Mercury 8.1S Horizon, Product Applications Manual

Page: 21 / 163

Transmission and Drive Line
Page 2A-2 90-863758060 AUGUST 2006
General Information
Gear Ratio Selection
The transmission gear ratio and propeller must provide the optimum match between the
engine and the boat application. Gear ratio and propeller selection are affected by
numerous factors including engine horsepower, WOT RPM, hull design, propeller
clearance, boat weight, and other factors. Several computer programs are available to
assist in analyzing all of these variables and selecting the proper propeller and gear ratio.
Typically, a higher reduction transmission coupled with a slower turning, larger diameter
propeller provides the best efficiency for larger, heavier boats. Conversely, a lower
reduction transmission and a smaller diameter, faster turning propeller will generally
provide superior performance on lighter, faster applications (35 mph and above).
Engine and Propeller Shaft Installation Angle
The transmission and engine should be mounted so that the angle relative to horizontal is
within the range shown on the installation drawings. As a general rule, the propeller shaft
angle should position a properly sized propeller at least one propeller diameter below the
waterline.
Installing the engine with the front end too high can cause aeration of the transmission fluid
and engine oil. Special care must be taken on 350 cid/5.7L and 377 cid/6.2L V-drive
applications with propeller shaft angles of less than 9° to avoid water reversion problems
through the exhaust system (See
Exhaust System ). The exhaust elbow outlets will have
less than their prescribed amount of downward slope with the exhaust elbows installed in
their normal orientation. On these applications, reverse the exhaust elbows to get the
proper slope. The engine can be obtained from the factory with this orientation.
NOTE: 5.7L, 6.2L, and 8.1L models have an increased slope in the exhaust elbow outlets
and can accommodate any propeller shaft angle (within the specified range) in V-Drive
applications without the need to reverse the elbow.
Propeller Shaft Rotation and Spacing on Dual Installations
Best all-around performance is typically obtained by installing the engines so that the
propellers turn outboard (looking from the stern). As a general rule, the centerline to
centerline spacing between the propellers should be a minimum of 2.5 times the propeller
diameter. Insufficient spacing will reduce efficiency and increase vibration. Shaft spacing
must provide the minimum distances between engines as specified in the table following
to allow for servicing.
a
b
c
d
c
d
14884
a - Inboard propeller rotation
b - Outboard propeller rotation
c - RH rotation
d - LH rotation