Practical Ship Hydrodynamics

Resistance and propulsion 69
course by special wires at the ship ends. Usually, models are free to trim and
sink. After the initial acceleration, some time has to pass before a stationary
state is reached. Then the remaining measuring time is determined by the
remaining towing tank distance and the deceleration time of the carriage.
Therefore, towing tanks are usually several hundred metres long to allow
sufficient measuring time (in most cases).
The model size is determined by a number of boundary conditions:
ž The model should be as large as possible to minimize viscosity scale effects,
especially concerning laminar/turbulent flow and flow separation.
ž The model should be small enough to avoid strength problems (both internal
strength of the model and loads on the test carriage).
ž The model should be small enough such that the corresponding test speed
can be achieved by the carriage.
ž The model should be small enough to avoid noticeable effects of restricted
water in the test basin.
This leads to a bandwidth of acceptable model sizes. Typically models for
resistance and propulsion tests have a size 4 m Lm 10 m. Model scales
range between 15 45. In practice, often the selected stock propeller
decides the exact model scale.
Tests are performed keeping Froude similarity, i.e. Froude number of model
and full scale are the same. The Reynolds numbers differ typically by two
orders of magnitude. The scale effect (error of not keeping the Reynolds similarity)
is then compensated by empirical corrections.
The models are made of special paraffin wax or special tropical wood that
hardly changes volume and shape with time or temperature. Wax models are
cheaper, but less robust. Wooden models receive a smooth finish of paint.
Yellow is the preferred colour for regular models as this colour contrasts
nicely with the (blackish) water which is important for visual observations,
e.g., of the wave profile. For icebreakers, often for similar purposes red is the
preferred colour as it appears to be a good compromise for contrasts of water
and ice.
Models operate at considerably lower Reynolds numbers. (Typically for
models Rn ³ 10 7 and for full-scale ships Rn ³ 10 9 .) This means that in the
model the transition from laminar to turbulent flow occurs relatively further aft.
As a consequence, the resistance would be more difficult to scale. Therefore,
the model is equipped with artificial turbulence stimulators (sand strip, studs,
or trip wire) in the forebody. One assumes that the transition from laminar
to turbulent flow occurs at a length corresponding to Rn D 0.5 Ð 10 6 from the
stem. In practice, often the turbulence stimulators are located somewhat further
aft. Then the reduced resistance due to the longer laminar flow compensates
(at least partially) the additional resistance of the turbulence stimulators.
3.2.2 Resistance test
Resistance tests determine the resistance of the ship without propeller (and
often also without other appendages; sometimes resistance tests are performed