Feature 7 | ADVANCED HULLFORMS
cross structure of a ‘catamaran section’
seems more reasonable,” said the authors
of the paper.
Notionally, the locking system described
in the paper consists of opposed pairs
of horizontal sliding wedges driven (for
example) by a jack screw. To lock the centre
hull, the wedges are driven outboard and
engage corresponding ‘buckets’ fixed
in the cross structure. Two assemblies,
separated longitudinally, transmit the
vertical bending moment and shear, and
also resist longitudinal forces and torsion of Adjustable-length trimaran concept; section showing possible location of locking and
the centre hull about the longitudinal axis. retraction systems.
One pair of locking wedges is located near
the aft end of the centre hull structure. Th e
other pair is located near the mid-length effects of weight distribution and trim, habitability spaces. Th e cross-structure of
of the centre hull, and engages the cross which can vary widely depending on fuel the catamaran section contains all the cargo
structure as far forward as practicable. and cargo loads, will be at least one possible stowage areas, cargo access arrangements,
Th us, the lever arm between the two sets driver of load requirements on the guides, accommodations and ship control spaces,
of locks is approximately half the length of even in calm water. in addition to propulsion machinery rooms
the centre hull. Types of actuators to move the centre and most of the auxiliary machinery. Th ere
In oblique seas and during manoeuvres hull could include rack and pinion (similar is no superstructure on the centre hull
there are, of course, also signifi cant lateral to systems used for jack-up drill rigs), section because of the need to retract it
bending moments and side forces acting on traction wheels, winches and cables, ball under the cross structure. Th is limitation
the centre hull. Th ese forces also have to be screws, or other linear actuators. Some eff ectively restricts the freeboard of the
transmitted by the locking wedges, although care is obviously needed with regard to centre hull to the wet deck height of the
primarily as loads along the axis of the buoyancy and trim in an adjustable length catamaran section. Th is in turn can have
actuating screw, rather than as shear in the vessel. Just because the entire trimaran is in a considerable infl uence on performance
wedges. Model test results in oblique seas static equilibrium with the sections locked and seakeeping. Th e solution to excessive
have only been obtained in irregular waves in a certain position, it does not follow that wetness of the centre hull forward, without
up to 4m signifi cant height. Consequently, when the locks are released each section excessive height of the catamaran section,
fi rm conclusions on the appropriate design will still be in equilibrium individually. remains a practical challenge for design
loads cannot be drawn yet. Th us, the guide system may be carrying trade-off s.
Whilst ATB coupling systems must both a force and a moment when the locks
incorporate bearings to permit rotation are released, and there would be situations Other design issues
under load, the locking wedges of the when the centre hull would tend to move In principle, the adjustable-length
trimaran do not, and this simplifi es the along the guide simply because of an excess trimaran may (and probably will) have
design of the system. Th e dimensions of of buoyancy or a trimming moment in one propulsion machinery located in all three
the wedge (diameter or largest dimension section or the other. hulls. Intakes and exhausts from centre hull
of the cross section) would be on the order Weights for the locking and retraction propulsion engine(s) present an unusual
of 1500mm, and the material thickness systems were included in estimates for the arrangement and gas plume problem.
around the perimeter of the section about adjustable-length trimaran, and amount It can be assumed that retraction and
120mm. Internal diaphragms could also be to somewhat more than 250tonnes, extension operations will be performed
required inside the wedge to help maintain including additional foundations. Th is is with the centre engines secured, as this
the desired distribution of contact forces, not an insignifi cant weight. However, the would only be done at low speeds in
and to transfer forces to the jack screw. penalty has to be compared with the added sheltered water. Even so, on arrival the
Apart from locking the centre hull in propulsion machinery and fuel that would centre engines and machinery spaces
its extended position at sea, the system be associated attempting to meet speed and would still be hot, and the machinery and
must also provide a means of retracting range requirements on an over-all length enclosures would still be radiating.
and extending the centre hull. Guides for limited to 170m, instead of 50% longer. Several alternative approaches for
this motion may take the form of rails with In the arrangement described in the air management have been considered,
wheeled carriages or slides. Th e loads on SNAME paper, the centre hull contains according to the authors of the SNAME
the guides are expected to be modest by fuel tankage and propulsion machinery paper. Th e most promising of these is to
comparison with the loads on the locking spaces, but for reasons that should be route intake and exhaust for the centre
system, but certainly not negligible. Th e apparent it does not contain cargo or hull engines through vertical trunks
66 Warship Technology October 2008
WT_Oct08_p61+64+66+68+70.indd Sec3:66 10/10/08 1:20:27 PM
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