A change in previous naval practice was considered necessary at an early period, because vessels moving at anything but high speed, particularly in the case of a number of ships in company, ran very considerable risks in waters where we might expect enemy submarines to be operating, unless screened by a strong force of destroyers. This conclusion affected the movements and operations of the Battle Fleet, since the number of destroyers we possessed was quite inadequate to form a screen for a Battle Fleet and a Battle Cruiser Squadron which constantly kept the sea. The number required, for such a screen to be effective, was at least 40 for the Battle Fleet alone as then constituted, and that number, 40, was all that we had stationed at the Northern Base. The fuel capacity of destroyers was only sufficient for them to remain at sea in company with a fleet for some three days and nights, whereas the Fleet itself could remain out for three or four times that period. Moreover, the destroyers could not be kept nearly so constantly at sea as the large ships owing to their requirements in the way of boiler-cleaning and the refit and adjustment of their more delicate machinery, and the necessity for giving not only the machinery, but the personnel, periods of rest. The heavy ships, then, had two alternatives, either to remain at sea without a destroyer screen or to return to harbour with the destroyers. In the early days the first alternative was adopted, the risk being accepted, but minimised as far as possible, by keeping the ships in the northern part of the North Sea.