Rear Admiral Ralph Earle, Chief of Bureau of Ordnance to Admiral William S. Benson, Chief of Naval Operations
CONFIDENTIAL Washington July 18, 1917.
BUREAU OF ORDNANCE.
To- - - Chief of Naval Operations.
Subject: - - - -Submarine Mine Barriers: Material For:
1. - - - The Bureau has developed a new type of mine, at present referred to as Mark VI, Type X, which it is confidently believed will facilitate the establishment of submarine barriers. The mine is radically different from other mines in its firing gear, which has been tested out with excellent results, and the Bureau is now proceeding with the design of the mine as a whole, and expects to complete it within two weeks.1
2. - - - The new mine will be as easily planted as the ordinary types of naval defense mines, and therefore the time and number of vessels required to establish a barrier will be reduced to a minimum. This mine can be rigged so as to be safe as regards surface vessels, but effective against craft operating below THE surface.
3. - - - The mine will be comparatively simple in design and it is believed that it can be manufactured at a minimum rate of 1000 per day, which means that the number required for about 300 miles of barrier can be produced within three months from the beginning of deliveries or within four months from the placing of the orders.
4. - - - The Bureau requests that a decision be reached at the earliest practicable moment as to the desirability of establishing complete barriers to prevent enemy submarines from gaining access to the Atlantic. The Bureau assumes that such a project is desirable as no other means of stopping the submarine peril appears to be in prospect, and, since it is going to take four months to obtain the necessary material, the Bureau believes it should be authorized to proceed immediately with arrangements for procuring materials.
5. - - - Theoretically only 72,000 mines will be required for 300 miles of barrier, but 100,000 should be provided to allow a reasonable excess for replacements, etc. In addition a number say 25,000 should be provided for our own coast defenses, it is believed, making a total of 125,000 mines, which, at an estimated cost of $320 each, gives a total cost of $40,000,000. This estimate is decidedly liberal.
6. - - - The Bureau is of the opinion that the design, manufacture, and assembly of the new mine should be carried out with the utmost secrecy and is taking the necessary precautions accordingly, since advance information of such a mine would be of the greatest aid to the enemy in devising means to counteract it.
7. - - - The above estimate as to time is based upon our success in securing the necessary quantity of T.N.T. or other high explosive.
8. - - - In considering this project the use of high speed mine laying vessels such as liners and merchantmen in addition to destroyers and light cruisers will be required, and such vessels must be provided. The mines can be dropped accurately at any speed by time devices. The whole barrier should be laid as one operation and be protected as far as possible. If isolated mines are planted it is probable that a device to defeat the mine firing mechanism will be developed by Germany.
(signed) Ralph Earle.
Source Note: Cy, DNA, RG 45, Entry 517B.
Footnote 1: The most important mine developed by the United States in World War I, the Mark VI was conceived for the Northern Mine Barrage, a strategic plan to blockade the route U-Boats took around Scotland to reach their patrol zones in the Northern Atlantic Ocean. As early as April 1917, the Bureau of Ordnance floated the idea of mine barrages in the North and Adriatic Seas to block the movement of U-Boats. The Royal Navy found this proposal impracticable, leading to a new suggestion in May 1917 from the United States Navy Office of Operations for a mine barrage in the North Sea; the British again expressed doubt that this plan would prove effective. See: Earle to Benson, 12 June 1917. The key to ultimately convincing the British of the success of the Barrage, and thus moving the plan forward, was the development of a new firing mechanism, the K-pistol, which posed a considerable threat to submarines over a wide area, but hardly affected surface craft. Although VAdm William S. Sims shared the British Admiralty’s skepticism, the enthusiastic support of Assistant Secretary of the Navy Franklin D. Roosevelt secured the Navy's approval. Creation of the Northern Barrage was agreed to at the Allied Naval Conference of 5 September 1917. The laying of mines began in October 1917 with twelve more minelaying excursions to follow, the last of which took place in October 1918. The barrage was never completed, however, as the approaching end of the hostilities led to the cancellation of additional excursions. In total, 56,571 out of a planned 70,177 mines were laid. Friedman, Naval Weapons of World War I: 375-376.