It was further always possible for the enemy to send reinforcements of additional flotillas from German ports, or to send heavier craft with minesweepers to sweep a clear channel, timing their arrival to coincide with an intended attack, and thus to place the German forces in a position of overwhelming superiority.
Our own Dover force at the commencement of 1917 consisted of one light cruiser, three flotilla leaders, eighteen modern destroyers, including several of the old "Tribal" class, eleven old destroyers of the 30-knot class (the latter being unfit to engage the German destroyers), and five "P" boats.

Of this total the average number not available at any moment may be taken as at least one-third.

This may seem a high estimate, but in addition to the ordinary refits and the time required for boiler cleaning, the vessels of the Dover Patrol working in very dangerous, foggy and narrow waters suffered heavy casualties from mines and collisions.

The work of the Dover force included the duty of escorting the heavy traffic between Dover and Folkestone and the French ports, this being mostly carried on during daylight hours owing to the prevalence of submarine-laid mines and the necessity for sweeping the various channels before the traffic--which included a very large troop traffic--was allowed to cross.

An average of more than twenty transports and hospital ships crossed the Straits daily during 1917, irrespective of other vessels.