These general anti-salvage considerations, however, did not furnish us with all the data required. They required to be dealt with in greater detail, and the matter of dimensions was another important factor.
It was essential to render impossible the passage of the German naval craft out of the canal over the top of the sunken blockships. The tide at Zeebrugge rises fifteen feet between its low and high levels. Allowing six feet as the minimum depth required to float small naval craft, it will be seen that the upper portion of each blockship should reach to within six feet of high tide level, or, at least, nine feet above low tide level, when resting on the bottom. The height of the blockship's hull, therefore, would need to be equal to the depth of the sea at low tide level plus, at least, nine feet. Now, the choice of vessel is naturally limited. In the midst of war it is unlikely that a navy would possess many craft, if any, which were not already in use for other purposes. Thus, the dimensions just referred to would have to fall within certain limits, namely, those corresponding to the dimensions of the only vessels from which one is likely to be able to choose. That part of the total height due to the rise of tide was beyond control; it would be the same anywhere in the same locality. Thus, the position chosen for the blocking must necessarily have a low tide depth of such an amount as would make the total depth at high water correspond to the total height of the available hulls.
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.
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 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.
Plan of CANAL ENTRANCE CHANNEL