To ensure destruction, very large charges were used. To intensify and channel the explosion, an oblong "fire chamber" was constructed on each ship, 1 metre in diameter. The bay was fitted with a brick floor, 30 centimetres thick and 5 metres wide; the walls of the chamber were 1.5 metres thick; the roof consisted of old tombstones, stacked vertically and sealed with lead. The chambers with a length of 12 metres were each filled with a charge of about of high quality corned gunpowder. On top of the chambers a mixture of rocks and iron shards and other objects was placed, again covered in slabs; the spaces next to the chambers were likewise filled. The whole was covered with a conventional wooden deck.