Commander Thomas A. Kearney, Acting Chief, Bureau of Ordnance, to Admiral William S. Benson, Chief of Naval Operations
(N3) MC July 30, 1917.
“To: Chief of Naval Operations.
Subject: Proposed British-American joint offensive operations;
Submarine barriers; Mark VI mines.
1. In its letter No. 32957 of July 18, 1917, the Bureau announced the development of a new type of mine that is peculiarly adaptable for use against submarines.
2. The firing mechanism of this mine is based on a very recent discovery in the electrical field and although there has been little time for development, the tests which have been carried out with an experimental mine by a submarine leaves no doubt, in the Bureau’s opinion, of the success of this invention.
3. The mine will have the following characteristics:
(a) A spherical mine case carrying a charge of 300 pounds of TNT having a destructive radius of about 100 feet against a submarine.
(b) The anchor may be either the automatic type such as that now in use or a simple mushroom type, depending upon the conditions under which mining operations shall be carried out.
(c) The firing mechanism comprises an electrical device carried within the mine case an antenna of any desired length, the end of which will be supported by a small buoy as near the surface of the water as may be desired. A second antenna may be suspended from the mine where the depth of water renders this necessary.
(d) A steel vessel coming in contact with the antenna will fire the mine.
4. The mine has the following advantages over other types:
(a) In depths of less than 100 feet, it may be planted on the bottom where it is least affected by wave action and current. In this case a buoyant mine is not necessary or desirable and it can be made smaller and cheaper than a buoyant mine. In such circumstances there is no possibility of its getting adrift and it can not be swept up in the usual way. It can, however, be fired by a mine sweep.
(b) In depths greater than 100 feet, it is proposed to submerge the mine to a depth of 100 feet since 100 feet is about its destructive range against submarines. At this depth the mine itself is entirely protected from wave action, and only the light float or buoy is exposed to such action.
(c) Where conditions permit, the antenna may take the form of a net; or the antenna of adjacent mines may be connected by horizontal wires forming an impassable barrier.
(d) If a floating mine be desired, this mine may be suspended from a buoy in such manner as to be harmless to surface craft but deadly to submarines submerged.
(e) It may be used as a towing mine with antennas to give it a very large danger space.
(f) It can almost entirely replace submarine nets of present types.
(g) It can be used for mining very deep water more easily than can other types.
5. The mine, with its anchor, antenna, and buoy will be assembled and launched as a unit, so that it can be launched at high speed from destroyers if desired.
6. The Bureau believes that with this mine it becomes practicable to close the North Sea, Adriatic, and other exits of enemy submarines, and that it gives us our opportunity to cooperate in carrying into execution, a major offensive operation of a decisive character. Even if the proposed barriers should prove to be only fifty per cent effective the enemy’s submarine campaign would surely fail.
7. It is suggested that the North Sea barriers must extend from the coast of Scotland to Norway and across the English Channel. The proposed line from Scotland to Norway must, to be at all effective, extend into the territorial waters of Norway thereby involving the questions of Norway’s neutrality. It would seem that if the German submarine is permitted by Norway to use her territorial waters it becomes incumbent upon the Allies to take measures to prevent such use.
8. The proposed mine barrier scheme does not infringe upon the neutrality of Holland, Denmark and Sweden, except in the restricted sense that the vessels of those powers, as well as of Norway, would be required to pass through a gate in the barriers under the control of the Allied forces. In effect, this would amount to the establishment of additional danger zones to be avoided by neutrals.
9. The Bureau understands that the British Admiralty has objected to any barrier in the North Sea that would interfere with the freedom of the British Fleet. It is suggested that a gate should be left in the barrier at an appropriate place near the Scotch coast, not only for British Naval vessels, but also for neutral merchant vessels. This gate would be, say, eight miles long, with mines so planted that their antennae would not come within forty feet of the surface at low water. In other words, the sub-surface would be mined against submarines and the surface left open. This gate could be effectively patroled with a very few vessels and submarines attempting to pass on the surface could be destroyed.
