Mercury 100 FourStroke, Service Manual

Page: 498 / 574

6B-8 - LOWER UNIT 90-830234R3 DECEMBER 1997
Transom Height of the Boat
Outboards with jet drives will be mounted approxi-
mately 7 inches higher on the transom than propeller
driven outboards. This requires outboards that have
a 15 in. shaft length to be installed on boats having
a 22 in. transom height and outboards that have a 20
in. shaft length to be installed on boats having a 27
in. transom height.
If the boat transom is of insufficient height, and the
outboard cannot be installed to the recommended
height, contact the boat manufacturer for recom-
mended procedure to build up the boat transom.
Locate Centerline Of The
Outboard
B A
D C
a
a - Centerline of Transom
Outboard Mounting Height
The initial outboard mounting height setting will work
good for most applications, however, because of dif-
ferent boat/hulls designs, the setting should be re-
checked by test-running the boat. Refer to Water
Testing.
•
Installing the outboard too high on the transom
will allow the water intake to suck in air and cause
cavitation (cavitation will cause the engine to
overspeed in spurts and reduce thrust). This
condition should be avoided by proper height set-
ting.
•
Installing the outboard too low on the transom will
allow excessive drag.
SETTING OUTBOARD MOUNTING HEIGHT ON
BOATS WITH “V” BOTTOM HULLS
1. Measure the width of the leading edge on the wa-
ter intake housing. Make a horizontal line (a) on
the transom up from the “V” bottom the same
length as the width of the water intake housing
(b).
a
b
2. Place (center) the outboard on the boat transom
so that the transom brackets are resting on top of
the transom. Temporally fasten the outboard to
the transom using two C-clamps.
3. Position the outboard in a vertical position.
4. Line-up a straight edge (c) along the bottom of
the boat with the horizontal line made in Step 1
and measure the distance between the horizon-
tal line and top front edge of the water intake
housing (d).
c
d
5. Raise the outboard up on the transom the dis-
tance measured in Step 4. Use a straight edge
and recheck the mounting height. The top edge
of the water intake housing should be lined-up
with the horizontal line made in Step 1.
6. Fasten outboard to the transom at this height.