During the 18th century the Greenland whale-fishery vessels were normally around 350 tons and were constructed in such a manner as to resist, so far as possible, the pressure of the ice. The crew normally numbered about fifty and the vessel carried six or seven whale-boats. The vessels left London at the beginning of April, and, after touching at the Shetlands, reached the whaling-grounds before the end of that month. In approaching a whale, silence was essential. The harpoon had to be delivered within a distance of a few yards. The moment the wounded whale disappeared, a flag was hoisted in the boat to give notice that assistance was required from the ship. It was vital that the whalers tended to the line, for if it became entangled with their boat they could be easily dragged under water by the whale.