Gemini 105Mc Owner’s Manual
30Copyright © 2004 Performance Cruising Inc.
Construction
Hull Construction
The hull of Gemini is produced using a large, one-
piece mold with no secondary seams or joints. Simi-
larly, the deck is produced from a single mold with
no removable or add-on parts. The centerboard
and rudder cases are each integral to the mold and
have an elaborate air blowing release system. In
this way, the centerboard case and rudder case are
all molded in at the same time as the hull is
molded. This gives a dimensionally accurate, leak
free and strong structure.
The hulls have a high quality Cooks marine gel coat
(953WA411) with a 20mm veil and vinylester barrier
coat. The main lay-up is all done at once, with no
chance of weakness as a result of interlaminate
bonds. The main lay-up is comprised of layers of 24
oz stitched bi-directional woven, alternating with
high stiffness 1 ½ ounce mat. The overlap in layers
increases the strength in strategic places such as
the keel. The keel is strong enough to support the
boat on a point load, and ends up with fiberglass
totaling 5/8" thick with additional wood reinforcing.
The bridge deck has the same lay-up as the hull but
balsa core is also integrated into all flat surfaces.
The reason for using Balsa is because the bridge
deck is flat and subject to heavy pounding loads.
Reinforcing to the hull is by strategic use of bulk-
heads and interior molds.
Deck Construction
The deck is a one-piece mold with all the horizontal
surfaces balsa cored. The balsa core provides a
solid, stable foundation underneath all walking sur-
faces. In the case of all deck fittings, the Balsa is
replaced with extra glass and plywood to avoid the
possibility of water infiltrating into the balsa core.
The use or non-use of backing plates is often mis-
understood by customers and even some boat build-
ers. An item bolted on that takes sheer such as a
chain plate must not have a backing plate.
The most important design consideration is to have
the materials of such a thickness that they have the
same percentage elongation under the same load so
that the bolts are uniformly loaded. A backing plate
would change the situation and put the entire load
on the first bolt.
Most items on the deck of a sailboat are subjected
to torsion such as the stanchions. The most impor-
tant design consideration is to transmit the load
over a large area. This is most easily accomplished
by molding in plywood; again simply making the
fiberglass thick also helps transmit the load. Back-
ing plates are academic because a failure would
almost certainly be a large chunk of deck being
ripped out, much larger than the actual backing
plate. The only time there is a good reason to have a
backing plate is for items that are being pulled at
right angles to deck or hull, such as rudder gud-
geons. A backing plate would stop the bolts pulling
through the hull. Unfortunately consideration should
be given as to whether it would be better to have,
for instance, four bolt size holes in the event of a
sudden unexpected load like running aground or a
large hole if backing plates are used. The small holes
can be stopped up easily while a large hole might
sink the ship.
Stress and Movement in Catamarans
A monohull, which is basically a tube, is automati-
cally stiff enough to take the mast and rigging loads.
Of course, the Achilles heel of a monohull is the
enormous point load of the narrow keel, which is
almost half the weight of the boat.
A particularly wide catamaran requires cross arms to
connect the hulls together. These cross arms have
high point loads at connections, particularly when
the boat is subject to twisting.
Gemini is a narrow catamaran and can be considered
as an upside down channel with the flat of the chan-
nel being the bridgedeck and the inside hulls being
the walls of the channel. The rest of the outer hulls
just keep the water out. The narrow catamaran with
full cabin is like a box and easily resists the twisting
forces.
As the mast pushes down on the center of the chan-
nel, the headstay and backstay pull up the ends. A
channel subjected to high loads can only bend if the
sidewalls deflect out. With Gemini the only signs of
deflection are the side hulls moving up perhaps ¼".
This movement is impossible to eliminate and is defi-
nitely a result of headstay and backstay loads and
not side shroud loads. The side shrouds are all con-
nected to the bridgedeck and not the outer hulls.
With the centerboard down this movement is only on
the windward hull because the side push of the cen-
terboard counteracts the outward movement of the
channel side. The windward hull when lifted out of
the water should be assumed to bend down, not up.
This is further proof that the upward movement of
the outer hull is induced by the backstay and head-
stay loads.
As a result of this upward movement it is not advis-
able to keep the backstay tight for a prolonged of
time. For racing there does not appear to be any
damage if the backstays are tightened to the maxi-
mum just for the duration of the windward leg and
then released, provided that the upward movement
of ¼" of the hulls does not damage the door jamb of
the main bulkhead.
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