth.
The depth Froude number Fnh should not be greater than 0.8 to be free of
significant shallow-water effects.
The frictional resistance is usually computed from the frictional resistance
of a flat plate of similar length as the length of the wetted underwater body
of the model. This wetted length at test speed differs considerably from
the wetted length at zero speed for planing or semi-planing hull forms. In
addition the correlation requires that the boundary layer is fully turbulent.
Even when turbulence stimulators are used, a minimum Reynolds number
has to be reached. We can be sure to have a turbulent boundary layer for
Rn > 5 Ð 10 6 . This gives a lower limit to the speeds that can be investigated
depending on the used model length.
Figure 3.12 illustrates, using a towing tank with water depth H D 6mand
a water temperature 15°, how an envelope of possible test speeds evolve
from these two restrictions. A practical limitation may be the maximum
carriage speed. However, at HSVA the usable maximum carriage speed
exceeds the maximum speed to avoid shallow-water effects.
2.0
1.5
Fn 1.0
0.5
0
max. speed in 6 m tank
min. speed at 15 degrees water
1 2 3 4 5 6 7 8 9 10
Model length
Figure 3.12 Possible speed range to be safely investigated in a 6 m deep towing tank at 15°
water temperature
ž Planing hulls
In the planing condition a significant share of the resistance is frictional
and there is some aerodynamic resistance. At the design speed, the residual
resistance, i.e. the resistance component determined from model tests, may