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CD670

Propeller Selection

IMPORTANT: Installed propeller must allow engine to run at the upper end of the specified throttle

operating revolutions per minute (rpm) range, with a normal load aboard the boat. Use an accurate

service tachometer to verify engine operating rpm.

It is the responsibility of the boat manufacturer and/or the selling dealer to equip the power package with the

correct propeller(s). Specified engine wide-open-throttle (WOT) and operating rpm range are listed below and

in “Specifications.”

IMPORTANT: The engines covered in this manual, depending upon the model, are equipped with either

a governor or a device that limits engine rpm. Be sure that propeller being used does not allow engine

to run against the governor or limiter, as a significant loss in performance will result.

Engine rpm Limits

MCM Model Engine Specified Operating Rpm Range

Rpm Governor or Limiter Setting

(Begins At: )

D3.6L 3600-3800 3830± 20

D4.2L 3400-3600 3630± 20

D4.2L LD 3400-3600 3650± 50

D2.8L D-Tronic

D4.2L D-Tronic

3600-3800 3875

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 specified 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 specified operating rpm

range will cause higher than normal wear and/or damage. Generally, there is a 200 rpm change between propel-

ler 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 (not as significant on D-Tronic models).

• Operating in a higher elevation causes an rpm loss (not as significant on D-Tronic models).

• 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.

Mercury D4.2L D-Tronic User Manual

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