Marine Propeller Pitch Selection
The pitch to swap to so the engine reaches the top of its rated RPM band at wide-open throttle.
Example
You enter
- Current prop pitch (in) 19
- Measured WOT RPM now 5000
- Target WOT RPM (rated band) 5400
- RPM change per inch of pitch 200
You get
- Pitch change 2.0 in less
- New pitch 17
Checked against published worked examples from: Boating magazine (B. Becker).
Details, formula, and sources
ScopeEach inch of pitch changes WOT RPM by ~200 rpm, so pitch change = (target - current WOT RPM) / rpm-per-inch and new pitch = current - change. A 19 in prop hitting 5000 rpm against a 5400 target drops to a 17 in prop; a boat over-revving to 6000 goes up to 22 in. Under-rev needs less pitch, over-rev needs more. A selection aid; a WOT test with the new prop and the dealer's prop chart govern.
Formula or tablepitch_change = (target_wot_rpm - current_wot_rpm) / rpm_per_inch; new_pitch = current_pitch - pitch_change; default rpm_per_inch = 200.
Source and editionMarine propeller pitch selection, standard boating / outboard-manufacturer practice (Mercury / Yamaha prop selection guides) by name; the ~200 rpm per inch of pitch rule of thumb.
Free accessThe rpm-per-inch rule and the pitch/RPM relation are published free in outboard-manufacturer propeller selection guides.
What governsEstimate. AHJ and licensed professional govern.
API field namescurrent_pitch_in, current_wot_rpm, target_wot_rpm, rpm_per_inch, pitch_change_in, new_pitch_in
- RPM per inch each inch of pitch changes WOT RPM by ~200 rpm (150-250 typical)outboard-manufacturer prop guides
- Pitch relation new pitch = current - (target - current RPM) / rpm-per-inchpropeller selection practice
- Other factors diameter, cupping, gear ratio, and load also affect WOT RPM; confirm with a WOT testscope of this tile