The advantages were:
(1) That, except for the difficulty of preventing the submarines from using Norwegian territorial waters for egress, a North Sea Barrage would be a menace to submarines using the Kattegat exit as well as those coming from North Sea bases.
(2) That the enemy would be unable to sweep up the minefield, owing to its distance (over 200 miles) from his bases.
The disadvantages were:
(1) The immense number of mines required—some 120,000, excluding reserves—and the improbability of producing them in Great Britain.
(2) The great depth of water in which many of them were to be moored, a depth in which no mines had ever been successfully laid before; time would be required to devise arrangements that would enable the mines to be laid at such depths.
(3) The very large number of patrol craft that would be needed to force submarines to dive into that portion of the minefield which was safe for surface vessels and the difficulty of maintaining them at sea in bad North Sea weather.
(4) The difficulty of preventing egress by the submarines in Norwegian territorial waters, in which, even if mines were laid, they would have to be moored at such a depth as not to constitute a danger to vessels on the surface.
Shortly after the subject was broached to us we learned that the United States Navy had devised a mine that it was expected would be satisfactory for the purpose of the barrage. An experienced mining officer was at once sent over by us to inspect the mine and to give to the United States officers such assistance as was possible due to his great knowledge of mining under war conditions.
When he arrived in the United States the mine was still in the experimental stage, but later he reported that it promised to be successful, and in view of the great manufacturing resources in America, it appeared that a considerable proportion of the mines for the barrage could be provided by the United States Navy. Our own efforts to produce a mine suitable for very great depths were also proving successful and anticipations as to manufacture were optimistic. Accordingly plans were prepared for a barrage across the North Sea, which were given to Admiral Mayo before he left England on his return to the United States. Without seriously relaxing our mining operations in the Heligoland Bight, and without interfering with our mine barrage on the Folkestone-Grisnez line, we anticipated at this time that we could provide mines for our portion of the North Sea Barrage by the time that the United States supply of mines was in readiness to be laid.