There remains the third method, namely, piece-meal removal by means of "cutting away." Cutting away can be accomplished, in the ordinary course of events, by means of acetylene gas cutters or by pneumatic tools. Acetylene gas will cut through steel with little more effort than a knife cutting through india-rubber. But acetylene gas cannot be used under water and cannot cut through large thicknesses of cement. Pneumatic tools provide a very laborious and {48} tedious means of cutting large quantities of steel. Work under water entails the use of divers. Thus, the removal of a ship by the piecemeal process is an exceedingly prolonged undertaking, especially as each piece must be lifted out when cut away; for reasons already stated the pieces must on no account be allowed to fall to the sea-bottom. From the foregoing remarks we arrive at the following conclusions. The blockships should be too large to lift off the bottom by the hawser method. They should be extensively damaged and sunk in such a manner that they would rest on the damaged portion of the hull. They should be fitted to counter "cutting away" tactics, and should be sunk in positions where silt would render impracticable the explosive method of dispersion; the damage should be so situated as to give the silting sand access to the hull through the holes in the latter. 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 {49} 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.