Mercury 200 OptiMax Jet Drive, Service Manual, Page: 37 / 391

General Information
Page 1C-12 90-8M0050731 MAY 2011
Conditions Affecting Performance
Weather
It is a known fact that weather conditions exert a profound effect on the power output of internal combustion engines. Established
horsepower ratings refer to the power the engine will produce at its rated RPM under a specific combination of weather conditions.
Corporations internationally have settled on adoption of International Standards Organization (ISO) engine test standards, as set
forth in ISO 3046 standardizing the computation of horsepower from data obtained on the dynamometer. All values are corrected
to the power the engine will produce at sea level, at 30% relative humidity, at 25 °C (77 °F) temperature, and a barometric pressure
of 29.61 inches of mercury (in. Hg).
Summer conditions of high temperature, low barometric pressure, and high humidity all combine to reduce the engine power. This,
in turn, is reflected in decreased boat speeds as much as 3 to 5 km/h (2 to 3 MPH) in some cases. Nothing will regain this speed
for the boater but cooler, dry weather.
Pointing out the consequences of weather effects, an engine running on a hot, humid day may encounter a loss of as much as
14% of the horsepower it would produce on a dry, brisk day. The horsepower that any internal combustion engine produces,
depends upon the density of the air that it consumes. The density of air is dependent upon the ambient air temperature, the
barometric pressure, and the humidity (water vapor) content.
Weight Distribution (Passengers and Gear) Inside the Boat
Shifting weight to rear (stern):
•
Generally increases speed and engine RPM
• Causes bow to bounce in choppy water
• Increases danger of following wave splashing into the boat when coming off plane
• At extremes, can cause the boat to porpoise
Shifting weight to front (bow):
•
Improves ease of planing
• Improves rough water ride
• At extremes, can cause the boat to veer back and forth (bow steer)
Bottom of Boat
For maximum speed, a boat bottom should be nearly a flat plane where it contacts the water and particularly straight and smooth
in the fore and aft direction.
•
Hook:
Exists when the bottom is concave in fore and aft direction when viewed from the side. When the boat is planing, hook
causes more lift on the bottom near the transom and allows the bow to drop, thus greatly increasing wetted surface and
reducing boat speed. Hook frequently is caused by supporting the boat too far ahead of the transom while hauling on a trailer
or during storage.
•
Rocker:
The reverse of hook and much less common. Rocker exists if the bottom is convex in fore and aft direction when
viewed from the side, and boat has strong tendency to porpoise.
•
Surface roughness:
Moss, barnacles, etc., on the boat or corrosion of the motor's gear housing increase skin friction and
cause speed loss. Clean surfaces when necessary.
•
Jet Unit:
If the unit is left in the water, marine vegetation may accumulate over a period of time. This growth must be removed
from the unit before operation, as it may clog the water inlet holes in the gear housing and cause the engine to overheat.
Water Absorption
It is imperative that all through‑the‑hull fasteners be coated with a quality marine sealer at time of installation. Water intrusion into
the transom core and/or inner hull will result in additional boat weight (reduced boat performance), hull decay, and eventual
structural failure.
Cavitation
Cavitation occurs when water flow cannot follow the contour of a fast‑moving underwater object. Cavitation is caused by water
vapor bubbles forming either from sharp turns or from an irregularity in the impeller blade itself. These vapor bubbles flow back
and collapse when striking the surface of the impeller blade resulting in erosion of the impeller blade surface. If allowed to continue,
eventual blade failure will occur. Common causes of cavitation are:
•
Weeds or other debris snagged on the water intake grate
• Bent impeller blades
• Raised burrs or sharp edges on the impeller
• Sharp turns of the boat