GASOLINE ENGINE 8.1 S INBOARD MODELS
Page 8 of 58
APPLICATION / RATIO SELECTION
The propeller shaft speed is determined by engine speed and the transmission ratio. Every
boat has a best shaft speed that directly relates to boat speed. If propeller shaft speeds are
too high an inordinately small propeller must be used resulting in poor performance. If they
are too low then too large a propeller must be used. Fast boats do best with direct drive or
small reductions. Heavier and slower boats require corresponding greater ratios of
reduction. 100 rpm of propeller shaft speed for each mph of boat speed is a rough rule of
thumb for selecting the drive ratio.
COUPLING
The coupling is a flange type coupler (available through Quicksilver Accessories). All
coupler bolts must be SAE Grade 8 (Metric Grade 10.9) or better, with a shoulder (grip
length) long enough to pass through the face mating plane of couplers. All coupler bolts
must be torqued to 50 lb-ft (68 Nm).
ENGINE/PROPELLER SHAFT INSTALLATION ANGLE
The transmission and engine should be mounted so that the angle relative to horizontal is
as shown in the installation drawings. Refer to individual installation drawings for each
specific engine and transmission.
IMPORTANT: Relative to horizontal, never install the engine with the front (pulley
end) down.
IMPORTANT: On all engines, a high angle of installation [front (pulley end) of engine
up] along with low transmission oil levels can permit transmission pump cavitation
on some models when operating in rough water.
PROPELLER SHAFT DIAMETER
The required propeller shaft diameter can vary significantly depending on the material used,
strut and bearing design, engine horsepower and shaft rpm. Information is available from
the propeller shaft manufacturer and in marine handbooks for selecting the proper diameter.
Sufficient shaft diameter is critical for durability and to minimize vibration. As a guideline,
the propeller shaft should be a minimum of 1/14 the diameter of the propeller.
VELVET DRIVE 72C TRANSMISSIONS
CAUTION
Velvet Drive 72C In-Line Only – Use of proper rotation propeller (specified on trans-
mission output flange rotation decal) is critical since the transmission must be op-
erated in forward gear selector position only to drive boat forward. If the wrong rota-
tion propeller is installed and transmission is operated in reverse to propel the boat
forward, transmission failure WILL occur. On engines which are equipped with Vel-
vet Drive In-line transmissions, a LH propeller is required.
