At the same time it should be recognized how severe a strain was imposed on officers, particularly the masters, of vessels sailing in convoy.
The matter was kept constantly under review.

In February, 1917, the Germans commenced unrestricted submarine warfare against merchant ships of all nationalities, and as a consequence our shipping losses, as well as those of Allied and neutral countries, began to mount steadily each succeeding month.

The effect of this new phase of submarine warfare is best illustrated by a few figures.
During the last four months of 1916 the gross tonnage lost by _submarine attack_ alone gave the following monthly average: British, 121,500; Allies, 59,500; neutrals, 87,500; total, 268,500.
In the first four months of 1917 the figures became, in round numbers: British.

Allies.

Neutrals.