The protection of the vessels employed in the French coal trade was entrusted very largely to trawlers, as the ships composing the convoy were mostly slow, so that in this case more screening vessels were available, although they were not so efficient, being themselves of slow speed.
For the introduction of a system of convoy which would protect merchant ships as far as their port of discharge in the United Kingdom, there were two requirements: (a) A sufficient number of convoying cruisers or armed merchant ships, whose role would be that of bringing the ships comprising the convoy to some selected rendezvous outside the zone of submarine activity, where it would be met by the flotilla of small vessels which would protect the convoy through the submarine area.

It was essential that the ships of the convoy should arrive at this rendezvous as an organized unit, well practised in station-keeping by day, and at night, with the ships darkened, and that the vessels should be capable also of zigzagging together and of carrying out such necessary movements as alterations of course, etc.; otherwise the convoy could not be safely escorted through the danger area.

(b) The other essential was the presence of the escorting flotilla in sufficient strength.
It has been mentioned that there was an insufficient number of vessels available for use as convoying cruisers.

It was estimated that about fifty cruisers or armed merchant ships would be required for this service if the homeward-bound trade to the British Isles alone was considered.

An additional twelve vessels would be necessary to deal with the outward-bound trade.