Captain Reginald R. Belknap to Admiral William S. Benson, Chief of Naval Operations
January 24, 1918.
For Chief of Operations:
SUBJECT : Anti-submarine mining.
Reference : (a) Commander H. Laning’s letter of Dec. 14, 1917 Bureau of Ordnance letter (N3) 32767-297 of January 3, 1918.
(b) Bureau of Ordnance letter (N3) 32767-297 of January 3, 1918.
I. Reference (a) proposed to establish sea lanes which shall be safe for surface craft but dangerous for submarine craft by reason of deep mines. This plan, on first consideration in the Planning Section of the Office of Operations, was regarded favorably, both for the submarine zones abroad and for approaches to the principal harbors of our own coast.
2. The question first involved is – can such a sea lane be safely used by surface vessels? This depends on the mine itself. The British mine contains an automatic device which renders the mine inoperative in case it fails to plan as deep as intended or deeper than a certain minimum – about 45 feet from the surface. Unrestricted navigation over deep mines is contemplated by both merchant and war vessels in the present operations abroad. The American Mark VI mine has no such device but it could have one. There is nothing complicated nor mechanically difficult about it. Hydrostatic devices of various kinds are commonly used in mines. The American Mark VI mine itself is planted very deep so that it may be possible to provide safety devices which will retain the dangerous qualities of the mine even if planted shallow, provided it were as deep as 40 or 45 feet.
3. The Bureau of Ordnance, in its letter of comment, has apparently considered the sea lane plan largely by comparison with the plan already in hand for the northern barrage, or as an alternative to it, rather than on its own merits alone. In regard to the questions of technical suitability and material supply, however, the Bureau’s letter is favorable, except as to the time within which suitable mines can be made available, should their employment for this purpose be decided upon. Briefly, the Bureau of Ordnance’ verdict is that the necessary deep regulation and safety devices are fairly reliable, though not absolutely so, but that no mine explosive could be obtained in quantity from sources now existing in this country during the remainder of this year.
4. Assuming that no safety device is absolutely safe, still the percentage of loss from that cause is extremely small, and the danger to shipping resulting from such occasional non-functioning of safety devices would be far less than the danger now and in the future existing to surface craft everywhere, as long as submarines can navigate submerged without restriction.
5. The question therefore as to the practicability of the proposed method of employing mines is answered in the affirmative.
6. As to whether or not this method can and should be employed in the present material situation, reference (b) holds to the contrary. The present output of mine material is obligated in very large part, if not altogether, for the northern barrage project as a whole. For the employment proposed by Commander Laning, a large number of mines would be required, - about 200 to the running mile of sea lane eight miles wide at one deep level only, and in many cases more than one level would be needed. Further, several months in the present situation would be needed to develop the requisite safety device. Hence, there is small possibility of obtaining complete mines in any sufficient number for such an employment before the end of the present year of the beginning of next without diverting material from operations now in progress.
7. Although practicable, therefore, thi s [i.e., this] employment of mines could not be undertaken without radical alteration of extensive operations already in progress. Such an alteration would be prejudicial to the prosecution of the war as a whole and should not be contemplated. For future operations however, for both the present war broad and also especially in the plans for mine defense on our own coasts and in the West Indies, the method of employing mines proposed by Commander Laning promises to give a greater protection by a smaller number of mines than would result from employing a larger number of mines in the ordinary way and at the same time this method would expose our own vessels to less risk.
8. The action indicated at the present time therefore would seem to be to devlop the American Mark VI mine to be fully suitable for the use proposed, so as to be in a position to undertake its manufacture for such use in suitable numbers as soon as plans therefor shall be completed and their execution ordered. At the same time, in making plans for the use of mine fields on our coasts and in connection with our operations in the West Indies the proposed method of employment should be fully considered and adopted where appropriate.
9. It is further recommended to refer a copy of this correspondence and the Department’s action to Vice Admiral Sims for consideration in connection with the further employment of mines abroad.