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Catamaran Characteristics
General
The hull form is a semi-planing type catamaran. It employs a
combination of symmetrical and asymmetrical sponson shapes, thereby
combining the attributes of both shapes in one hull. The symmetrical
bow-section ensure directional stability in short swell conditions and
following seas, whilst the asymmetrical midships and aft sections ensure
softness of ride and reduced wetted area which enhances comfort and
economy.
The catamaran hull has a high tunnel ceiling with a large opening
between the sponsons, which allows free movement of wind-waves without
slamming on the wet-deck. Horizontal steps on the inside of the tunnel
walls act both as chines to deflect green water from the hull surface,
and to break up the solid water into spray.
Comfort
The hull is particularly soft riding and is ideal for passenger
ferries where comfort is of great essence. This is mainly due to the
vertical inside shape of the sponsons, which reduces the planing area,
thereby reducing the vertical acceleration forces.
However, a further important feature in enhancing passenger and crew
comfort is the action of the longitudinal chines on the inside of the
tunnel walls. As solid green water is broken up into spray whilst being
deflected from the hull, it mixes with air streaming down the opening
between the sponsons. This mixture of spray and air creates a
high-density medium inside the tunnel, which causes a dampening effect
each time the hull moves through a trough of a wave.
Since the vertical accelerations caused by wave action on this type
of hull is lower than most other types of craft, the vessel can maintain
service speeds in relatively rough conditions without compromising the
comfort of its passengers.
Performance and
speed
The action of the longitudinal chines inside the tunnel, as well as
wide chines on the outside, both deflecting water away from the hull,
reduces the wetted area and therefore the resistance of the hull. The
vertical inside shape of the sponsons minimizes wave interference
between the sponsons, which further reduces drag. The combined effect is
a hull with low resistance, low vertical accelerations and therefore
excellent performance. A further benefit of the vertical inside shapes
is that the direction of forces imposed on the hull during turning,
causes the vessel to bank inwards on turning, similar to a monohull,
thereby increasing high speed turning performance and safety.
In applications where speed is of essence, a hydrofoil is fitted to
the hull. It consists of an underwater wing profile spanning the tunnel
at approximately midships position. The lift produced by the hydrofoil
reduces the hull resistance, which increases speed, whilst at the same
time increasing the load-bearing capability. The foil action reduces the
power needed to maintain service speed, and therefore fuel consumption
and running costs are reduced. Unlike conventional hydrofoils, which
lift the hull completely out of the water, the hydrofoil is designed to
only partly reduce the draft, thereby reducing resistance, but still
maintaining good seakeeping by having the hull still partly submerged.
The hydrofoil further enhances the softness of the ride, especially in
choppy seas.
Probably the most outstanding feature of this hull-form is its soft
riding characteristic. The ability to maintain high speed in rough water
conditions makes the hull particularly well suited for para-military
and passenger ferry applications, when voyages or scheduled crossings
need to take place in all weather conditions.
Wake
The height of the wake produced by catamarans type hulls is much
reduced as compared to conventional monohulls, due to the reduced wave
making resistance of the long, slender sponsons. The lift action of the
hydrofoil reduces the draft and therefore the amount of water which need
to be displaced by the underwater part of the hulls. This results in a
further reduction of the size of the wake. Typically, a Teknicraft
catamaran of 63' waterline length, displacing 27t and running at 27 kn
will create a maximum wake height (peak to trough) of 19", with a rms
wake height of less than 6".
Safety
Catamarans are generally the safest type vessels due to the high
transverse stability of the hull shape as well as having two separate
hulls contributing to positive buoyancy in case of damage.
Watertight bulkheads divide the hull into several compartments, all
capable of supporting a substantial displacement, in case of flooding of
other compartments.
Design work is done in accordance with rules of various
classification societies, with statutory safety requirements adhered to
as well. When necessary, plan approvals are obtained from the relevant
authorities prior to construction of a vessel, and the construction is
surveyed to ensure proper building standard.
Economy
The lower resistance of the hull necessitates less power and
therefore less fuel to attain a given service speed. Ensuring that all
systems are simple to run and easy to maintain further reduces running
cost.
The hydrofoil, being permanently fixed in one position and
constructed of high strength corrosion resistant steel or aluminium
alloy, need no maintenance. It is fixed above the keels of the sponsons
and is therefore no more vulnerable to damage than the hull itself.
Space
The high beam to length ratio of the catamaran hull provides a large
deck area for placing of passenger seating, deck cargo, fish stowage,
industrial equipment or for entertainment. Due to the high stability of
the hull, second and third tier decks can be fitted to increase space
without compromising on comfort or safety. |
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