Practical Ship Hydrodynamics

Resistance and propulsion 65
3.1.2 Decomposition of resistance
As the resistance of a full-scale ship cannot be measured directly, our knowledge
about the resistance of ships comes from model tests. The measured calmwater
resistance is usually decomposed into various components, although
all these components usually interact and most of them cannot be measured
individually. The concept of resistance decomposition helps in designing the
hull form as the designer can focus on how to influence individual resistance
components. Larsson and Baba (1996) give a comprehensive overview
of modern methods of resistance decomposition (Fig. 3.1).
Wavemaking
resistance R WM
Residual resistance R R
Pressure resistance R P
Wave resistance R W
Wavebreaking
resistance R WM
Figure 3.1 Resistance decomposition
Total resistance R T
Form effect on skin friction
Viscous pressure resistance R PV
Total resistance R T
Skin friction resistance R FO
(equivalent flat plate)
Friction resistance R F
Viscous resistance R V
The total calm-water resistance of a new ship hull can be decomposed into:
ž Friction resistance
Due to viscosity, directly at the ship hull water particles ‘cling’ to the
surface and move with ship speed. A short distance away from the ship, the
water particles already have the velocity of an outer, quasi-inviscid flow.
The region between the ship surface and the outer flow forms the boundary
layer. In the aftbody of a container ship with Rn ³ 10 9 , the boundary layer
thickness may be 1 m. The rapid velocity changes in normal direction in the
boundary layer induce high shear stresses. The integral of the shear stresses
over the wetted surface yield the friction resistance.
ž Viscous pressure resistance
A deeply submerged model of a ship will have no wave resistance, but its
resistance will be higher than just the frictional resistance. The form of the
ship induces a local flow field with velocities that are sometimes higher and
sometimes lower than the average velocity. The average of the resulting
shear stresses is then higher. Also, energy losses in the boundary layer,
vortices and flow separation prevent an increase to stagnation pressure in
the aftbody as predicted in an ideal fluid theory. Full ship forms have a
higher viscous pressure resistance than slender ship forms.