The Blocking of Zeebrugge

by Alfred Francis Blakeney Carpenter

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 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.

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 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 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.