Page 260 - Introduction to Naval Architecture

PROPULSION 245

The unknown ao can be eliminated by adding successive pairs of
equations, yielding three equations for 2V. Adding successive pairs of
these eliminates a 2 and so on, giving, finally:

8V = V l + SV^ + 3V S + V 4, from which Vfollows.
If the tide varied linearly with time three runs would be enough. A
higher order equation for tide can be used if more runs are made.
Usually four runs are adequate.

Trial condition
Ideally trials would be carried out for each of the likely operating
conditions. This would be expensive and time consuming. The key
condition is that for which the contract speed is defined which is
usually the deep load condition. If this level of loading cannot be
achieved some lesser load is specified with a correspondingly higher
speed to be obtained. In some ships, oil tankers for instance, the load
condition can be achieved by water ballasting.
The trial is carried out in calm conditions which are easy to define for
contract purposes but are not representative of the average conditions
a ship will meet in service. Increasingly it is realized that it is this speed
that is of real interest and this has led to a lot of effort being devoted
to obtaining and analysing voyage data. Also the advent of accurate
positioning systems facilitates measurement of speed, albeit relative to
land, in a whole range of weather conditions during the service life,

Plotting trials data
The results from the ship trial can be plotted as in Figure 9.22. The
revolutions will be found to plot as a virtually straight line against

Figure 9.22 Trials data