Rear Admiral Joseph Strauss, Commander, Mine Force, Atlantic Fleet, to Vice Admiral William S. Sims, Commander, United States Naval Forces Operating in European Waters
HIGHLY SECRET England.
20 August 1918.
From : Commander U.S. Mine Force.
To : Force Commander.
SUBJECT : Proposals for Adriatic Mine Barrage.
Reference : (a) Allied Conference on Mediterranean Mining –
Malta August 6-9, 1918.
(b) Memorandum from Vice Admiral Cusani of the
(c) Cable # 229 of August 17 from Alusna, Rome to
Rear Admiral Strauss, via Force Commander.
Inclosures : (A) Chart showing proposals.
(B) Copy of reference (b)
(C) Copy of reference (c)
I. Reference (a) is being printed at Malta by Vice Admiral Calthorpe, R.N. President of the Conference, and copies will be supplied the Force Commander. The final recommendations of the Conference in regard to the Adriatic are quoted as follows :
“(a) The Conference recommends :-
I. (i) That the present mine net barrage now being laid by Italy and France be completed as designed between Otranto and Fano Island and that the United States should reinforce it by laying deep mines to complete its efficacy as a barrage to a depth of 85 meters from the surface.
(ii) Where the great depth of water does not permit of mines of present type being laid, the depth of the net o<b>struction should be increased to at least 85 metres.
(iii) With reference to the passage of submarines over the top of the nets the Conference were not satisfied that any present form of towed explosive obstruction had proved effective in anything but fine weather and that further experiments must be made before deciding on this question. In the meanwhile the present mobile barrage patrol should continue.
Admiral Ratye did not concur with the recommendations embodied in (iii). He was of opinion that the space on top of the nets should be guarded by moored mines or by towed de Quillac mines.
“2. That a mine barrage between Cape Santa Maria de Leuca and Fano Island or farther on the Italian coast up to Cape Otranto and across to Fano Island be laid by the United States conditional upon the successful production of a mine suitable for the depth involved. That this barrier extend from three metres from the surface down to a total depth of 85 metres. That a gate be left near the Western and not exceeding five miles in length and to be narrowed in future should experience warrant its reduction, such gate to be free of all mines down to a depth of 12 metres, but to be thoroughly patrolled by craft capable of fighting submarines and in sufficient number to compel the submarines to dive into the minefield. That inasmuch as the United States Government would undertake to furnish the mines and lay them, that the operation of laying, controlling and maintaining the barrage be under their jurisdiction.
Rear Admiral Salazar agreed on the condition that when battleships have to pass through the channel it may be swept to a greater depth.
3. If, however, the strategical objections raised by Italy can be overcome, we were of opinion that a complete mine barrier with suitable gate between Otranto and Cape Linguettashould be laid in preference to the one proposed betw een Santa Maria de Leuca and Fano Island . This would be done with the material already designed, as the water is shoaler, and moreover, the barrier would be less exposed to the sea.
Admiral Salazar did not concur in the mine barrier being placed in the position proposed above”.
2. Upon arrival in Brindisi August 13, Lieutenant Lais of the Italian Navy presented Inclosure (B) to me which was later approved by the Italian Chief of Staff as stated in Inclosure (C)[.] This in substance means that Italy withdraws her objections expressed in paragraph 3 of the quotation in paragraph I, and instead of, insisting on the mine barrage being placed so far to the south’ard as to involve depths of water greater than 500 fathoms which will probably require a new type of mine, they now propose the barrage extending from Brindisi to <Sasseno> Island which is to all intents and purposes the same as that which was most favored by the other nations represented at the Conference.
3. The following decisions with regard to the characteristics of the barrage were agreed upon at the Conference :
(a) The barrage to extend from 3 meters below the surface to 85 meters below the surface.
(b) A gate to be left near the western end not exceeding five miles in length and to be narrowed in future should experience warrant its reduction, such gate to be free of all mines down to a depth of 12 meters but to be thoroughly patrolled by craft capable of fighting submarines and in sufficient number to compel the submarines to dive into the minefield.
(c) That, inasmuch as the United States Government would undertake to furnish the mines and lay them, that the operation of laying controlling and maintaining the barrage be under their jurisdiction.
4. The enclosed chart has been prepared to meet the above requirements and show the necessities from a mining stand point, as to material which will be required and the area which will be occupied by the minefield. It also incorporates certain other features upon which <d>efinite decisions should be reached by the Allies involved at the earliest possible date and before any work is begun by the United States.
5. The area occupied by the minefield represents the minimum space in which the mines can be laid by the present Mine Force. The field will consist of :
10 complete rows with 35 foot antennae laid 45 feet below
4 complete rows with 70 foot antennae laid 125 feet below
4 complete rows with 70 foot antennae laid 205 feet below
4 complete rows with 70 foot antennae laid 285 feet below
All mines will be spaced 300 feet apart in each row. This will represent a theoretical destructive efficiency of 65.2 per cent for submarines passing through the barrage on the surface and 34.4. per cent for submarines passing submerged between depths of 45 feet and 300 feet (assuming beams of submarines to average 30 feet)
6. The total number of mines which will be required for this barrier are as follows :-
Present type (This includes anchors with mooring cables 240, 320, 400, 900 and 950 feet long.
3390 mines with 45 foot plummet cord and 35 foot antennae.
1430 mines with 125 ft. plummet cord and 70 foot antennae.
1430 mines with 205 ft. plummet cord and 70 foot antennae.
1430 mines with 285 ft. plummet cord and 70 foot antennae.
Total present type 7680 mines.
Special type. (Mooring cables for 500 fathoms).
5830 mines with 45 ft. plummet cord and 35 foot antennae.
2350 mines with 125 ft. plummet cord and 70 foot antennae.
2350 mines with 205 ft. plummet cord and 70 foot antennae.
2350 mines with 285 ft. plummet cord and 70 foot antennae.
Total special type 12,880 mines.
Grand total. 20,560 mines.
7. The decision reached by the Conference as to the characteristics of the gate (vide par 3.b) is not considered entirely satisfactory. A width of five miles is far greater than is necessary since practically no shipping exists in the Adriatic. One or possibly two miles of gate should be sufficient. Deep mines are not considered satisfactory for placing below a gate on account of the irregularity with which they take their depth when planted and from past experience it appears that the mental anxiety due to the uncertainty of the field is sufficient to keep ships from passing over it. A far more satisfactory solution for establishing a gate would be to place mine nets similar to those now being laid in the Straits of Otranto at each end of the gate, as shown on the chart, extending from 30 feet below surface to the bottom. This would form a more complete barrier below the gate than mines and besides having a higher destructive <value> makes passage through the gate much safer.
8. The passage between Sas<s>eno Island and the mainland to the eastward has been closed. The exact nature of the obstruction is no<t> know<n> though I think it is mine nets. It is quite possible that the French Commander-in-Chief in the Mediterranean will insist on a gate being left here to allow the French Fleet based at Corfu access to the Adriatic without going through the western gate of the barrage. If such a gate is required the same characteristics should govern it as were suggested for the western gate, namely:- as narrow as possible, mine nets from 30 feet below surface to the bottom, constant patrol.
9. Access to Valona Bay must be preserved on account of the shipping which supplies Italian troops in Albania. Therefore, in order to obtain the maximum efficiency from the barrage it is desirable that a mine net should be laid from Cape Linguetta to meet the barrage and thus prevent a clear channel for submarines which may have successfully passed through the northern part of the barrage.
10. Summarizing, it is considered desirable to incorporate the following points if possible for establishing a mine barrage in the Adriatic when this subject is presented to the Allied Naval Conference for final decision.
(1) Limits of barrage to be shown on chart enclosed.
(2) Gate at western end of barrage to be reduced as much as possible.
(3) Mine nets pre<f>erable to deep mines below gate at northern and southern extremity of barrage and off Cape Linguetta.
(4) Italians and French to supply and lay nets, being aided by the United States supplying such material as cannot be obtained by them.
(5) Gates to be constantly patrolled day and night to prevent submarines passing through on the surface.
(6) If French insist on <gate> being left between Sas<s>eno Island and mainland to the eastward, that gate be as small as possible and that it also be patrolled by mine nets and patrols.
(7) Responsibility for the provision and laying of the mine field rests entirely with the United States.
(8) Responsibility for laying mine nets to rest with Italy and France.
(9) Responsibility for maintaining patrols to be under the Allied Commander-in-Chief of the Mediterranean.
(I0) Upon request of the Commander U.S. Mine Force the Allied Commander-in-Chief will furnish anescort of approximately 12 destroyers to protect the U.S. Mine Force en route to and from and during a mining operation.
/s/ J. Strauss.
The Malta Commission (August 1918) considered the Otranto Channel barrage and proposes as follows :-
(I) –Surface explosion net obstruction between Otranto and Fano, reinforced below with a mine barra<g>e to a depth of 85 meters below the surface. Where the depth will not pe<r>mit planting mines, (greater than 500 fathoms) the net obstruction is to be carried to a depth of at least <85> meters.
(2) - Place a mine barrage from 3m, to 85 m. from the surface so as to close completely the Otranto Channel from one shore to the other, leaving only a single gate not longer than 5 miles.
The position which the Commission believes preferable for this barrage is that joining Otranto-Capo Linguetta, on which line the depths are not greater than 500 fathoms, thus permitting the rapid laying of the barrage.
If Italy does not accept this proposal, the barrage can be placed between Tricase and Fano, but only after the Americans have perfected mines that can be anchored in depths of 600 fathoms.
The Commander-in-Chief of the Fleet, having considered the above, accepts fully proposal (I) of the Commission. In regard to Proposal (2), he observed that the greatest advantages may be obtained by planting the barrage between a point 25°(true) distant 5 miles from Cape Cavallo (Lat.40<°>43<’>N long. 18<°>-40<’>30<″> E.) and the northern extremity of Sas<s>eno.
This solution would have the disadvantage of a greater length of line – 56 miles as ag<a>inst 45 miles of the Tricase – Fano Line -, but it would offer the following very great advantages :-
(a) Protection against an offensive coming from any of the Adriatic ports for all the Valona traffic, as well as for that of Taranto and Brindisi.
(b) Would not inter<f>ere with the work of the units of the m<o>bile barrage, leaving them sufficient room for hunting with hydrophones, which is to be continued until the proof of the efficacy of the barrage.
(c) This, together with the net obstruction at Otranto-Fano, would form a sheet of protected water between Val
lona-Fano and the Italian coast.
(d) It would not require a gate, passage being effected between the west end of the barrage and Cape Cavallo.
(e) The ends of the barrage being at two naval strongholds, it would not need any further protection for the ends of the gate.
(f) The depths are less than those of the Otranto-Linguetta line, showing a maximum of about 430 fathoms as against 470 fathoms.