All's under control at the sharp end
Docking a 65m long tall ship is a tricky business but a 75kW bow
thruster, powered by a variable speed drive ensures that the Jubilee
Sailing Trust's 586 ton square-rigged 'Tenacious' can safely be brought
alongside by a mix of able-bodied and physically disabled crew
Tenacious is the largest wooden tall ship to have been built in the UK in
the last 100 years, and she is owned and operated by the Jubilee Sailing
Trust, a leading organisation offering adventure sailing for people with
mixed abilities. Costing over £14m - mostly raised by the Trust's
supporters, corporate partners and a Lottery grant, Tenacious is designed
to allow a mixed crew (including ten permanent crew) of 50 able-bodied
and physically disabled people to share the adventure of tall ship
sailing on equal terms.
With masts 44m high, the tides and even moderate winds exert significant
lateral forces on the ship and during docking these can tend to drive her
away from the dock or, worse, drive her against the dock wall. With the
combination of twin 400hp main screws and the Torquemaster bow thruster,
however, the crew can bring her alongside in complete safety. The
joystick operated Torquemaster facilitates the docking process by slowly
swinging the bow of the vessel towards the dock for tying up. When
leaving port, it works in the opposite direction to push the vessel away
from the dockside.
The Torquemaster bow thruster comprises a 600mm diameter screw mounted in
a tube, which passes laterally through the bow of the vessel. Exerting an
overall lateral thrust of 1 ton, the screw can be driven in either
direction to swing the vessel to port or starboard; alternatively, it can
be closely controlled to hold the vessel stable against a lateral force.
The screw is driven from a 5:1 helical bevel gearbox, driven by a
two-pole ac motor, selected to limit torque through the gearbox. However,
wide, step-less variable speed control of the lateral thrust down to very
low screw speeds is essential for smooth docking.
Torquemaster chose a Danfoss VLT5000 series inverter to power its 75kW
bow thruster, not least for its various marine approvals, including that
of the Lloyds Register of Shipping. The requirement for close speed
control from a +/-10V joystick, and the ability of the inverter to
operate satisfactorily from the ship's 105kVA generator were also
critical - the VLT's integral filters ensuring compatibility with the
diesel-driven generator supply.
Danfoss inverters often turn up in Torquemaster products, as the latter's
Bill Hampton explains. The choice is as much to do with our long and
satisfactory relationship with them, as it is for technical reasons. The
reliability of the drive unit was an important consideration for the
Tenacious bow thruster, as was the availability of service in the many
ports around the world into which the vessel will call. With their
worldwide network, Danfoss can provide service anywhere she sails.
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