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

Sir Frederick Bramwell cont'd :

With regard to the telescopic arrangement of the cradle of the slip, the employment of hydraulic power, and the other matters mentioned in the Paper, they were, according to his recollection, quite accurate. Slips and dry docks were useful in cases where there was a suitable foundation ; but there were places in which it was desirable to repair a ship where it was impossible to get a good foundation, and in such places it was necessary to resort to a floating dock. He did not think it was possible to make a floating dock at the same small amount of cost for which a slip could be made where there was a good foundation ; but it was perfectly possible, by means of a floating dock, to raise very readily the largest ship, to get it up in a short time, to repair it, and to lower it again with regularity and success.

A dock that he designed twenty years ago, had been at work for many years at St. Thomas’s, in the West Indies, and although curtailed of its original dimensions, it was still competent to raise 3,000 tons dead weight with facility. The sides of the dock were not closed in the manner of an ordinary dock, but were open-girder work, and the lateral stability of the dock was obtained by means of floats which worked within the girder-work, and which might be said to rise and fall within that work, the fact being, that the floats remained on the surface of the water, and the dock rose and fell past the floats. The aides being open, there was as much ventilation for the workmen employed, as there was when the ship was upon a slipway.

Mention had been made of hydraulic keel-blocks, all coupled together, and subjected to one pressure. He knew that they had been used in London in the floor of an ordinary dry-dock ; but he believed they were abandoned because they were found to strain the ship. The notion was to give the ship an elastic bed, and thus to prevent strain ; but it would be seen on consideration that blocks of that construction must cause strain because they were all coupled up and were under one pressure, so that if they were all of one size they all had an equal tendency to rise out of the presses in which they were placed.

Therefore, whether the part of the ship over them weighed much or weighed little, the upward tendency of the block being the same, the ship had to keep down the block, say at the ends of the vessel where the weight was little, not by the weight of the ship itself, which it could not do, but by acting as a girder, and transmitting to the keel-block so situated a portion of the weight which resided in the central part of the ship.