manoeuvring technology
passenger ferries, further options are available which provide very high manoeuvrability, to the point of full axial vessel rotation. The Voith Schneider Propeller system (VSP), manufactured by Voith Turbo GmbH & Co KG, was first developed over 80 years ago but provides a radical solution in terms of being able to deliver thrust of any magnitude in any direction from a combined propulsion and steering unit. The VSP design is based on a rotor casing ending flush with the vessel hull, below which are a variable number of blades, vertically mounted, that are capable of being turned individually between zero and 360 degrees. Whilst the main blade carrier rotates as an assembly, individual blade angles are adjusted to provide the required thrust in any desired direction. The VSP is continuously and fully variable and, as a result of this, can be considered as offering the ultimate in propulsion and manoeuvring flexibility. Both magnitude and direction of thrust are
controlled by a system described by Voith as mechanical kinematic transmission. This both drives the main propeller assembly and facilitates the rotation of individual blades to provide the required direction and magnitude of thrust. Power can be provided from either a diesel engine or an electric motor and there are options for a range of drive train designs with reduction gearboxes and intermediate fluid couplings. The latter are recommended on all higher power installations and the coupling provides the major benefit of damping vibrations, isolating the driver from the propeller unit and preventing vibration transmission between the two. Due to the nature of the VSP design, reversing of the drive unit is unnecessary as thrust direction from the propeller can be changed entirely through adjustment of the propeller blade angles. Ship designs incorporating the VSP must be specific, as the propeller well, a cylindrical structure with a flange mounting arrangement, forms an integral part of the vessel design. The hull itself must be capable of carrying the propeller weight but also of taking the full reaction forces from the propeller operation in any direction without exceeding mechanical stress limits. The Voith Turbo VSP is available in a range of powers from 180kW to just under 4,000kW. The lower power units start with a four-bladed assembly at a pitch of 1m with the largest six-bladed
70 I Passenger Ship Technology I Spring 2012
High manoeuvrability is a major benefit for fast- turnaround passenger ferries
assemblies having an effective pitch of 3.8m and a dry weight of over 80 tonnes. The VSP system has proved a popular choice for ferries acting as ‘floating bridges’. The high levels of flexibility and manoeuvrability are of major benefit for vessels operating in narrow harbour areas and in locations where strong currents and high side winds are present. Short docking and cast off times ensure the ability of ferries to meet demanding operating schedules and this benefit also controls cost and minimises fuel use.
For double-ended ferry designs, the VSP offers significant advantage as effectiveness of propulsion and vessel manoeuvrability is equal, regardless of vessel direction. Propeller arrangements on these ferries can also be optimised for specific applications. The most usual and standard arrangement is with two VSPs on the ship’s centreline, but where there is a preferred direction of travel, the option of three VSPs is available. For higher power requirements, or where there are restrictions of vessel draught, four units can be fitted. One recent example of this is on the ferry Uthlande, where Voith Turbo has provided four 470kW VSP units with reduced length blades. This has limited vessel draught to 1.75m, enabling it to operate effectively in ferry ports such as Dagebüll, in Germany, where the available draught is only 1.85m when sea levels are at their lowest. PST
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