Mercury 9.9 EH Manual page 51

2. Twin engine: Position the outboard engines approximately
580 mm (22.8 in.) apart, measured from the center line of each
engine, and equal distance (a) from the center of the
transom.
3. Transom matching: Be sure that the anti‑ventilation plate of
the outboard is below the water surface when running with the
throttle wide open. If the above condition cannot be met due
to the shape of the bottom of the boat, please consult the
dealer.
IMPORTANT: If the anti‑ventilation plate is at a level higher than
the bottom of the boat, overheating may occur as a lack of cooling
water flow in the cooling systems.
4. Clearance between the anti‑ventilation plate of the motor and
the bottom of the boat must be 30–50 mm (1.2–2 in.). If the
height difference exceeds 50 mm (2 in.), engine power
performance is likely to be reduced as a result of increased
water resistance to the gearcase assembly.
a - Bottom of hull
INSTALLATION
580 mm
a
30–50 mm (1.2" 2")
41
a
37509
a
–
b
37510
b - Anti‑ventilation plate