Feature 7 | ADVANCED HULLFORMS
water resistance and seakeeping motion cancellation can be achieved and the wave
results were presented. Finally, the paper resistance of the trimaran can be less than
concluded with a summary of the merit of that of a slender monohull of equivalent
the diff erent numerical tools in predicting displacement and overall length.
vessel performance. Unfortunately, the overall length of the
trimaran with side-hull stagger exceeds
Adjustable-length trimaran the austere port length constraint, but
As the authors of the paper noted, the an unconventional solution can be
speed, range and payload requirements proposed, if it is supposed that the hulls Vessel geometry.
of high-speed sealift present a naval are structurally connected and locked
architectural challenge of a high order, in a staggered configuration during
even without the addition of severely transit, even though the overall length In the concept discussed in the SNAME
constrained length and draft for austere exceeds the austere port constraint. On paper, for an adjustable-length trimaran
port access. With these dimensional arrival, the vessel mechanically alters its with a retractable centre-hull section, the
constraints, especially the length constraint, confi guration for port access by retracting centre hull is locked in the extended position
the problem has repeatedly shown itself to to a more compact length. On departure, for transit, with locking mechanisms similar
be beyond the reach of conventional ship the vessel extends and locks into its transit to the devices that are used on integrated
designs with presently available material, confi guration again. tug barges, sliding wedges or rams - sized
machinery and fuel technologies. As the paper’s authors acknowledge, this to take the structural loads required for
Credible concept-level designs, whether is not a trivial design problem, but on the open-ocean operation at high speed. For
they have been of ‘conventional’ (long other hand it does not appear to be out of access to an austere port, the locks would
centre hull) trimaran, catamaran, or reach with present technologies. be disengaged and the centre hull would
monohull configuration, displacement, In the concept outlined in the SNAME be retracted along a system of guides that
heavy-planing, or cushion-borne, do not paper, transom confi gurations permit the need only take the loads required in port,
converge (carrying payload and fuel for installation of a single waterjet propulsor sheltered waters at relatively low speeds.
the desired range) except by exceeding the in each hull, an arrangement considered Based on current practice and
170m austere port length constraint by a most suitable for model testing. However, commercial systems used in integrated
considerable margin. propulsion-machinery selections (for an tug barges, a structural locking system
To fi nd a solution, the authors of the actual vessel) might ultimately favour can be developed at a reasonable weight,
paper envisioned a trimaran in which single or twin waterjets in one, two, or all provided that appropriate design loads can
the three hulls are of approximately equal three sub-hulls. be established.
length (the austere-port constrained The principal design challenge is, In seakeeping model tests, forces and
length) and of nearly equal displacements. obviously, that the mechanical systems moments were measured at the connections
A trimaran of this type, with the hulls required for structural connection of between the port and starboard cross
staggered (instead of directly alongside the retractable hull must be capable of structures and the sides of the centre hull.
one another), can achieve substantial supporting the design sea loads, and still Th e highest irregular wave case tested was a
wave-resistance advantages compared with be able to retract and extend under some representative Sea State 7 spectrum in head
other confi gurations. other (presumably less severe) set of loads. seas, at 25knots. Based on a preliminary
With an appropriate stagger for a given This is not completely without analysis of those results it is considered
transverse separation of the hulls, wave precedent, however, and locking and likely that vertical bending moments and
bearing mechanisms sized to carry loads shear forces will govern the design of the
(of magnitudes appropriate for primary locking system, at least in a confi guration
ship structures) have been used in where the centre hull retracts under the
commercial marine applications such as cross structure between the two side hulls.
integrated and articulated tug-barge (ATB) “Conceptually, there are other ways of
systems. Similarly, mechanical systems for performing this retraction. For example,
retraction and extension of large objects, the port and starboard cross structures
under substantial forces, have been and hulls could slide fore and aft along
developed for other marine and off shore the sides of the centre hull. In fact, such
applications, such as jack-up drilling rigs, an arrangement would permit at least one
off shore structure launching appliances, geometric advantage in terms of freeboard
and heavy-lift derricks and cranes. constraint on the centre hull. However,
Th e additional challenge for a high-speed in other important respects, especially
Adjustable-length trimaran concept with ship application is to provide the required structural continuity and arrangements for
retractable centre hull section extended system capabilities (locking, retraction and cargo stowage and access, a concept with
and retracted. extension) at an acceptable weight penalty. the centre hull connected underneath the
64 Warship Technology October 2008
WT_Oct08_p61+64+66+68+70.indd Sec2:64 10/10/08 1:20:27 PM
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