The keel is raised and lowered by a 6 metre stroke hydraulic cylinder designed and manufactured by Hunger Hydraulic, with a bore diameter of 320 mm and rod diameter of 190 mm. The cylinder is vertically mounted inside the keel, and due to installation envelope restrictions is ported through the rod. Fully sealed stainless steel self-aligning bearings connect the cylinder each end to the keel and hull structures. The keel weighs in excess of 150 tonnes, with the cylinder designed to cater for 2g inertia loads when the vessel is operating in heavy seas.
When the desired position is reached the keel is secured using short stroke hydraulic locking rams, also supplied by Hunger Hydraulic, that drive the keel forward a short distance into a tapered bearing area to lock it in position. To release the keel, for raising or lowering, there is a similar set of rams at the forward side of the keel box to drive the keel aft.
The keel is flooded at all times requiring the lift cylinder construction and materials to be suitable for continuous operation in the worst of marine environments. Stainless steel components are used externally, with the most critical item, the piston rod, being protected by Hunger PTA stainless steel welded overlay. The overlay, developed by Hunger Hydraulic specifically for cylinders operating in this type of marine environment, involves plasma transferred arc welding of a 3 mm thick layer of cobalt based stainless steel material onto the high strength carbon steel rod. Both Hunger and independent testing institutions, in particular DNV, have carried out extensive tests to verify the overlay properties when used in severe marine operating conditions. The tests have included chemical analysis, metallurgical examination, hardness, impact and bending testing, electro chemical porosity testing, saline droplet testing and pitting corrosion testing in accordance with ASTM G48, all passed with flying colours.
The vessel is now in fully commissioned and operating in the Mediterranean.