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

Mr. Bruce Bell cont'd :

The reason for introducing these modifications was, 1st. for the adoption of gates. The shortness of the available length of ground, laid out at the requisite inclination, brought the level of high-water up to the door of the purchase-house ; gates were therefore put in at the river line, which were shut, as in a graving dock, after the vessel had been hauled up, and the water had ebbed. 2nd. For adoption a curved line, in place of a straight line of vertical inclination, was to avoid the great cost of sinking into the bed of the Thames to the depth required for the extreme end of a straight incline, and the certainty of constant deposit of mud at such a depth.

It might be asked why not have made a graving dock at once?  The answer to this was the expense of excavation, and the constant expense, ay every opening of the dock, of clearing out the mud brought in from the river, and the cost of pumping, all od which evils were avoided in the dock as designed. This dock was unique of its kind, both in design and cost, the latter having been about £5,000 (equivalent to c. £612.5k in 2019) [771] At high water the tide came up to the purchase-house door, and the total length of the slipway was only 356 feet. On the ordinary mode of construction it must have been 500 feet at least.

Another dock, combining the dry-dock and slip-dock, was that constructed at Meadowside on the Clyde. It was laid on a straight incline, and had ample room above high-water ; but to gain the use of the submerged portion of slipway, a chamber for the reception of a caisson was placed a little above the low-water line. This slip received vessels of 1,500 tons. The cost of this dock was about £18,000, (equivalent to c. £2.2m in 2019) that of Kelvinhaugh Dock was about £10,000. (equivalent to c. £1.2m in 2019) [771]

Complete plans for a graving slip-dock with caisson were made in 1859, and were sent to Stettin to be erected by a Prussian engineer. For a long time after the introduction of slip- docks, the vessels for which they were intended not exceeding probably 300 or 400 tons, and the beaches upon which they were laid (principally on the east coast) having a good range of tide, rendered the laying of the ways a comparatively easy operation by tidal work ; but when larger ships came to be provided for, and slip-docks to be built on the Clyde with a range of only 9 or 10 feet of tide, the work of laying the outer end of the ways became difficult.