Volume Two - The Era of Optimism, Investment & Development - The gridiron and slipway of the Penarth Shipbuilding & Ship Repairing Co. . . .

For this purpose the bottom has been prepared, by laying down chalk or stone in suitable positions, and what are known as 'public hards' have been established in this way. The hards are usually managed by the river conservators or other public authority, or were vested in the lords and manors adjoining the shore. Many of these hards are of considerable antiquity, and were formerly used for seagoing vessels. They are still used on the Thames and elsewhere for tugs, barges, and smaller craft.

This system has been developed into the form of gridirons, in which, instead of a floor of chalk or stone, a timber framework is provided, supported by piles driven in the river bottom, and carrying wooden keel blocks on which the weight of the vessel is taken. One disadvantage of the hards was that, unless the vessel was very carefully placed, she was liable to lie over on one side or the other as the tide left her, and if any quantity of mud overlay the stone bottom there was a risk of her being held down by suction and not righting easily as the tide rose. In the case of gridirons there is no possibility of adhesion to mud in the bottom, and the vessel is kept upright by means of moveable bilge blocks.

The cross section is shown in Fig. 200, where the rise of tide at Springs is 24 ft. A ship with a draught of 20 ft. and a beam of 26 ft. could be accommodated on this gridiron at Spring tides, but if the repairs were protracted beyond one or two days, such a vessel would have to remain on the gridiron until the next Spring tides before she could be floated off.