The North Atlantic homeward-bound trade was brought under convoy in May, 1917, and the Gibraltar homeward-bound trade in July, but for some months it was impossible to provide for the institution of a complete convoy system.  At first some 40 per cent, of the homeward-bound trade was convoyed.  Then the system was gradually extended to include first 60 per cent., then 80 per cent., and finally 100 per cent, of the homeward Atlantic trade and the trade from Gibraltar, trawlers being used as escorts for the Gibraltar trade, as the majority of the ships therein engaged were slow.  But trawlers are unsatisfactory escort vessels.

In the early stages of the convoy system difficulties were experienced from the fact that all the available destroyers and most of the sloops were used as escorts, with the result that the ships not under convoy were left with but little protection.

CHAPTER V

THE CONVOY SYSTEM AT WORK

As has been mentioned in Chapter II., the first ships to be brought under a system of convoy were those engaged in the French coal trade and in the trade between Scandinavia and the United Kingdom.

In the case of the French coal trade, commencing in March, 1917, the steamships engaged in the trade were sailed in groups from four different assembly ports, viz.: 

Southend to Boulogne and Calais. 
St. Helens to Havre. 
Portland to Cherbourg. 
Penzance to Brest.

Between Southend and Boulogne and Calais the protection was given by the vessels of the Dover Patrol in the course of their ordinary duties, but for the other three routes special escort forces were utilized, and daily convoys were the rule.

Owing to the great demand for coal in France, sailing vessels were also used, and sailed under convoy from several of the south-west ports.

A large organization was required to deal with the trade, and this was built up under the supervision of Captain Reginald G.H.  Henderson, C.B., of the Anti-Submarine Division of the Naval Staff, working under Vice-Admiral (then Rear-Admiral) Sir Alexander Duff, head of the Division, in conference with the Commanders-in-Chief, Portsmouth and Plymouth, under whose direction and protection the convoys were run.  The immunity of this trade, carried out in the infested waters of the English Channel, from successful attack by submarines was extraordinary.  No doubt the small size of the vessels concerned and their comparatively shallow draught were a contributory cause to this immunity.  The figures for the period March to August, 1917, show that 8,825 vessels crossed the Channel under convoy, and that only fourteen were lost.