It was believed that the Mole garrison consisted of about one thousand men. But what of {82} reënforcements arriving from the shore? Access to the Mole would entail the crossing of the viaduct by such reënforcements. Therefore the viaduct must be destroyed. Consideration on this point led to a decision to utilise one or more submarines filled with explosives and to blow them up under the viaduct, so as to cut the latter in twain. This particular phase of the operation had not been included in the original Plan evolved at the Admiralty. As previously stated the first edition could not be expected to cover every single investigation of every point in the problem. The attack on the viaduct, after a large amount of experimental work ordered by Vice-Admiral Keyes, took the following shape. Two submarines, each carrying several tons of high explosive, were to accompany the expedition. They were each to carry a crew of two officers and four men, who, after securing their craft underneath the viaduct, were to light the time fuses and then to take to the boats. Each submarine carried a small motor-driven dinghey for this latter purpose. So much, then, for the blockships, storming vessels, and submarines at present. {83} CHAPTER VI THE VESSELS INVOLVED: THEIR DUTIES. THE RESCUE WORK. In addition to the special vessels mentioned in the preceding chapter, many other vessels and craft were required to assist in the operation. One can imagine the amateur reckoning up the probable number as follows. Three blockships at Zeebrugge and two at Ostende, three storming ships and two submarines at the former place. That makes ten vessels of sorts. Allow a few more for other purposes--say, fifteen altogether. As a matter of fact, there were one hundred and sixty-two. Let us see why so many were required. Take the requirements necessitated by the use of smoke screens. It has already been stated that the section of coast on which the Germans had established heavy gun batteries was twenty-one miles in length. Smoke screens were required to mask those guns so that the approach of the blockships and storming vessels should remain undiscovered until the latest possible moment. This meant that a large number of craft were necessary for smoke screening alone. Again, if the smoke screens were to be efficient the smoke would have to be emitted within a short distance of the coast; i.e., in comparatively shallow water. Thus shallow-draught vessels were necessary. Shallow draught goes hand-in-hand with small dimensions. The carrying capacity of small craft is very {84} limited; this constituted an additional reason for employing large numbers.