The principal doubt was as to the ability of the enemy to train submarine crews with sufficient rapidity to keep pace with his building programme.

However, it was ascertained that the Germans had evidently devoted a very great number of their submarines to training work during the period September, 1915, to March, 1916, possibly in anticipation of the unrestricted warfare, since none of their larger boats was operating in our waters between these months; this fact had a considerable bearing on the problem.

As events turned out it would appear either that the training given was insufficient or that the German submarine officer was lacking in enterprise.

There is no doubt whatever that had the German craft engaged in the unrestricted submarine warfare been manned by British officers and men, adopting German methods, there would have been but few Allied or neutral merchant ships left afloat by the end of 1917.

So long as the majority of the German submarine attacks upon shipping were made by gun-fire, the method of defence was comparatively simple, in that it merely involved the supply to merchant ships of guns of sufficient power to prevent the submarine engaging at ranges at which the fire could not be returned.  Whilst the method of defence was apparent, the problem of supplying suitable guns in sufficient numbers was a very different matter.  It involved arming all our merchant ships with guns of 4-inch calibre and above.  In January, 1917, only some 1,400 British ships had been so armed since the outbreak of war.

It will be seen, therefore, that so long as ships sailed singly, very extensive supplies of guns were required to meet gun attack, and as there was most pressing need for the supply of guns for the Army in France, as well as for the anti-aircraft defence of London, the prospect of arming merchant ships adequately was not promising.

When the enemy commenced unrestricted submarine warfare attack by gun-fire was gradually replaced by attack by torpedo, and the problem at once became infinitely more complicated.

Gun-fire was no longer a protection, since the submarine was rarely seen.  The first intimation of her presence would be given by the track of a torpedo coming towards the ship, and no defence was then possible beyond an endeavour to manoeuvre the ship clear of the torpedo.  Since, however, a torpedo is always some distance ahead of the bubbles which mark its track (the speed of the torpedo exceeding 30 knots an hour), the track is not, as a rule, seen until the torpedo is fairly close to the ship unless the sea is absolutely calm.  The chance of a ship of low speed avoiding a hit by a timely alteration of course after the torpedo has been fired is but slight.  Further, the only difficulty experienced by a submarine in hitting a moving vessel by torpedo-fire, once she has arrived in a position suitable for attack, lies in estimating correctly the course and speed of the target.  In the case of an ordinary cargo ship there is little difficulty in guessing her speed, since it is certain to be between 8 and 12 knots, and her course can be judged with fair accuracy by the angle of her masts and funnel, or by the angle presented by her bridge.