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Question:

Here’s a basic rule of thumb, every inch of pitch (for a constant diameter) is worth 200 rpm.

That holds true for many situations, but can’t be considered completely general. It works for RPMs around 4000 and pitches around 20", but won’t work at 11" pitch and 5400 RPM, for example. Increase the pitch, reduce the rpm.  

That is always true. A reasonable way to estimate RPM when moving to a new prop is approximating it with this inverse proportional relationship: new RPM X new pitch = old RPM X old pitch -or- new RPM = old RPM X (old pitch/new pitch)   But, since nothing is a constant the only way to determine which is best for your application is testing and an understanding prop shop.

AMEN. — —

Response:

The first number is the diameter of the prop and the second number represents the distance the prop will move forward in one revolution of the prop (asuming the water was a hard surface). The correct prop for you would be the prop that puts you in the top-most end of your motors Wide Open Throttle range.  

Don’t forget this is assuming your motor is operating properly. Don’t get caught up in the "re pitching when my 6 cyl motor  is running on 5 cyl" game.  Happens more than you think!! Bill

Response:

The pitch is also in inches.  Under no-slip conditions, this is the distance, in inches, that the prop would travel through in one rotation. — Regards Michael Smith M/V GRIN ‘N BEAR IT St. Stephen, N.B. Canada – Hide quoted text — Show quoted text – The first number is the diameter in inches, the other is the pitch in degrees. higher pitch gives you more speed with slow accelleration, lower pitch gives you a faster accelleration to a slower top-speed.

Response:

Recently inherited a mid 70’s bowrider with Evinrude 85 hp and am having fun with it, first boat and all. The prop on the motor is stamped 13 3/8 x 17.  It came with two other props, 1 is stamped 13 3/4 x 15, the other is stamped 13 1/4 x 17. Would some kind soul educate me as to what the difference is, and what do these numbers mean??? While no speed demon, the boat seems to perform nicely with the prop attached and pulls up a skier with ease. Also, the props all look about the same to me. Does one or another give better speed, quicker acceleration, or what???

Response:

I always thought the second number was the number of inches a prop would move if it rotated one revolution through a solid. Robert – Hide quoted text — Show quoted text – The first number is the diameter in inches, the other is the pitch in degrees. higher pitch gives you more speed with slow accelleration, lower pitch gives you a faster accelleration to a slower top-speed. Recently inherited a mid 70’s bowrider with Evinrude 85 hp and am having fun with it, first boat and all. The prop on the motor is stamped 13 3/8 x 17.  It came with two other props, 1 is stamped 13 3/4 x 15, the other is stamped 13 1/4 x 17. Would some kind soul educate me as to what the difference is, and what do these numbers mean??? While no speed demon, the boat seems to perform nicely with the prop attached and pulls up a skier with ease. Also, the props all look about the same to me. Does one or another give better speed, quicker acceleration, or what???

Response:

You are correct, the "pitch" as it’s called, is the number of inches the prop moves forward per revolution, in a solid (100% efficiency). Here’s a basic rule of thumb, every inch of pitch (for a constant diameter) is worth 200 rpm.  Increase the pitch, reduce the rpm.  Higher diameter props provide more surface area thus increasing the efficiency (in theory).  Larger diameter props allow some designs to use slightly less pitch thus providing better holeshot or midrange throttle response.  But, since nothing is a constant the only way to determine which is best for your application is testing and an understanding prop shop. — Mark Whatman http://home.att.net/~mwhatman I always thought the second number was the number of inches a prop would move if it rotated one revolution through a solid. Robert The first number is the diameter in inches, the other is the pitch in degrees. higher pitch gives you more speed with slow accelleration, lower pitch gives you a faster accelleration to a slower top-speed.

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Response:

The first number is the diameter of the prop and the second number represents the distance the prop will move forward in one revolution of the prop (asuming the water was a hard surface). The correct prop for you would be the prop that puts you in the top-most end of your motors Wide Open Throttle range.  Load the boat with your regular running gear and just you in the boat.  The run it up to WOT.  Check the specs for YOUR motor.  If, for example, it says your WOT range is 4500-5500, then you want your RPMs to be at 5500.  Then when you put people in your boat and a skier on the tail, you will still be in the WOT range.  Contrary to popular belief a larger pitch prop will not give you any more speed.  All that will do is lower your engine RPMs.  If it’s not in that upper most WOT range, your motor may not be making its full power as 2 strokes make more power at higher RPMs.  This also allows you to tune for best fuel economy and engine life-span. For more details and terminology check out this page:         http://www.omc-online.com/Prop/prop_main.html Hope this helps. Happy boating Big Smoke.

Response:

The first number is the diameter in inches, the other is the pitch in degrees. higher pitch gives you more speed with slow accelleration, lower pitch gives you a faster accelleration to a slower top-speed. – Hide quoted text — Show quoted text – Recently inherited a mid 70’s bowrider with Evinrude 85 hp and am having fun with it, first boat and all. The prop on the motor is stamped 13 3/8 x 17.  It came with two other props, 1 is stamped 13 3/4 x 15, the other is stamped 13 1/4 x 17. Would some kind soul educate me as to what the difference is, and what do these numbers mean??? While no speed demon, the boat seems to perform nicely with the prop attached and pulls up a skier with ease. Also, the props all look about the same to me. Does one or another give better speed, quicker acceleration, or what???

Response: