Rear Admiral Joseph Strauss, Commander, Mine Force One, Atlantic Fleet, to Vice Admiral Henry S. Knapp, Commander, United States Naval Forces Operating in European Waters
File No. 252-F Kirkwall,
10 May 1919.
From: Commander Mine Force.
To : Force Commander.
Subject: Report of First Minesweeping Operation.
1. The Minesweeping Detachment sailed from Inverness at 0700, Tuesday, April 29th. Divisions One and Two of the Minesweepers, accompanied by six submarine chasers, proceeded direct to the minefield while the remainder of the Detachment proceeded to Kirkwall.
2. The first Minesweeping Operation was experimental in nature, the object sought being to develop the safest practical means of sweeping the barrage. On account of the danger of counter-mining, pairs of sweepers operated at a sufficient distance from other sweepers to minimize the risk of counter-mines. The sweeps were also shortened to such a length as to prevent a sweeper from being directly above a line of mines before the sweep had completely crossed the previous line. This condition, however, depended upon the assumption that the mines had been laid in rows 500 yards apart, were still in position, and that the sweepers maintained practically perfect stations.
3. The results of the experiment which continued for two days showed seven separate instances of counter-mining. There were in all 221 mines cut up or exploded by the sweep. This represents slightly over 25 percent of the original umber of mines laid in the areas which were swept.
4. In addition to the counter-mining, an additional danger was encountered for which it has been so far impossible to account.
(a) The first case was when a middle level mine exploded approximately 600 yards ahead of the PELICAN. No other explosions had occurred within a period of eleven minutes.
(b) The second instance occurred when an upper or middle level mine exploded approximately four miles astern of a pair of sweepers and about 600 yards abeam of a submarine chaser which was engaged in sinking mines which had been cut adrift. It did not closely follow any other explosion.
(c) Observations made by some of the vessels on their hydrophones while hove to at night to the northward of the minefield indicated that mines were still exploding throughout the night at irregular intervals. This condition was apparently set up by the explosions caused by sweeping, and constitutes an ever present danger, which can only be lessened by crossing the minefield the minimum possible number of times.
5. From the sequence in which explosions occurred, together with a review of some of the reports of minelaying operations, it is concluded that the mines are not in sufficiently uniform rows to justify using a very small sweep, as was done on this first operation. This conclusion, together with the danger of mines exploding in unexpected places, at unexpected times, has made it desirable to sweep a field with the maximum sweep which can be carried by two sweepers without fouling the bottom and without the use of buoys on the sweep wire. Experiments are, therefore, being undertaken to determine the length of such a sweep wire, prior to the second operation. Inclement weather has delayed operations four days. Experiments are being carried out in the meantime to determine the feasibility of using un-insulated kite wire in place of the insulated kite wire supplied, which has not proven satisfactory. The original idea of using the charged sweep to explode the mines upon coming in contact with the mooring cable apparently was unsuccessful, judging from the results of the first operation, and had the additional disadvantage of setting up strong magnetic fields aboard the ships, which produced serious deviations in the compass.
6. The second sweeping operation will probably begin May 9th or 10th, the vessels remaining at sea (weather permitting) for a period of approximately twelve days.
7. There are enclosed two copies of Reports of Minesweeping by the Division Commanders.