In the above I have been describing the operations of the U-boats in the great area to the west and south of Ireland. But there were other hunting fields, particularly that which lay on the east coast of England, in the area extending from Harwich to Newcastle. This part of the North Sea was constantly filled with ships passing between the North Sea ports of England and Norway and Sweden, carrying essential products like lumber and many manufactured articles. Every four days a convoy of from forty to sixty ships left some port in this region for Scandinavia; I use the word "convoy," but the operation was a convoy only in the sense that the ships sailed in groups, for the navy was not able to provide them with an adequate escort--seldom furnishing them more than one or two destroyers, or a few yachts or trawlers. Smaller types of submarines which were known as UB's and UC's and which issued from Wilhelmshaven and the Skager Rack constantly preyed upon this coastal shipping. These submarines differed from the U-boats in that they were smaller, displacing about 350 and 400 tons, and in that they also carried mines, which they were constantly laying. They were much handier than the larger types; they could rush out much more quickly from their bases and get back, and they did an immense amount of damage to this coastal trade. The value of the shipping sunk in these waters was unimportant when compared with the losses which Great Britain was suffering on the great trans-Atlantic routes, but the problem was still a serious one, because the supplies which these ships brought from the Scandinavian countries were essential to the military operations in France.