Penarth Dock, South Wales - 150 years - the heritage and legacy  
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Volume Three - The Pontoon Era - Proposed off-shore dock . . .

Where keel-blocks occur normally, there is but little difference in the two designs, for both contemplate transverse vertical plates under the blocks that convey the load to the longitudinals. In this case of intermediate supplementary blocks however, where we provide three bearings in a space of 8'-0”, the new scheme only provides two, that in the unsupported space is twice as long. We fail therefore to see any advantage in departing from our usual practice, which has the merit of many years success.

Extra longitudinal strength is provided in the new scheme by thickening the shear strake on the top of the Wall. As you may know, it is our invariable practice to thicken the top flange (the deck) and the shear strakes when required, and we should quite concur in bringing the deck plating to ½”, and the shear strakes to 7/16”.

This will really give as great a resistance as the 5/8” shear strake and ½” chequered plates in the open soon cease to give protection against slipping, whilst there is the great disadvantage that they can only be obtained in small sizes, and we are always very adverse to building up the flange of the girder with a lot of little plates, when large and long ones can be readily obtained.

We have accordingly altered the drawings so as to show the above modifications. We have also added to the pumping plan the pipes and troughs required for the new outer wing pumping divisions, and we have taken the opportunity of showing the pumps with an 18” discharge.

We would refer to the other point, and that is the position of the central longitudinal bulkhead. In any one-sided Dock, once the dimensions of the Dock are fixed, and the design determined, there is only one possible position in which the vessel can be placed in the transverse sense, if the whole of the Dock's buoyancy is to be utilised as lifting power.

If the vessel is to be placed anywhere away from this position, water has to be left in some portion of the Pontoon, to compensate for such displacement and so to balance the Dock. In other words, it would be immaterial if this outer portion of the Dock was cut off altogether so far as lifting power is concerned. In the present Dock, the ship balances, as nearly as can be ascertained at present, at 32'- 6” from the face of the Wall. In this position, a 55'- 0” ship would have her outer bilge keels within two feet of the airbox.


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