Page 257 - Introduction to Naval Architecture

242 PROPULSION

Wind
The wind was the only means, apart from oars, of propelling ships for
many centuries. It has always been popular for pleasure craft. The rise
in fuel costs and public concern with conserving energy sources has
rekindled interest. Some ships have sails to use in place of their engines
when wind conditions are suitable. Other applications have harnessed
modern technology to use the old idea of rotating cylinders, the Flettner
rotor concept, more effectively.

SHIP TRIALS

A complete range of trials is carried out on a ship when complete to
confirm that the ship meets its specification. Amongst these is a speed
trial which has the following uses:

(1) To demonstrate that the desired speed is attained. There are
usually penalties imposed if a ship fails to meet the specified
speed but it would be uneconomic to provide too much power.
This illustrates the importance of a designer being able to
predict resistance and powering accurately in the design
stages.
(2) To provide a feedback on the effectiveness of prediction
methods and provides factors to be applied to overcome any
shortcomings in the methods.
(3) To provide data on the relationships between shaft revolutions,
ship speed and power for use by the master.

To meet the last two aims it is desirable to gather data at a range of
speeds. Therefore trials are run at progressively higher speeds up to the
maximum. For that reason they are often called progressive speed trials.
The engine designer may wish to take readings of a wide range of
variables concerned with the performance of the machinery itself. The
naval architect, however, is concerned with the shaft revolutions, thrust,
torque and speed achieved relative to the water. Thrust is not always
measured. It can be measured by a special thrust meter but more
commonly by a series of electrical resistance strain gauges fitted to the
shaft. Torque is measured by the twist experienced by an accurately
known length of shaft. This leaves the problem of determining the
speed of the ship.

Speed measurement
Ships are provided with a means of speed measurement, usually in the
form of a pitot tube, or pitot log, projecting below the keel. This is not