Rosyth Dockyard – Calculation of the force needed to open or close the emergency opening caisson.
< Return to 16 – 1916 Fitting the caissons to the entrances
The emergency opening (direct opening) is only ever opened at high tide, and the water level in the basin is adjusted by opening the basin sluices to match the river level before the caisson is opened. But this still presents a wide range of conditions.
At Extreme High Water – 11 ft above the Ordnance datum, the caisson experiences maximum flotation so it “weighs” a mere 11.25 tons and can be moved with a push or pull of 12.06 tons. (It weighs 906 tons in the dry.)
Caisson in place at extreme high water – maximum flotation. The caisson weighs 11.25 tons
At High Water Ordinary Neap tide – i.e. the lowest high tide, the water level is almost 7 feet lower at 4.33 feet above the Ordnance datum, so the caisson displaces less water and now “weighs” 34.75 tons. The force need to slide the caisson increases from 12.06 tons to 27.33 tons.
Caisson in place at High Water Ordinary Neap Tide, the caisson now weighs 34.74 tons
The caissons in the graving docks will normally operate with the fixed depth of water in the basin, as will the basin end caisson in the entrance dock. The river end caisson in the entrance dock has to operate at any state of the tide from Spring High Water (8.75 feet above Ordnance datum) to Spring Low Water (7.55 feet below Ordnance datum) a variation of 16.3 feet (5 m.)