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.
It will be seen, then, how easy was the problem before the German submarine officers, and how very difficult was that set to our Navy and our gallant Mercantile Marine.
It will not be out of place here to describe the methods which were in force at the end of 1916 and during the first part of 1917 for affording protection to merchant shipping approaching our coasts from the direction of the Atlantic Ocean.
The general idea dating from the early months of the war was to disperse trade on passage over wide tracts of ocean, in order to prevent the successful attacks which could be so easily carried out if shipping traversed one particular route.