Gemini 105Mc Owner’s Manual
24Copyright © 2004 Performance Cruising Inc.
Engine
The standard Gemini 105Mc engine is the Wester-
beke 27 horsepower, Model 30B. The 27-hp diesel is
linked to a sonic outdrive leg and provides excellent
fuel economy and battery charging from the 55-amp
alternator.
The use of diesel is safe and can be used for other
appliances on the boat such as heaters and genera-
tors. This engine weighs about 400 lbs. The fuel
economy will range from 6 to 12 miles to the gallon
depending on speed and conditions.
The Westerbeke engine is mounted on the bridge
deck under the center cockpit hatch. The gearbox at
the back of the engine is connected through a flexi-
ble coupling to the Sillette sonic out-drive leg. The
engine is accessible through all three cockpit lockers
and aft deck hatches.
The oil dipstick, oil filter, and water pump are lo-
cated behind the starboard side breakaway panel.
Access to the port side is by unscrewing the side
panel. Please refer to your Westerbeke manual for
oil change information and specific operating in-
structions including service periods.
The 30B engine uses about 3 1/8 quart of SAE 10W-
30 weight oil. The transmission uses about 7/8
quart of the same type oil.
Refer to the back of the manual for an addendum
and for how to contact your closest authorized
Westerbeke Service Center.
Cooling Water
The cooling water for the engine comes into the ship
through a seacock under the starboard aft bed. The
seacock has (3) positions: In from the sea, closed,
and a third position which enables connecting a hose
to the open barb on the seacock for winterizing.
There is a water filter by this seacock. The engine is
a freshwater-cooled engine with salt water-cooling
the freshwater in a heat exchanger. There is a fresh
water header tank on the wall above the engine.
Gear Lever / Throttle
The Gemini 105Mc comes with a single lever throttle
and gear control (UFlex
B73 Single Lever Top Mount
Engine Control)
located on the starboard side of the
wheel. With the lever in the neutral position, the
handle can be pulled out 1/4" to disengage the gears
so the throttle can be operated without moving the
boat.
Once the lever drops back to the neutral position,
which is vertical, the gears are engaged. Pushing the
lever forward 45º engages forward. The rest of the
Engine, Drive Leg, Fuel
movement increases the engine speed. Moving the
lever back engages reverse.
Fuel Tanks / Gauges
The Gemini 105Mc has (2) two 18 gallon fuel tanks,
each located either side of the engine under the aft
deck. The fuel gauges are located on the instrument
panel. There are (2) two fuel selector valves located
under the engine hatch on the center stern. One is
fuel supply to the engine. The other is the return
valve returning the unused diesel fuel back to the
tank. The valves simply point to the tank in use.
Both valves should point to the same tank.
Following the fuel valves is a glass Racor fuel filter
on the rear wall. Following the fuel filter, the fuel
goes to the engine electric pump and then to an-
other fuel filter. The Westerbeke uses an electric
pump and is self-priming.
Diesel fuel in tanks can grow a fungus, akin to a
black sooty deposit. This deposit can block the
hoses, filters, pumps and injectors. Even a filter will
not help resolve the problem as the hoses leading to
the filter can get blocked. To stop this potential
problem, a diesel additive can be added to the fuel.
Diesel fuel is also subject to water condensation. It
is advisable to leave the tanks full to reduce the
occurrence of water condensation.
Hot Water and Cabin Heater
Just inside the engine compartment are (2) two 3/8”
black hoses that connect to a brass junction with a
valve. Continuing from the brass junction are two
(2) 5/8” hoses. Each hose connects to the hot water
heater and the optional cabin heater.
With the valve at the junction in the horizontal posi-
tion, fresh water from the engine is heating the wa-
ter in the water heater. With the junction valve
vertically down, the water supply is shut off.
With the optional cabin heater there is an additional
valve in the port aft cabin under the bed. This valve
can open the engine hot water to the cabin heater as
well. The optional heater is the Heater Craft 500H
unit capable of 28,000 BTU.
Sillette Sonic Drive Leg
Please read through the Sillette manual thoroughly.
The sonic drive leg will provide many years of de-
pendable service if used correctly and properly
maintained. Improper use can result in breakage.
Slamming the leg down into forward with the full
throttle before the leg has locked into place can
cause damage. Always bring the leg down in a slow
and controlled manner and do not throw the throttle
completely forward until the leg clicks into place. If
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