Outside the German mine-fields, and in any inshore areas which were unmined, German patrol craft would probably be stationed. Patrol craft, in comparatively narrow waters, are effective for discovering the approach of surface vessels in clear weather by day or night. The _minimum_ harm that they could do to the attacking force would be that of reporting the latter's approach. A single alarm rocket might be sufficient. It is, therefore, almost inconceivable that the patrol vessels could be passed without the alarm being given. Any gun-firing would, of course, act as an alarm; ramming, a much more silent method, would be the best course open to the attacking craft if they encountered the patrols. There were two other forms of patrol, however, which could provide even more serious obstacles. Submarines, stationed on the route between the attacker's base and the objective, could patrol at periscope depth. The passing of the squadrons, viewed through the periscope of the unseen submarine would be reported by wireless telegraphy immediately the submarine could come to the surface. Thus, long before the attack commenced, the defenders would be perfectly well aware of the attacker's approach, whereas the latter would imagine that their mission was unsuspected. This use of a submarine, as a lookout, would be of infinitely greater importance, in such an event as this, than her use as a torpedo vessel. Aircraft patrolling off the coast--say at a height of five thousand feet--would be able to see as far as the southeast coast of England, provided the atmosphere were clear. Under average conditions of {41} visibility there would be no difficulty in discovering a naval force several miles distant. Such discovery would be immediately reported to the defences with the same result as that just described in the case of the submarine. The Germans had a strong force of seaplanes based on the Flanders coast. These machines were generally patrolling the vicinity--_provided the Allied aircraft were not about_. [Sidenote: Summary of Obstacles] We have now arrived at the stage where we can make a summary of the main obstacles in the way of a blocking enterprise at Zeebrugge. There were (_a_) the aerial patrol; (_b_) outlying submarines; (_c_) surface patrol vessels; (_d_) mines; (_e_) uncharted shoals; (_f_) lack of navigational aids; (_g_) coast defence batteries and illuminating apparatus; (_h_) the guns on the Mole; (_i_) the obstruction booms; (_j_) the harbour defence craft; (_k_) the shore batteries defending the canal; (_l_) the difficulties of seamanship in a tideway; and lastly (_m_) the vagaries of the weather. In connection with a blocking enterprise at Ostende the same obstacles applied with the exception of those resulting from the presence of the Mole. The list is undoubtedly formidable _though not yet complete_. The operation, on the face of it, did not seem to be altogether simple.