No trace of a submarine was discovered, and subsequent investigation showed that the alarm _may_ have been false, the evidence not being conclusive either way. The incident, however, made it clear that protection against submarine attack was an absolute necessity, as the Fleet could not remain at a base that was as open to this form of attack as Scapa Flow. The only possible action, in the event of an alarm being given of the presence of a submarine, was to take the Fleet to sea, and, in addition to the dangers arising at that time from a hurried departure in thick weather, a feeling of insecurity was created, which would be bad for moral, and the ships were deprived of opportunities for cleaning boilers, refitting machinery, etc., which experience was showing was essential if the steaming efficiency of the Fleet was to be maintained. The matter was at once represented to the Admiralty and proposals were made for blocking all the entrances to Scapa Flow, except the Hoxa and Hoy entrances, by sinking old merchant ships in the channels. Pending the supply of reliable defences for the main entrance, Rear-Admiral Miller was directed to requisition a large number of net-drifters. The plan was to lay drift nets in the entrance to be watched by the drifters, the nets having indicator buoys attached to them and floating on the surface, so that the presence of a submarine might be indicated by the buoy moving with the net. The navigational difficulties of the narrower entrances were, meanwhile, the only obstacles to the passage of submarines through them, but it was realised that these were considerable. The opinion was formed, however, that the Hoxa entrance presented no insurmountable difficulty of this nature to a determined submarine commander. By this date the local defence flotilla of destroyers consisted of vessels of the E class. A request was made for basing a hunting flotilla of 16 trawlers on Kirkwall, these vessels being intended to guard the approaches to the Pentland Firth and to act as submarine hunters generally. It was also suggested that a monetary reward should be offered to the personnel of any trawler through whose agency a submarine was captured or destroyed. The mine-sweeping force at Scapa had been strengthened by the addition of a flotilla of trawlers, and the mine-sweeping gunboats and trawlers were continuously employed in keeping certain fixed channels of approach to the Pentland Firth swept and clear of mines. These vessels were, therefore, not available for anti-submarine work, except to the detriment of their mine-sweeping duties. On September 1st the available cruiser squadrons were employed in Area No. 6, supported by the 3d Battle Squadron and the 1st Battle Cruiser Squadron.