Gemini 105Mc Owner’s Manual
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Copyright © 2004 Performance Cruising Inc.
Centerboards and Rudders
Centerboards
In terms of performance, the centerboards are Gem-
ini's secret weapon. Fixed keels, which are used in
most cruising catamarans, are a detriment to wind-
ward ability, down-wind speed, and draft, which
dangerously lowers the center of buoyancy and af-
fects their offshore ability and storm survival. In
addition, fixed keels that increase the draft to 3’
have to be engineered to take striking an unseen
rock without sinking the boat whereas centerboards,
with their pivot at the front, simply push up when
they strike a rock. Daggerboards are the most effi-
cient, but, are not a cruising option because when
they hit something they have to break.
On the Gemini, there is a centerboard in each hull.
The centerboards are on the outside of each hull 7"
off center to give foot room in the hull and to keep
the centerboard slot to one side of the keel so that
stones are not forced into the slot when grounding.
The top of the centerboard case is level with the
working surface in the galley and navigation station.
The centerboard winch handle socket is in the wall
of the galley and navigation station.
The Gemini 105Mc has asymmetric foam cored fi-
berglass centerboards as opposed to the older
wooden boards. These boards yield remarkable per-
formance and are incredibly strong. Unfortunately,
for best performance using asymmetric center-
boards, only the leeward board should be used. This
necessitates continuously changing the board when
tacking.
The drawbacks to centerboards are that they need a
long slot. As this slot is narrow, the boards must also
be narrow. The narrowness of the boards has a ten-
dency to create turbulence on the low-pressure side
of the board. Designing a centerboard shape to
overcome this tendency is a challenge.
With experienced design, Gemini’s' centerboards
create minimal turbulence and do not affect the
interior accommodation. They are less accident
prone than any other cruising catamarans with vul-
nerable fixed keels, and because they can be used
one at a time, getting rid of the parasitic drag of the
windward keel, they are dramatically more efficient
than low aspect ratio fixed keels.
Operating the Centerboards
The centerboards are 7' long with the pivot 6" back
from the front of the case in line with the main bulk-
head and level with the floor. When the center-
boards are fully down the leading edge is approxi-
mately 45º back and the exposed centerboard is in
the shape of a quadrant or fin coming out of the bot-
tom of the hull. Most of the slot is still filled with
centerboard; this prevents turbulence and gives a
strong bearing surface. The centerboards go down 4’
below the keel.
There is a 3 1/2" solid PVC drum inside the case up at
the top, 2' back from the front of the case. 3/8" rope
is connected to the top front of the centerboard. The
rope then comes back across the top of the center-
board under a bolt and then up to the drum. When
the centerboard is up, the rope goes approximately
one turn round the drum on the outboard side,
through the drum to the inboard side (where it is se-
cured) then round the drum an additional 3 turns.
From the drum it then goes under the bolt and back
to pad eyes on the top of the centerboard.
Inserting a winch handle in the socket and turning
the drum counterclockwise approximately 1 3/4 turns
lowers the centerboard. Turning clockwise raises the
centerboard. You will know the board is completely
up when raising once you hear the board bump
against the top of the case.
The centerboard is held in position by sliding a 1 1/4"
wing nut over a 3/4" nut behind the winch handle
socket. Clockwise turning of the wing nut jams the
drum to the side and prevents the board from moving.
The direction of rotation is such that if the center-
board was down and it struck bottom it would push up
and simply undo the locking nut.
Centerboard Maintenance
The Gemini’s design calls for very little tolerance
between the centerboard and the centerboard case.
As such, the boards are designed to be in the water
only when necessary. When not necessary, raise and
lock the boards in the “Up” position. If the boards
are left in the water for prolonged periods, growth
will occur on the board’s surface and make raising
impossible until the board has been cleaned.
The centerboard pivot is visible on the floor side and
accessible from behind the case. In the event of a
leak the pivot can be tightened a little. 3M 5200 is
liberally used to seal around the pivot and at worst
can be resealed. The boat should be taken out of the
water to do this. In over 800 boats we have found that
pivot leaks are rare.
The mechanism in the case at the top can be reached
by unscrewing the cap. The rare problems that can
happen are for the rope to slip off the drum. This is
near impossible because bolts hold the case together
around the drum leaving nowhere for the rope to go.
It is very rare for the rope to break. This rope can be
changed with the boat in the water. The winch
socket, shaft, and drum can also be changed with the
boat in the water.
