Rundquist Propeller Tools, Inc.
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Rundquist Propeller Tools, Inc.

How to Change Propeller Pitch

The pitch on most propellers can be changed to obtain better performance if necessary. Aluminum propellers can be changed two (2") inches up or down in pitch. Bronze propellers can be changed two (2") inches up or down in pitch. Stainless Steel can be changed one (1") inch up or down. We do not recommend changing the pitch any more than one (1") inch.

When changing pitch you need to know how the propeller performs before you change it. By lowering the pitch one (1") inch the motor will gain 200 RPM. By increasing the pitch the motor will lose 200 RPM. Do not attempt changing the pitch of a propeller in towards the hub. The metal in this area usually will not bend. It is only necessary to change 1/3 to 1/2 of the propeller blade to the new pitch.

1" Pitch = 200 RPM
Lower pitch to gain RPM
Increase pitch to lose RPM

Single cup = 200 RPM
Double cup = 300 RPM

1/4" Diameter = 200 RIM
Decrease diameter - gain RPM
Increase diameter - lose RPM

Progressive Pitch

Progressive pitch on a propeller makes it more versatile and helps the motor adapt to different loads. A progressive pitched propeller accelerates better than most straight pitch propellers and also develops very good top end.

 

Most propellers can be changed from straight pitch to progressive pitch. For better take off - lower the pitch over the leading edge. For better top end - increase the pitch over the trailing edge. See example below.

To figure out which pitch will be the most effective on a given boat, follow this formula.

  1. Gauge the existing propeller and determine what pitch it is.
  2. Find out what RPM the prop turns.
  3. Find out what the motors recommended RPM range is.

Example

  1. Motor - Mercruiser 120 HP @ 4400 - 4800 RPM
  2. Motor has 1:68 to 1 Gear Ratio
  3. Prop has true 17" pitch
  4. Motor currently runs 4600 RPM
Pitch in Feet 17" pitch / 12" per foot
= 1.42
x RPM at prop
= 2738
= Speed in feet per minute
= 3888
x 60 (60 minutes per hour)
x 60

 

233280
/ 5280 feet (number of feet per mile)
/ 5280
= Theoretical Speed
= 44.18 MPH
x .82 (average slip = 18%)
x .82
= Probable Speed
= 36.22 MPH

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Rundquist Propeller Tools, Inc. | Naples, FL 34104