Volume Ten - Pre-Victorian to the present day - even more aspects - The Patent for the Penarth Slipway - 1881 . . .

Mr. Bruce Bell cont'd :

The construction of a cofferdam was too serious a matter to contemplate, and in the first slip-docks constructed on the Clyde, those for Messrs. Barclay, Curie &Co., about forty years ago, the lower ends of the ways were laid upon a long platform of timber, as described by the Authors, and were sunk upon a bed prepared for them by dredging. This was also done with the Kelvinhaugh slip, the platform being about 300 feet long, but in some of the larger slips since made in the Clyde and elsewhere, a much stronger frame- work of timber was required ; and in the case of the Meadowside slip, the bed had first to be piled, and the pile-heads cut off to the required line by sawing under water, and the frame-work set on top of them.

The best foundation for the upper end of the large, slip-docks where not water-borne, and in fact for the whole of the slipway if made within a cofferdam, was ashlar stones set continuously upon solid masonry or concrete, and the cast-iron rails laid upon them without any timber intervening, the rails accurately fitted to the stone and bedded on flannel soaked in tar. The action upon the rails being a continuous steady motion by rollers of small diameter, closely set together, required a firm unyielding foundation.

The advantages and disadvantages of a slip-dock as compared with a dry-dock were less cost, better circulation of air around the vessel, better light, and no pumping for leakage. Against greater length of ground occupied, the larger amount of scaffolding required, and the great depth to which the after end required to he sunk beyond what was necessary for the draught of vessel. Where ground was of little value, and ample length could be obtained without inconvenience, the great length required for slipway was of little consequence ; but where ground was valuable, and powers could not be got to project the slipway into the harbour or river, it became a more serious consideration.

In the case of a dry-dock the only portion of the works unavailable for dock-floor was the entrance and gate chamber, not many feet in all. But in the case of a slip-dock a very large extent of dock-floor was rendered valueless. For example, supposing a slip were required for vessels 3OO feet in length, such a vessel could be trimmed with ballast so as to have only 11 feet depth of water at the bow, about 3 feet more were required for the cradle and keel-blocks, or 14 feet in all, as depth of water on the rails, at a point 300 feet from the extremity of the slipway.