METEOROLOGICAL AND TIDAL CONDITIONS. VISIBILITY
The periods during which the conditions would be favourable for our
enterprise depended upon various factors. The extent to which we could
make use of any particular date during one of those periods depended,
in turn, upon meteorological conditions.
It has been shown elsewhere that, for the purpose of utilising the
artificial fog, we required a wind blowing more or less toward the
Belgian coast from seaward. It was also necessary for the wind to be
light so that the small craft would not be hampered by rough seas.
Light winds are often accompanied by fog, especially in the North Sea,
and fog would be a serious obstacle. Rain would be detrimental to the
use of aircraft.
The depths in the entrances to the canals at Zeebrugge and Ostende were
such that the blockships could only navigate during the period around
high water. It was, therefore, necessary for the vessels to arrive at
about that state of the tide.
Again, it was essential to carry out the operation during the night for
reasons already mentioned. In this respect, however, the word
operation must not be used too loosely; the periods of approach to the
objectives and of retirement therefrom must both be included in that
term. The approach had to be {132} undiscovered till the latest
possible moment. That necessitated darkness throughout the approach,
which latter may be considered as comprising the passage during the
last twenty miles of the oversea voyage. Obviously the greater part of
the whole passage conducted during darkness the less would be the
chance of losing the element of surprise.
Likewise the retirement must be made before daylight if the
concentration of the shore batteries was to be avoided. I have already
stated that the German guns could make things very uncomfortable for
ships up to a maximum distance of fifteen to twenty miles, provided
that the ships could be seen. The retiring forces should, therefore,
be outside that range before there was sufficient daylight to see so
far; the latter state of visibility would obtain at least half an hour
before sunrise. Allowing a speed of about ten knots for retirement,
this meant that the ships must leave the coast about two and a half
hours before sunrise after completing the operation. The attack itself
was expected to continue for about one and a half hours. The reader
does not require to be an advanced mathematician to realise that the
attack must commence not later than about four hours before sunrise,
that is, during the middle portion of the night. Of course, the attack
was the all-important matter; any question of safe retirement must be a
secondary consideration. Nevertheless, the question of retirement,
similarly to that of rescuing the blockships' crews, had to be taken
into account.
[Illustration: Specimen Diagram for ascertaining Available Period.
(Transcriber's note: this diagram occupied page 133 of the source book)]
From the foregoing it will be seen that the attack should be preceded
and followed by a considerable period of darkness, and should more or
less coincide {134} with the time of high tide. The latter only occurs
about every twelve hours and takes place roughly fifty minutes later
each day. The number of consecutive days on which high tide would
occur in the middle portion of the night was, therefore, very limited.
With regard to visibility, as far as naval operations are concerned,
strong moonlight is almost as disadvantageous as daylight. Half the
nights per month may be termed moonlight nights in that respect. The
state of the moon on any particular night is known beforehand, the
state of the clouds affecting moonlight may change from hour to hour.
Just one more calculation. In mid-April the period sunset to sunrise
is about eleven hours in length. Allowing two hours for the
approach-passage through the danger zone, one and a half hours for the
attack, and two hours for the retirement (to be completed half an hour
before sunrise), it follows that the maximum number of after-sunset
hours available for the open sea passage would amount to five. That
was the best possible condition for us, but could only be utilised if
high water occurred about four hours before sunrise. If high tide
occurred any later our time of arrival would necessarily be later and
our retirement could not be completed before daylight. If high tide
occurred any earlier our time of arrival must be earlier, in which case
there would be less dark hours available for our open sea passage on
the way across. The length of the open sea passage would be
approximately six hours. Thus even on the most favourable date, some
of the open sea passage would have to be made in broad daylight. Six
days before that (the tide being five hours earlier) the whole of the
open sea {135} passage would have to be made in daylight, only the
approach-passage, i.e., the last two hours of the whole trip, taking
place after sunset.
A rather complicated set of conditions, astronomical, tidal, and
meteorological, was thus required.
It was almost too much to expect that everything would be favourable
during the possible period; it is practically certain that a commander
who refused to move until all conditions were exactly as desired would
never accomplish anything. Nevertheless we sincerely hoped that
fortune would be kind to us.
[Sidenote: Keenness of the Men]
The men were kept informed of the chances as forecast from the current
weather conditions; their eagerness for favourable predictions was
manifest. There is much advantage to be gained by a commander taking
his men into his confidence. In this particular case the men realised
that leakage of information would entail disaster; that was sufficient
to ensure that the confidence would be respected; the rigorous
censorship was there to make doubly certain. It is always more irksome
for those who wait in ignorance than for those who know the reasons for
delay. So after prayers each morning the latest forecast was divulged,
other items of interest were made known, and the keenness of the men
was maintained.
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