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10.8m
eco cat – 2009
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10.8m eco-cat
This design was initially
conceived as a private commuter – to and from the Hauraki Gulf Islands independent of ferry
services – and an in-town apartment. The concept called for a
displacement cat that would use an absolute minimum amount of fuel
and be able to make light work of rough water. Skinny hulls and a
high tunnel clearance were essential. High top speed was considered
less of a priority than consistent moderate speed – and
15-20kn constant operating speed is very good.
The design has developed
into a versatile vessel that could be optimised for many purposes
for example: small ferry, yacht club committee boat, charter
fishing or diving, pleasure cruising, commercial survey, super
yacht tender (with the addition of inflatable tubes) or even
commercial fishing vessel. It would also be suitable for powering
or testing and optimisation of small hybrid power sources such as
diesel electric.
Constructed is intended
to be composite or strip planked timber glassed construction. But
it could be adapted for aluminium construction.
Hulls
At 10.8m the eco cat is
big enough to be viable for a displacement hull form and 4.6m of
beam is generous – a big platform. The near vertical stems
are “right” for this type of boat – even if they
don’t look like a planning monohull – as every bit of
waterline length counts and increases speed potential. In keeping
with the efficiency theme, the 16:1 waterline length to beam ratio
is optimum. The hull could actually be termed semi displacement as
it will push on past theoretical hull speed without a problem. Any
tendency to squat in the stern has been counteracted by a flat area
above the canoe “bustle” generates lift – as is
well proven and a common stern treatment on displacement cats.
Careful attention to weight distribution results in the tanks and
Yanmar 4JH engines being concentrated around the centre of
buoyancy. And because the engines are well forward the aft 3.8m of
the hulls are available for lightweight storage and/or long range
tankage.
Wing deck clearance can
be an issue and a focus of this design has been to keep clearance
as high as possible – approximately 800mm of clearance
– to prevent slamming under the wing deck in rough
conditions. Other seakeeping features include the foredeck well
between the hulls up frd that is drained by grates straight out
through the underside of the wing deck. This provides a firm
standing platform with the advantages of instant drainage. A
central “nacelle” (partial middle hull frd) and stepped
hulls provide additional reserve buoyancy for the fine bows; while
the forefoot (very frd part of the hull underwater) has been cut
away slightly. This minimises any bow steer that can come with a
deep knife type bow, but maintains the advantages of the longest
possible waterline length. The last hull shape feature is the
transom steps providing excellent access onboard.
Layout
The cabin extends almost
to the gunwale. Instead of side decks there is a small ledge that
gives a detail crease line. Access to the foredeck is through the
front of the cabin which enables bow or stern boarding. There is
accommodation for a couple in a double cabin on the main level and
three single berths in the hulls with the head and shower in the
stb hull frd. The main double cabin has screens that can be
retracted during the day when the boat is in use so that the
skipper has uninterrupted visibility to port. In addition the
dinette can be converted to a double berth for occasional use.
There is good space for the helm station and associated
electronics.
Cockpit
In the huge cockpit a variety
of layouts are possible, the one shown has a deep freeze in the
locker behind the main bulkhead to port and storage to stb. The
seating in the cockpit aft could alternatively be higher and be
locker space. Access steps in the aft end of the hull lead straight
up to the cockpit without any winding treads. Davits aren’t
shown but the suggested arrangement for cruising is to have davits
to suspend an RIB at the aft end of the cockpit. The davit may be
fixed or folding.
Machinery
In the hulls the well proven
and efficient Yanmar 4JH engines are located quite high up under
the aft bunks with conventional shaft drive to protected
propellers. There is good access, ventilation and soundproofing.
Tank capacity is excellent – 600 litres diesel, 560 litres
water – without using the reserve space in the aft end of the
hulls. The choice of engines can range from 39hp x 2 to 125hp x 2
depending the intended operating speed.
Construction
Construction is intended
to be composite foam or strip planked timber core and can be
optimised to suit builder and budgetary constraints. We suggest a
single planked female hull mould that will enable both hulls to be
moulded from it using gelcoat and any resin type. The hulls can
then be bonded to the separately moulded wing deck and nacelle
using a flanged system. A simple female timber mould is required
for the cabin top and the rest of the cockpit and superstructure
may be moulded or fabricated from composite panels laid up on a
table.
Key dimensions
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Length
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10.8m
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Waterline
length
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10.75m
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Beam
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4.6m
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Displacement
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5200kg
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Draft
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0.67m
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