UMC Design & Consultancy Division has developed a procedure and necessary equipment to carry out an afloat replacement of the Voith Schneider Propellers fitted to Royal Navy Minehunters. A recent trial in Portsmouth Dockyard proved that this procedure could be performed with minimal impact on the ship and in a time frame equivalent to or less than a conventional dry-docking. Replacement of the propellers or even single blades has to date always required the vessel to be dry-docked.

The equipment comprises of a large circular cofferdam which provides a water-tight enclosure around the Voith Schneider blades, and a special lifting frame that sits on the after deck and provides a stable structure to carry out the initial lift of the 10 tonne propeller unit. The cofferdam is made from aluminium and incorporates buoyancy tanks that allow the unit to be made neutrally buoyant and easily manoeuvred by one diver using lifting tackle secured to the ship’s hull. Windows in the cofferdam also allow the diver to confirm the position of the cofferdam and witness the water level change as the cofferdam is drained down using an eductor system. With the hull sealed, the Voith Schneider unit is initially lifted from the lift frame, and then once clear of it’s mounting, totally removed from the ship using a dockside crane. Once ashore the unit is placed in a stand, which is supplied as part of the kit. All the lift frame and stand can be disassembled for transport along with the cofferdam which is supplied with it’s own lift frame and blanking plates used inside the ship.