Voith Propulsion Unit Swap-Out
Voith replacement in just 6 days - May 2013
Red Funnel's Raptor Class ro-pax ferries are propelled, not by conventional propellors but by a highly efficient Voith Schneider Propulsion System. There are two 5 bladed Voith units installed in each ferry making them highly manoeuvrable as the direction of thrust can be changed almost instantaneously.
Normally each unit is swapped-out every 4 years for overhaul and a spare is installed whilst the vessel is out of the water in a dry-dock. In April 2013 the no.2 unit in Red Osprey developed a bearing noise, which was progressively getting worse. The decision was taken to carry out an unscheduled swap-out of the unit before the start of the main summer season.
Unfortunately no dry-dock was available until June so the engineering team, led by Peter Gould (Technical Manager) designed a novel way of ballasting the no.1 end of the ship to the extent that the Voith housing at the no.2 end the hull was clear of the water so the unit could be replaced without dry-docking the vessel.
Removal of faulty unit
1) Sat 11 May, vessel is ballasted at the no.1 end using seawater
2) A large steel access panel on E deck is unbolted and craned out of the way.
3) A similar steel cut-out is removed from C deck to enable the Voith unit to be craned out of the ship.
4) The Voith unit is uncoupled from the engine's propeller shaft along with electrical and hydraulic connections.
5) Bolts holding it to the hull were then unfastened and the unit was craned out of the ship using two heavy-lift cranes and landed ashore onto a special cradle.
6) Unit will be stripped down to find the fault and given an overhaul. It will then act as a spare unit ready for the next scheduled swap-out on either Red Osprey, Red Falcon or Red Eagle.
Completion time: 3 days
(images courtesy of Dave Booker)
Installation of new Voith Unit
1) Spare Voith unit is taken to site in a specially constructed cage
2) Heavy-lift cranes lift it from the quayside and it is gently lowered first through the hatch in C deck and then through the hatchway in E deck
3) Once in the Voith compartment it is carefully manoeuvred into position and then bolted to the hull
4) The shaft connecting the unit to the Wartsila diesel engine is then re-coupled.
5) Electrical and hydraulic connections are re-established and the unit is tested briefly
6) The steel hatch covers are then re-fastened (E deck) and re-welded in the case of C deck and the joints recoated.
7) Vessel to undergo extensive sea trials before returning to service on Fri 17 May 2013.
Estimated completion time: 3 days
(images courtesy of Mark Dear)
How it works
Over 80 years ago this type of ship’s propulsion system, the only one of its kind in the world was developed by Voith from an idea by the Austrian engineer Ernst Schneider. It allows thrust of any magnitude to be generated in any direction quickly, precisely and in a continuously variable manner. It combines propulsion and steering in a single unit.
This solution is as convincing as it is straightforward: on the Voith Schneider Propeller, a rotor casing which ends flush with the ship’s bottom is fitted with a number of axially parallel blades and rotates about a vertical axis.
To generate thrust, each of the propeller blades performs an oscillating motion about its own axis. This is superimposed on the uniform rotary motion.
Blade excursion determines the amount of thrust, while the phase angle of between 0° and 360° determines its direction. As a result, the same amount of thrust can be generated in any direction, making this the ideal variable-pitch propeller. Both variables – the magnitude and the direction of thrust – are controlled by a mechanical kinematic transmission.