Practical Ship Hydrodynamics

76 Practical Ship Hydrodynamics
is in self-propelled equilibrium. Usually the speed of the towing tank carriage
is kept constant and the rpm of the propeller varied until an equilibrium is
reached. A propeller dynamometer then measures thrust and torque of the
propeller as a function of speed. In addition, dynamical trim and sinkage
of the model are recorded. The measured values can be transformed from
model scale to full scale by the similarity laws: speed Vs D p Ð Vm, rpm
ns D nm/ p 3 4 , thrust Ts D Tm Ð ⊲ s/ m⊳ Ð , torque Qs D Qm Ð ⊲ s/ m⊳ Ð .A
problem is that the propeller inflow is not geometrically similar for model and
full scale due to the different Reynolds number. Thus the wake fraction is also
different. Also, the propeller rpm should be corrected to be appropriate for the
higher Reynolds number of the full-scale ship.
The scale effects on the wake fraction are attempted to be compensated by
the empirical formula:
ws D wm Ð cFs
� �
cFs
C ⊲t C 0.04⊳ Ð 1
cFm
cFm
t is the thrust deduction coefficient. t is assumed to be the same for model and
full scale.
The evaluation of the propulsion test requires the resistance characteristics
and the open-water characteristics of the stock propeller. There are two
approaches:
1. ‘Thrust identity’ approach
The propeller produces the same thrust in a wake field of wake fraction w
as in open-water with speed Vs⊲1 w⊳ for the same rpm, fluid properties
etc.
2. ‘Torque identity’ approach
The propeller produces the same torque in a wake field of wake fraction w as
in open-water with speed Vs⊲1 w⊳ for the same rpm, fluid properties etc.
ITTC standard is the ‘thrust identity’ approach. It will be covered in more
detail in the next chapter on the ITTC 1978 performance prediction method.
The results of propulsion tests are usually given in diagrams as shown
in Fig. 3.9. Delivered power and propeller rpm are plotted over speed. The
results of the propulsion test prediction are validated in the sea trial of the
ship introducing necessary corrections for wind, seaway, and shallow water.
The diagrams contain not only the full-load design condition at trial speed, but
also ballast conditions and service speed conditions. Service conditions feature
higher resistance reflecting the reality of the ship after some years of service:
increased hull roughness due to fouling and corrosion, added resistance in
seaway and wind.
3.2.8 ITTC 1978 performance prediction method
The ITTC 1978 performance prediction method (IPPM78) has become a
widely accepted procedure to evaluate model tests. It combines various aspects
of resistance, propulsion, and open-water tests. These are comprehensively
reviewed here. Further details may be found in section 3.2.5, section 3.2.7
and section 2.5, Chapter 2. The IPPM78 assumes that the following tests have