Setting Outboard Mounting Height On Boats With Flat Bottom Hulls; Water Testing; Checking For Cavitation - Mercury 40 Service Manual

Setting Outboard Mounting Height
on Boats with Flat Bottom Hulls
1. Place (center) the outboard on the boat transom
so that the transom brackets are resting on top of
the transom. Temporarily fasten the outboard to
the transom using two C-clamps.
2. Position the outboard in a vertical position.
3. Place a straight edge along the bottom of the boat
as shown and measure the distance between the
bottom of the boat and top front edge of the water
intake housing.
a
b
a - Straight Edge
b - Top Edge of Water Intake Housing
4. Raise the outboard up on the transom the dis-
tance measured in Step 3. Use a straight edge
and recheck the mounting height. The top edge
of the water intake housing should be in line with
the bottom of the boat as shown.
5. Fasten outboard to the transom at this height.

Water Testing

Checking for Cavitation

Making the initial outboard height setting should be
close to the optimum setting for the outboard. How-
ever because of the hull design of some boats, ob-
structions or imperfections in the hull ahead of the
water intake may require this setting to change in or-
der to prevent cavitation at running speeds.
90-852572R1 JANUARY 1998
When operating the boat, the outboard drive shaft
should be vertical when planing or tilted toward the
boat in order to provide a scooping angle on the water
intake. Tilting the outboard out beyond a vertical
position reduces the scoop angle and can cause im-
peller slippage and cavitation. If the angle of the boat
transom does not allow the drive shaft to be posi-
tioned vertical a Wedge kit should be installed behind
the transom brackets to increase the tilt-in angle.
NOTE: Slight cavitation in sharp turns and rough wa-
ter is acceptable but excessive cavitation is harmful
to the outboard and should be avoided.
Test run the boat. If cavitation occurs (air enters the
pump causing loss of thrust, engine over-speeds er-
ratically). the first thing to try is lowering the outboard
height 1/4 in. This can be accomplished by elongat-
ing the drilled mounting holes in the boat transom by
1/4 in.
If cavitation still exists after lowering the outboard 1/4
in., it maybe helpful to seek advice from the boat
manufacturer.
A number of other options are available to further re-
duce cavitation.
1. Water intake fin kit – Available from the Specialty
Mfg. Co. for jet models 30 thru 140. The purpose
of these fins is to ram more water into the intake
and shield the forward sides of the intake from the
entrance of air. This kit will help reduce cavitation
when running with the wind in a chop.
a - Intake Fin Kit
Water Intake Fin Kit Part No.1186 for jet models
45 thru 140 and Part No. 1185 for jet model 30
is available from.
Specialty Mfg. Co.
2035 Edison Ave.
San Leandro, CA 94577
a
LOWER UNIT - 6D-11