Then again the number of ships required would depend on the relation between their horizontal dimensions and the breadth of the channel to be blocked. For instance, a single vessel whose beam dimensions were approximately equal to the breadth of the lock gateway would be sufficient to block the latter, provided that the height of her hull also agreed with the conditions just mentioned above. Now, it had to be borne in mind that if a vessel was sunk in the lock gateway the "cutting-away" method would be greatly facilitated by the erection of cranes and machinery, within a few feet of the vessel, on dry land. This position, being so far removed from the tidal current which runs parallel with the Belgian coast, was unaffected by silt. Thus, although the lock gateway, by reason of its small breadth, could be completely blocked by any suitable vessel sunk therein, the work of salvage would be very much less difficult here than elsewhere. {50} Further out, between the wooden piers at the canal entrance, the navigable channel was approximately one hundred and twenty feet in breadth; i.e., slightly over one-third of the whole distance between the piers. A vessel of one hundred and twenty feet in length, therefore, would require to be turned dead across the navigable channel before sinking if she was to block {51} every inch of it. Obviously, a vessel of three hundred feet in length would not require to turn herself to anything like the same extent. The maximum depth in this position was believed to be about thirty-six feet at high tide level. Thus, we arrive at the conclusion that a blockship sunk between the wooden piers would need to have a hull whose height was not less than thirty feet, and to have a length of at least one hundred and twenty feet. [Illustration: Plan of CANAL ENTRANCE CHANNEL] [Illustration: SECTIONAL SKETCH of SUNKEN BLOCKSHIPS] In this position the silt was known to be very active. That fact, taken in conjunction with the exposure to rough seas, the presence of the tidal current, and the impracticability of erecting salvage plant on the land within easy reach of the vessel, rendered it obvious that, all things considered, the position between the wooden piers would be the ideal blocking position if suitable vessels were available for the purpose, and if such vessels were damaged and sunk with due regard to anti-salvage considerations. {52}