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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.
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.
Accompanying the effects of weather inspired loss of power is a second, but more subtle loss. Consider a boat rigged during
cooler, less humid weather with a propeller that allowed the engine to turn within its recommended RPM range at full throttle.
Higher temperatures with high humidity weather will consequently decrease the available horsepower. The propeller, in effect,
is too large for the atmospheric conditions and the engine operates at less than its recommended RPM.
The engine‑rated horsepower is a direct relation to the engine's RPM. An engine with too large of a propeller will have a further
loss of horsepower and subsequent decrease in boat speed. This secondary loss of RPM and boat speed can be regained by
switching to a smaller pitch propeller that allows the engine to run at recommended RPM.
For boaters to realize optimum engine performance under changing weather conditions, it is essential the engine has the
proper propeller to allow it to operate at, or near, the top end of the recommended maximum RPM range at wide‑open throttle
with a normal boat load. Not only does this allow the engine to develop full power, but equally important, the engine will be
operating in an RPM range that discourages damaging detonation. This enhances overall reliability and durability of the engine.
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 fore and aft direction.
•
Hook: Exists when bottom is concave in fore and aft direction when viewed from the side. When boat is planing, hook
causes more lift on bottom near transom and allows bow to drop, thus greatly increasing wetted surface and reducing boat
speed. Hook frequently is caused by supporting boat too far ahead of transom while hauling on a trailer or during storage.
•
Rocker: The reverse of hook and much less common. Rocker exists if 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 boat or corrosion of outboard's gear housing increase skin friction and
cause speed loss. Clean surfaces when necessary.
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, such as a gear housing or a
propeller. Cavitation increases propeller speed while reducing boat speed. Cavitation can seriously erode the surface of the
gear housing or the propeller. Common causes of cavitation are:
•
Weeds or other debris snagged on the propeller
•
Bent propeller blade
•
Raised burrs or sharp edges on the propeller
90-8M0105568 eng MARCH 2016
© 2016 Mercury Marine
General Information
Page 1C-11
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