Propeller Pitch - SailNet Community
Old 03-24-2005 Thread Starter
moderate?

Join Date: May 2002
Location: East Coast
Posts: 13,877
Thanks: 0
Thanked 6 Times in 6 Posts
Rep Power: 17

Propeller Pitch

Can anyone explain to me how propeller pitch is measured? I have a 20x13 propeller and don''t know what the 13 stands for. Just curious...Thanks!

Old 03-25-2005
Member

Join Date: Jul 2002
Posts: 68
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 15

Propeller Pitch

that is the theroretical advance thru the water in one revolution of the prop. realistically of course because of slippage this can''t happen.
capnjim02 is offline
Old 03-25-2005
Senior Member

Join Date: Dec 2002
Posts: 552
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 15

Propeller Pitch

Most props are stamped on the forward end with diameter, pitch and shaft size. Diameter is always first, pitch second. Size is often repeated on the side of the hub with the manufacturer''s name, type prop, hand, and date of manufacture.

For your 20 x 13 prop’, the first number is the diameter (the prop’ is 20" tip-to-tip circle), and the second number designates the 13" propeller pitch.

Pitch is the angle of the blades, expressed in inches of theoretical travel in one revolution of the propeller. This propeller has a pitch that will theoretically* result in 13" of forward travel in one revolution.

“SLIP”:
*As capnjim02 says , the propeller experiences slippage, so that its actual travel per revolution is less than the stated pitch. “Slip” is the difference between the actual distance traveled by a boat, and the theoretical distance computed by the number of revolutions and propeller pitch.
Actually, there are several kinds of slip, including:
Apparent slip is the difference between the advance observed and that calculated by pitch times revolutions.
True slip is greater than apparent slip, due to wake moving with the vessel. The wake that affects slip is a body of water surrounding the propeller and moving along with the vessel. The amount of wake is determined by the amount of friction produced by the hull moving through the water.
Sailing auxiliaries generally experience about 35 to 40% slip.

“PITCH”
Increasing or decreasing propeller pitch directly affects engine load throughout its RPM range.
A high load (high pitch and/or diameter) on an engine will result in lower engine RPM throughout its entire RPM range for a given throttle setting. Adding 1 inch of propeller pitch will reduce full-throttle (WOT) RPM by about 150 to 200.
A light load (low pitch and/or diameter) on the engine will result in higher engine RPM throughout its entire RPM range for a given throttle setting. Reducing pitch by 1" will reduce full-throttle (WOT) RPM by about 150 to 200.

More interesting info’ at:
RBBI PROP CALCULATOR: http://www.rbbi.com/folders/prop/propcalc.htm
THE PERFECT PITCH: http://www.nautica.it/info/motore/eliche_e.htm

FWIW,
Gord May
GordMay is offline

Old 03-25-2005 Thread Starter
moderate?

Join Date: May 2002
Location: East Coast
Posts: 13,877
Thanks: 0
Thanked 6 Times in 6 Posts
Rep Power: 17

Propeller Pitch

Thanks Gord and Cap''n Jim. The relation between pitch and RPM''s was particularly useful. Curiosity satisfied! <grin>

Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.

## Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.

User Name:
 Please enter a valid email address for yourself. Email Address:
OR

## Log-in

 User Name Password Remember Me?
Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

 Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

 Similar Threads Thread Thread Starter Forum Replies Last Post ddilman Gear & Maintenance 26 4 Weeks Ago 04:18 PM ddilman General Discussion (sailing related) 3 02-12-2006 12:45 PM dick/happyours Gear & Maintenance 4 03-11-2005 08:41 AM ddilman Gear & Maintenance 1 10-31-2004 04:31 AM Scurvy_Dog General Discussion (sailing related) 4 02-19-2003 04:59 PM

 Posting Rules You may post new threads You may post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OnTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

For the best viewing experience please update your browser to Google Chrome