Selecting Inboard Transmission Geometry
Selecting the proper reduction ratio for a par-
ticular combination of boat, engine, and pro-
peller is a critical part of propeller selection.
This is really a propeller-sizing issue. Besides
selecting the reduction ratio, however, you
must also select a gear or transmission with
the proper geometry to install the engine
where you need it for arrangement-plan and
weight-and-trim considerations, as well as to
make the drivetrain fit properly in the boat.
There are several standard gear geome-
tries (Figure 3-1):
In-line coaxial
The output shaft is in-line with and on the
same axis as the engine driveshaft. This is
usually the smallest and most compact gear
configuration and is usually suited to shoal
hulls with the engine near the hull bottom.
Parallel drop-shaft, shallow case
The output shaft is parallel to the engine
driveshaft, but is offset down by a few inches.
This is a common configuration that can help
you raise an engine a bit to clear the bilge bet-
ter. The shallow case means the drop isn’t too
great.Parallel drop-shaft, deep case
The output shaft is parallel to the engine
driveshaft, but is offset down by several
inches. This common configuration can help
you raise an engine significantly to clear the
bilge better. You must have sufficient clear-
ance over the engine to do this. The deep
case means the drop is substantial. This is a
common configuration for reduction gears
over 3: 1 in larger displacement vessels.
Down-angle gear
The output shaft angles down relative to the
engine driveshaft. Angles are usually between
10 and 15 degrees. Down-angle gears permit
installing a more level engine on a steep
shaft. In doing this, they also lower the for-
ward top corner of the engine, permitting a
lower engine-compartment overhead or fit-
ting a higher engine in a lower engine
compartment.
Direct-mount V drive (also called UV drive
or U drive)
The gear mounts directly on the engine bell
housing as usual, with the output shaft re-
versed and angled down and under the
engine in a V. Angles are usually available
between 10 and 15 degrees. This permits
keeping the engine well aft and facing
backward. Direct-mount V drives are greatCHAPTER 3