Against this form of attack the gun armament of cruisers or armed merchant ships was practically useless, and, however powerfully armed, ships of this type were themselves in peril of being torpedoed.
Small vessels of shallow draught, possessing high speed, offered the only practicable form of protection.

Shallow draught was necessary in order that the protecting vessels should themselves be comparatively immune from successful torpedo fire, and speed was essential for offensive operations against the submarines.
Convoy sailing was, as has been stated, the recognized method of trade protection in the old wars, and this was a strong argument in favour of its adoption in the late war.

It should, however, be clearly understood that the conditions had entirely changed.

Convoy sailing for the protection of merchant ships against torpedo attack by submarines was quite a different matter from such a system as a preventive against attack by surface vessels and involved far greater difficulties.

In the days of sailing ships especially, accurate station keeping was not very necessary, and the ships comprising the convoy sailed in loose order and covered a considerable area of water.