(1) A method of blocking either mechanically or by mines all the exits of the submarines from their North Sea or Baltic bases.

(2) A barrage of mines at different depths, from near the surface of the sea to near the bottom.

(3) A combination of deep mines with a patrolling force of surface craft and aircraft whose object would be to force the submarines under the surface into the minefield.

(4) A force of surface craft and aircraft patrolling an area of sufficient extent to prevent submarines coming to the surface to recharge their batteries during the hours of darkness.

Admiral Mayo was informed that in our opinion the first scheme as given above, viz. that of absolutely sealing the exits, was the only radical cure for the evil, but that there were very great difficulties to be overcome before such an operation could be successfully carried out.  He was shown the plan that had been prepared for a mechanical block of all the enemy North Sea bases, and he entirely concurred in the impracticability of carrying it out.  Such a plan had been advocated by some officers and by other people; it was, of course, most attractive in theory and appealed strongly to those who looked at the question superficially.  When, however, a definite operation came to be worked out in detail the difficulties became very apparent, and even enthusiastic supporters of the idea were forced to change their views.  It was not a matter for surprise to me that the idea of sealing the exits from submarine bases was urged by so many people on both sides of the Atlantic.  It was, of course, the obvious counter to the submarine campaign, and it appealed with force to that considerable section which feels vaguely, and rightly, that offensive action is needed, without being quite so clear as to the means by which it is to be carried out.

In this particular case I informed the clever and able officers to whom the planning of the operation was entrusted that they were to proceed on the assumption that we intended to seal the enemy’s ports somehow, and that they were to devise the best possible scheme, drawing up all the necessary orders for the operations.  This was done in the most complete detail and with great care and ingenuity, but at the end there was no difference of opinion whatever as to the inadvisability of proceeding with the operations.

It is to be observed in connexion with this question that sealing the North Sea bases would not have been a complete cure, since submarines could still make their exit via the Kattegat, where we could not block channels without violating the neutrality of other nations.

The final conclusion arrived at was to use a combination of the last three alternatives provided that a satisfactory type of mine could be produced in sufficient numbers and a sufficient supply of small craft provided by ourselves and the United States.