Rear Admiral Ralph Earle, Chief of the Bureau of Ordnance, to Secretary of the Navy Josephus Daniels
Bureau of Ordnance.
January 3, 1918.
To : The Secretary of the Navy, (Operations)
Subject : Anti-submarine mining.
Reference : (a) Commander H. Laning’s letter of Dec. 14, 1917 to Sec. Nav.
I. A copy of reference (a)having been referred to the Bureau for comment and recommendation, the following remarks are submitted:
2. The plan of anti-submarine operations suggested by Captain Laning is set forth by him apparently as a substitute for the barrage plan, and comparisons of the two are made to the disadvantage of the latter. The Bureau is of the opinion that the two cannot be properly compared, as they are essentially different in the purpose sought.
3. The barrage plan has for its object the botting up of the enemy’s submarines and therefore their banishment from the high seas. The sea-lane plan, on the other hand, would merely safeguard friendly shipping in comparatively restricted areas. The former is an offensive-defensive on the order of a strict blockade; while the latter is a local protective measure.
4. As Captain Laning states, “ocean traffic is congested in the waters adjacent to France and England, and it is in these waters that the vast majority of submarine attacks occur”. But these waters are of very considerable area, and to mine sealanes to even half a dozen ports would require a prohibitive number of mines. Captain Laning’s estimate of 20,000 mines for each I00 miles of lane is taken as a basis for consideration though a lesser number might serve. Assuming that I,000 miles of sea-lane would suffice, 200,000 mines would be needed, or considerably more than the proposed barrages will require. As the number of mines that can be manufactured and planted is limited, the adoption of the sea-lane plan would necessitate the abandonment of the barrage plan. There will not be enough mines for both.
5. Enemy submarines have attacked shipping a very considerable distance off shore, probably 500 miles, and we have also the exploit of the U-53 off Nantucket to show that it would not be enough to merely protect shipping near port. We must expect that as long as enemy submarines are permitted access to the high seas they will find opportunities to attack our ships.
6. The 220-fathom line runs nearly north and south only a few miles off the west coast of Ireland. The proposed mined sea-lines could not be extended to the westward of that line, using present types of mines, and it is not believed practicable to design mines for much greater depths than this.
7. It is reliably reported that the British have not yet been able to make the English Channel very unsafe for enemy submarines, and that the great majority of these vessels enter the Atlantic through this Channel. The Channel is only 25 to I00 miles wide and it would appear that a barrage across it should be the first step in any anti-submarine mining operation. If such a barrage cannot be made effective it is quite hopeless to attempt anything of the magnitude of the proposed sea-lane plan or the Scotland-Norway barrage.
8. An English Channel barrage, if fully effective, would alone be of far greater value, in the Bureau’s opinion, than the proposed sea-lane project since it would force the enemy to send his submarines around the northward of the British Isles, a distance of I200 to I500 miles, to attack our shipping. This would reduce their time on station by half at least, which would be equivalent to a similar reduction in the number of submarines.
9. Referring to paragraph 4 of reference (a) the Bureau does not believe that Captain Laning’s criticisms of mine barrages are well founded. They will be very briefly considered here in the order mentioned by him.
(a) The mine barrage need not be laid in the enemy’s sphere of action. The English Channel is not, or should not be, in the enemy’s sphere of action. The Aberdeen-Norway barrage which was proposed by this Bureau, is sufficiently removed from German bases and could be protected from British bases. As for the proposed Orkney-Bergen barrage, it would be about 500 miles from the nearest German base, and to the northward of the base of the British Grand Fleet.
(b) A vigilant patrol and a sufficiently heavy supporting force at nearby bases, to prevent the enemy clearing passages through a barrage is required: but every military line either offensive or defensive, whether on land or sea, naturally requires force to maintain and protect it; and if the barrages should be effective, all naval operations elsewhere would sink to insignificance since the enemy vessels would be enclosed, and all lines of communication would be safe from end to end except insofar as an occasional raider might cause some damage.
(c) A large and constant patrol of anti-submarine craft would be required as stated above, to prevent submarines crossing the barrages on the surface, but not a large force is now employed on convoy and other anti-submarine service. A supporting force in readiness is essential.
I0. In paragraph 5 of reference (a) three advantages are claimed for the sea-lane plan, the first and second of which apply, without qualification, to the barrage plan as adopted.
II. It has been mentioned above that the number of mines required for the sea-lane plan would be prohibitive. This is to be explained by the fact that the quantity of high explosives obtainable during the next year will be insufficient for more than I00,000 mines plus the depth charges, bombs, etc. that must be supplied. Any considerable increase in the supply of high explosives can be had only by the creation of new sources of supply. As an indication of present conditions, it may be stated that the Army requirements of high explosives are twice as great as can be produced from existing sources in this country, and that no foreign supply is available. The Bureau, after acquiring all TNT obtainable since the beginning of the war will have barely enough to charge I00,000 mines by April, 1918, and the available supply throughout the remainder of the year will suffice only for existing projects.
12. One more possible objection to the sea-lane plan of mining should be mentioned, though the Bureau believes it may be overestimated, and that is, the danger of such mining to the shipping that must pass over it. Depth regulation of mines and safety devices to render mines inoperative in case they rise too near the surface are fairly reliable but probably not absolutely so, and occasionally accidents from mines insufficiently submerged would be inevitable. The British mines of latest type have devices for rendering them safe should they rise above a predetermined depth. The American Mark VI mine is not so fitted at present as it was designed primarily for mining to the surface. But by planting our mine sufficiently deep the upper extremity of the antenna may be submerged to any desired depth. The safety of the mine then depends upon the certainty and accuracy of the depth setting mechanism in the mine anchor.
13. In conclusion the Bureau recommends that the mined sea-lane plan be not favorably considered as it does not appear to offer a solution of the submarine menace equal to the Dover and North Sea barrages and entails an expenditure of an immensely greater quantity of mine without similar effectiveness.
(S) Ralph Earle.