Page 244 - Introduction to Naval Architecture

PROPULSION 229

OPEN WATER TESTS

Open water tests of propellers are used in conjunction with tests behind
models to determine the wake and relative rotative efficiency. Also
methodical propeller testing is carried out in a towing tank. The
propeller is powered from the carriage through a streamlined housing.
It is pushed along the tank with the propeller ahead of the housing so
that the propeller is effectively in undisturbed water. Records of thrust
and torque are taken for a range of carriage speeds and propeller
revolutions, that is advance coefficient. Such tests eliminate cavitation
and provide data on propeller in uniform flow. This methodical series
data can be used by the designer, making allowance for the actual
flow conditions a specific design is likely to experience behind the hull
it is to drive.

Figure 9,15 Propeller diagram

There have been many methodical series. Those by Froude, Taylor,
Gawn, Troost and van Lammeren are worthy of mention. The reader
should refer to published data if it is wished to make use of these series.
A typical plot for a four bladed propeller from Troost's series is
presented in Figure 9.15.