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

Such a slipway consists : first, of a foundation, the nature of which must, of course, be specially decided by the engineer for each particular situation ; secondly, of longitudinal timbers on cross-sleepers carrying cast-iron rails (Plate 5, Figs. 3 to 7) ; thirdly, of a cradle provided with cast-iron rollers and sliding bilge-blocks for receiving the vessel ; and fourthly, of the hauling machinery. The modes of construction of the foundations, ways and cradle, present little variation in slipways of different systems ; but the hauling gear, though now generally actuated by water-pressure, is subject to variation according to the ideas of the manufacturer or designer.

It is proposed to consider briefly each of the four sections, and to describe the operation of working when it is required to take on a vessel for repairs.

After deciding upon the construction of a slipway, it first devolves upon the engineer to fix the inclination of the ways. This can generally be at once determined by a consideration of the amount and value of land at his disposal, by the depth of water it is necessary to provide over the cradle when at its lowest position, and sometimes by the natural slope of the ground. As a rule, gradients of from 1 in 15 to 1 in 22 have been adopted ; but these by no means form a limit, as it is obvious that no special engineering difficulties attend the introduction of either a greater or lesser inclination. The most suitable description of foundation can only be arrived at after a careful survey of the ground, and it is of course best to take borings from top to bottom of the site. In some cases no further preparation than an mere surface-dressing will be wanted, while in others it will be necessary to provide a bed of concrete or stone filling, or even to drive piles under the ways. As a rule, however, the foundation of a slipway is not a difficult or an expensive matter, as the weight of the vessel and cradle is spread over such a large area that the pressure per unit of surface is very little, not more indeed than 7 to 8 tons per lineal foot of way. The Authors are of the opinion that piling should never be resorted to except in cases where it is absolutely necessary, and even then only with great caution, unless the whole length of way is to be piled. When part of the ways are supported on natural ground and part on piles, a difficulty is very likely to arise at the junction, for, the ground being more or less yielding, the cradle drawn on to more rigid piled-portion, is subjected to excessive local stresses, which would probably cause breakage of the rollers, carriage or rails. What is to aimed at is to have a uniform support for the ways throughout their entire length, and where the formation of the ground varies, this uniformity must be obtained by excavation and filling-in with suitable materials.