10. If a decision should be reached immediately to proceed with the assembling of the material for these barriers, it would require approximately six weeks to complete the designs, place the orders and start production on a large scale. After starting production mines could be obtained at a minimum rate of 5,000 a week, and if the project were given the importance due it, there is no doubt that the manufacturers could be depended upon to increase this figure. In this connection, it is assumed that the British Admiralty would be willing to cooperate to the extent of furnishing a portion, at least, of the mine anchors, but it is believed that we should supply all of the mines, with the exception of the anchors.
11. It would require approximately 72,000 mines to establish barriers around the North Sea, assuming that the barrier will be composed of four lines of mines, placed 100 feet apart in each line, in other words, a barrier would require a mine for every 25 feet. To this 72,000 should be added at least 28,000 for renewals and as a reserve. If it should be decided to place the barrier across the Adriatic and to close the Dardanelles about 50 miles of barrier, or about 15,000 additional mines would be required.
12. It is estimated that 125,000 mines can be manufactured at a cost of $40,000,000.
13. The Bureau has made every effort to keep the discovery and development of this mine a military secret, and it is believed that this secrecy can be maintained by proper organization and administration until such time as it becomes necessary to assemble the completed mines to ship them to Europe. To this end, the various parts of the mine will be manufactured by different companies and no manufacturer need be informed as to the characteristics of the mine as a whole. The company which will manufacture the firing gear has taken such precautions that only three members of the company will know that the electrical apparatus used in the mine is intended for a mine.
14. In view of the importance of keeping this matter a military secret, it is considered desirable that the British Admiralty should not be informed as to the features of the mine until the mines shall have been manufactured and shipped. This view is taken because it is inevitable that information will leak out regarding the design, if any considerable number of persons should become informed of it, and since it is proposed to manufacture the mines complete in this country, it would seem unnecessary to send any information regarding it abroad and would only invite the possibility of such a leak.
15. If the enemy should learn of this invention it would be easy for him to evolve a similar mine which he could use to blockade the British ports. The principle of the firing mechanism is so simple that only the slightest clue would enable the enemy to duplicate it.
16. If this project should be carried out, the Bureau is of the opinion that its execution will bring about a general engagement with the German Fleet, which it is supposed, is desirable.
17. The following is a summary of the cooperations deemed necessary to carry out this plan:
(a) Provide mines, except anchors.
(b) Send mines to England.
(c) Assist in assembling mines in England.
(d) Provide a number of mine layers.
(e) Assist in laying.
(a) Provide anchors.
(b) Assemble mines on anchors.
(c) Organize and equip mine laying force.
(d) Lay all mines with U.S. Assistance.
18. In the above it is suggested that Great Britain provide the anchors for the reason that about 30,000 tons would be required and that the transportation of this tonnage should be avoided, if possible.
19. Regarding the mine laying part of this project, it is understood that Great Britain has about eighteen regular mine layers and that the United States could probably furnish four, giving a total of twenty-two, not including destroyers. A number of British destroyers are fitted to carry eighty mines and probably some of ours could readily be fitted to carry forty to eighty each, so it is assumed that forty destroyers may be available. The mine laying program may then be assumed to be approximately as follows:-
(a) 22 mine layers could lay 200 mines per day each. If they take one day to reload, they would lay an average of 100 per day each.
(b) Forty destroyers could average 50 per day each.
(c) All combined could lay 4200 per day.
(d) For the Northern barrier, about 60,000 mines are required. These, at the rate of 4200 per day, could be laid in about 15 days.
(e) For the English Channel barriers, assumed lengths 50 miles, 12,000 mines would be required. At the rate of 4,200 per day, these could be laid in three days. It is assumed that two barriers each 25 miles long would be required in the channel to fully protect the Channel crossing.
20. Lacking definite information as to the mine laying facilities in the Mediterranean, but, assuming that ten vessels could be made available, the Adriatic barrier, 40 miles, could be laid in about one week; and the Dardenelles barrier in a shorter time.
21. As the manufacture and assembling of the material will be an immense undertaking, and as time is precious at this juncture in the war, a decision should be reached at the earliest moment practicable.
22. If this plan be adopted, it will be necessary to expedite manufacture by giving, this work priority over certain other Government work, particularly in the matter of obtaining a sufficient supply of T.N.T. This will be made the subject of special report if the general plan be adopted.”