Page 51 - REPOWER REFERENCE GUIDE (2020)

Propeller Selection

Pitch and Diameter
Select model and initial pitch propeller to begin testing using information provided in the Mercury Precision Parts and
Accessories Guide or the Propeller Selector tool found at http://www.mercurymarine.com/propellers/prop‑selector/#. A broad
array of propeller models and pitches are available to suit virtually any boat application. Initial selection is typically done by boat
weight. Using a propeller with a higher pitch may give slightly more speed and fuel economy, but generally, this will cause the
engine RPM to be on the low side or even below the specified WOT RPM of the engine, which can damage the engine.
IMPORTANT: On sterndrive models, do not use excessive power trim angle when determining correct propeller pitch.
NOTE: Engines with a rev‑limiter may appear to have an ignition misfire as the limit is reached. When the engine RPM returns
to within the specified WOT RPM range, the ignition misfire will disappear.
Operate the boat at wide‑open throttle and adjust the power trim to achieve optimum speed without using excessive trim. If
RPM is above the specified range, a higher pitch propeller must be used. A lower pitch propeller must be used if RPM is too
low. There is a change in RPM between propeller pitches that is generalized as follows:

Diameter or Pitch Change RPM Change
1 inch 200–225
2 inch 400–450

Example: Changing from a 21‑in. pitch to a 19‑in. pitch propeller increases engine RPM at WOT. Switching from a 19‑in. pitch
to a 21‑in. pitch propeller decreases engine RPM.
On inboard or ski engines, a change in propeller diameter or cupping a propeller will also change engine RPM.
For better acceleration, such as is needed for waterskiing, the next lower pitch propeller can be used. Care must be exercised
when not pulling skiers to prevent overrevving the engine. When cruising, the next highest pitch propeller should be used.

21”

23880

Additional Variables

Because of the many variables of boat design, only testing will determine the best propeller for a particular application. Just
because a certain size propeller is used on a given model of boat does not mean all boats like that model can use the same
pitch propeller. Variances in the boat and accessories can require that a different pitch propeller be used to get the engine to
operate at the maximum RPM of the specified RPM range.
The following common factors may also cause a loss of engine RPM and require the use of a lower pitch or smaller diameter
propeller:
• Warmer weather and greater humidity

90-8M0149179 eng NOVEMBER 2018 © 2019 Mercury Marine Page 4B-3