Vice Admiral William S. Sims, Commander, United States Naval Forces Operating in European Waters, to Secretary of the Navy Josephus Daniels
12 March 1918.
S E C R E T.
From: Commander, U.S.Naval Forces in European Waters.
To: Secretary of the Navy (Operations)
Subject: Hour for Troop Convoys to arrive at destination.
Reference: Opnav Cable 3486 of 5 March, 1918.
1. There are two dangers to troop convoys arriving in European Waters – one the submarine, and the other mines. Of these the submarine has always been considered the graver menace, as his attack may be delivered a long way from the coast, resulting in probable loss of the transport, and possibly considerable loss of life. Mines are always near the coast, and in an area from which a maximum of help may be obtained. In many cases a ship that is mined could be beached if necessary, and both ship and personnel be saved. It has therefore been the practice to make the best disposition possible to prevent submarine attack, and to accept the minor disadvantages that may follow from such disposition.
2. In order to avoid the submarine attack, advantage is taken of darkness, for during darkness in the open sea contact with a submarine could take place only by chance. It has been customary, therefore, for all ships, whether in convoy or not, to pass through the most dangerous submarine areas during darkness, in order to avoid the possibility of submarine attack. The only exception to this is in narrow or constricted waters where submarines know the route that must be taken by ships, and are conceded to have an advantage on the surface at night under such conditions.
3. In the case of moonlight nights, the situation is somewhat different. The submarine may be able to sight ships from 5 to 10 miles. In addition, he enjoys the advantage of being able to deliver an attack with much less risk than during daylight. During daylight the submarine has a greatly increased range of visibility, but he is subjected to greater danger in delivering an attack. On the whole, there would seem to be little advantage whether convoys cross an area during moonlight or daylight. There is another factor, however, that modifies this conclusion. Submarines of late have shown an increasing tendency to operate in maximum numbers during moonlight periods. All things considered, therefore, it would seem best for convoys and shipping in general during a period approximating six days on either side of full moon, to cross the most dangerous submarine areas during daylight. During moonlight periods the submarine is relatively safer from attack while on the surface than he is during dark nights, for his radius of visibility is generally several miles on a moonlight night. It is probably for this reason that more submarines are now operating during moonlight periods than during darkness, and it is for this reason that I have recommended modification in the former practice in the hour of arrival of troop convoys.
4. The following comments are submitted on the points raised in Reference (a):-
“First. Convoys often set ahead by current and compelled to slow just before making landfall”.
So far as I am aware, this has occurred on only one or two occasions, and there is no record of the convoys having slowed. It is a general practice that convoys must at all times while in the zone, maintain full speed, reversing course if necessary to adjust hour of arrival.
“Second. If stars not visible no opportunity to get sun for position”.
Prevailing conditions on the French Coast make it difficult to count on either sun or stars in the early morning. Lighthouses, however, can be seen many miles, even during hazy weather and furnish excellent marks for making landfall. To assist convoys in making their landfall, it is customary for the most dangerous points to be marked by patrol vessels acting in the capacity of lightships. Only one case is recorded in which a group had difficulty in making a landfall, and this difficulty occurred on the first trip of this group to France.
“Third. To afford minesweepers better opportunity to clear channel of mines laid on previous night”.
I fully concur. Mines are a menace and ships may be easily sunk by them. It seems preferable, however, to follow the practice recommended in my cable and to count on the port authorities to keep the channels clear, or if mines are discovered to divert the convoys to a safe anchorage until channels are swept.
“Fourth. Making landfall at daylight on moonlight nights particularly dangerous as submarines locate them on light circles of powerful light.”
This is not considered a serious objection, except in the cases of vessels very near the coast. In any event as the approach is made directly toward the light the submarine enjoys no advantage until the convoy passes between him and the light, by which time the submarine is left astern.
“Fifth. To give aircraft from shore stations opportunity to inspect water through which convoys must pass.”
Throughout the winter aircraft have been of very little assistance. Their activities have been confined to areas in immediate proximity to bases, and these areas are rarely dangerous ones. It will be noted that the attacks on the ANTILLES, FINLAND and ALCEDO were well beyond the area covered by aircraft.
“Sixth. Submarines are now probably aware of practice of arriving at daylight and will await convoys that hour.”
Although convoys are routed to arrive on the coast about daylight, due to weather and other causes, there has been no regularity in the hour of arrival of convoys. The majority have arrived before noon, but several have arrived in the afternoon and a few have arrived at daylight. The average hour of arrival on the coast for vessels bound for St. Nazaire is 11 a.m. and for those bound for Brest is 9:30 a.m. Convoys have sailed similarly sometimes before noon and sometimes after noon.
The recommendation in my cable will, in any case, make the hour of arrival still more irregular.
WM. S. Sims