4 Care should be taken to ensure that the observations,
especially those of period, are not influenced by the waves generated
by the motion of the ship.
4.1 DIRECTION FROM WHICH THE WAVES COME. This
is easily obtained either by sighting directly across the wave front
or by sighting along the crests of the waves and remembering that
the required direction differs from this by 90º. Direction is
always recorded true, not magnetic.
4.2 PERIODfootnote.
For measurements of period, a stopwatch is desirable. If this is not
available, an ordinary watch with a seconds hand may be used or, alternatively,
a practised observer may count seconds. The observer selects a distinctive
patch of foam or a small object floating on the water at some distance
from the ship, and notes the time at which it is on the crest of each
successive wave. The procedure is repeated for the larger waves of
each successive group until at least twenty observations are available.
The period is then taken as the average time for a complete oscillation
from crest to crest. In a fast ship it will be found that the “patch
of foam” method will rarely last for more than one complete
oscillation and that many waves should be observed separately. With
practice, suitable waves can easily be picked out and the timing from
crest to crest becomes quite simple. When it is desired to use an
object (an empty beer can is usually conspicuous against the sea and
will remain afloat long enough to serve its purpose) it should be
thrown as far forward as possible. Another method available to the
observer with a stopwatch is to observe two or more consecutive “central”
waves of a wave group while the watch is running continuously, then
to stop the watch until the central waves of the next wave group appear,
the watch being then restarted. This procedure is repeated until at
least twenty complete oscillations have been observed. The period
is then obtained by dividing the total time by the number of oscillations.
It is important to note that the periods between times of crests passing
a point on the ship are not the ones required.
4.3 HEIGHT. Although wave-recorders are fitted
to a few research ships, there is at present no method of measuring
the height of waves suitable for general use on merchant ships, but
a practised observer can make useful estimates. The procedure to be
adopted depends on the length of the waves relative to the length
of the ship. If the length of the waves is short in comparison with
the ship’s length, i.e. if the ship spans two or more wave crests,
the height should be estimated from the appearance of the waves at
or on the side of the ship, at times when the pitching and rolling
of the ship is least. For the best result, the observer should take
up a position as low down the ship as possible, preferably amidships
where the effect of pitching is least, and on the side of the ship
towards which the waves are coming.
4.3.1 This method fails when the length of the
waves exceeds the length of the ship, for then the ship rises bodily
with the passage of each wave crest. The observer should then take
up a position in the ship so that his eye is just in line with the
advancing wave crest and the horizon, when the ship is vertical in
the trough. The height of eye above the ship’s waterline is
then the height of the wave. The nearer the observer is to an amidships
position, the less chance will there be of the measurement being vitiated
by pitching. If the ship rolls heavily, it is particularly important
to make the observation at the moment when she is upright in the trough.
Exaggeration of estimates of wave height is mostly due to
errors caused by rolling (see figures 2.1 and 2.2). When the ship
is rolling (figure 2.2), the observer at “0” should take
up a higher position to get a line on the horizon than when she is
upright (figure 2.1).
4.3.2 The observation of height of waves is most
difficult when the length of the waves exceeds the length of the ship
and their height is small. The best estimate of height can be obtained
by going as near the water as possible, but even then the observation
can only be rough. In making height estimates an attempt should be
made to fix a standard of height in terms of the height of a man or
the height of a bulwark, forecastle or well-known dimension in the
ship. There is generally a tendency to overestimate the height of
short waves and underestimate the height of long waves.
4.3.3 Estimating the height of a wave from a high
bridge in a fast ship is a difficult job and much will depend on the
skill and ingenuity of the observer; in many cases all one can hope
for is a very rough estimate. All estimates of wave height should
be made preferably with the ship on an even keel so that the observer’s
height of eye is consistent. The inherent difficulties already mentioned,
together with the practical difficulties of estimation, make it essential
that the recorded height be the average value of about twenty distinct
observations. These observations should be made on the central waves
of the more prominent wave groups.