Page 217 - Introduction to Naval Architecture

.RESISTANCE 203

early design stages and it is desirable that their consequences should be
known. One way of achieving this is to run a set of three models early
on. One is the base model and the other two are the base model with
one parameter varied by a small amount. Typically the parameters
changed would be beam and length and the variation would be a
simple linear expansion of about 10 per cent of all dimensions in the
chosen direction. Because only one parameter is varied at a time the
models are not geometrically similar. The variation in resistance, or its
effective power, of the form can be expressed as:

The values of a] etc., can be deduced from the results of the three
experiments.

MODEL EXPERIMENTS

Full scale resistance trials are very expensive. Most of the knowledge on
ship resistance has been gained from model experiment. W. Froude was
the pioneer of the model experiment method and the towing tank
which he opened in Torquay in 1872 was the first of its kind. The tank
was in effect a channel about 85 m long, llm wide and 3 m deep. Over
this channel ran a carriage, towed at a uniform speed by an endless
rope, and carrying a dynamometer. Models were attached to the
carriage through the dynamometer and their resistances were meas-
ured by the extension of a spring. Models were made of paraffin wax
which is easily shaped and altered. Since Froude's time great advances
have been made in the design of tanks, their carriages and the
recording equipment. However, the basic principles remain the same,
Every maritime nation now has towing tanks.
Early work on ship models was carried out in smooth water. Most
resistance testing is still in this condition but now tanks are fitted with
wavemakers so that the added resistance in waves can be studied.
Wavemakers are fitted to one end of the tank and can generate regular
or long crested irregular waves. They may be oscillating paddles or
wedges or use varying pneumatic pressure in an enclosed space. For
these experiments the model must be free to heave and pitch and these
motions are recorded as well as the resistance. In towing tanks, testing
is limited to head and following seas. Some discussion of special
seakeeping basins was presented in Chapter 6 on seakeeping. Such
basins can be used to determine model performance when manoeuvr-
ing in waves.