Propeller Selection; How Elevation And Climate Affect Performance - Mercury Bravo One Series User Manual

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Propeller Selection

It is the responsibility of the boat manufacturer and/or the selling dealer to equip the power package with the
correct propeller(s).
Select a propeller that will allow the engine power package to operate at or near the top end of the recommended
wide-open-throttle operating rpm range with a normal load. High rpm, caused by an excessive trim angle, should
not be used in determining correct propeller selection.
If full throttle operation is below the recommended range, the propeller must be changed to prevent loss of perfor-
mance and possible engine damage. On the other hand, operating an engine above the recommended operating
rpm range will cause higher than normal wear and/or damage. Generally, there is a 200 rpm change between
propeller pitches.
After initial propeller selection, the following common problems may require that the propeller be changed to a
lower pitch:
Warmer weather and greater humidity cause an rpm loss.
Operating in a higher elevation causes an rpm loss.
Operating with a damaged propeller or dirty boat bottom causes an rpm loss.
Operating with increased load (additional passengers, pulling skiers, etc.).
For better acceleration, such as is needed for water skiing, use the next lower pitch propeller. However, do not
operate at full throttle when using the lower pitch propeller but not pulling skiers.
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How Elevation And Climate Affect Performance

Elevation has a very noticeable effect on the wide-open-throttle power of an engine. Since air (containing oxy-
gen) gets thinner as elevation increases, the engine begins to starve for air. Humidity, barometric pressure and
temperature do have a noticeable effect on the density of air. Heat and humidity thin the air. This condition can
become particularly annoying when an engine is propped out on a cool, dry day in spring and later, on a hot, sultry
day in August, doesn't have its old zip.
Although some performance can be regained by dropping to a lower-pitch propeller, the basic problem still exists.
In some cases, a gear-ratio change to more reduction is possible and very beneficial.
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 2 or 3 miles per hour in some
cases. Nothing will regain this speed for the boater, but the coming of cool, dry weather.
In pointing out the practical consequences of weather effects, an engine -- running on a hot, humid summer day
-- may encounter a loss of as much as 14% of the horsepower it would produce on a dry brisk spring or fall day.
With the drop in available horsepower, this propeller will, in effect, become too large. Consequently, the engine
operates at less than its recommended rpm. This will result in further loss of horsepower at the propeller with
another decrease in boat speed. This secondary loss, however, can be somewhat regained by switching to a
lower-pitch propeller that allows the engine to again run at recommended rpm.
For boaters to realize optimum engine performance under changing weather conditions, it is essential that the
engine be propped 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 is the fact that the engine also
will be operating in an rpm range that discourages damaging detonation. This, of course, enhances overall reli-
ability and durability of the engine.
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