# Thread: Propeller Pitch

1. Member
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Jul 2006
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## Propeller Pitch

As mentioned in another thread, I am looking for another prop. I have found several that might work. My question is - Is it possible to visually determine the pitch of a prop? If so, how?

Thanks again.

Ed Everett

2. Its usually stamped on them Ed, sometimes a bit hidden,covered with old paint, perhaps ground down, sanded down or similar.
If it was me, I'd go off to a boatyard, find one with the pitch stamped on it and take a bevel gauge and a square across the average of the blade for comparison.( say.. 40 % or so in from the blade tip?) Or if you have the prop take it to a prop shop and ask.

3. Originally Posted by Capm Ed
As mentioned in another thread, I am looking for another prop. I have found several that might work. My question is - Is it possible to visually determine the pitch of a prop? If so, how?

Thanks again.

Ed Everett
Yes.
Imagine a round stick of butter. If the propeller is placed in it and turned and it has a pitch of 7 that means that for each revolution the the propeller will move 7inches forward thru the butter.

make a straight line on a surface.(line of advance) Place the propeller fore and aft over the line. Now draw the angle of the blade to that line.
The length of the blade when looking down will be how far it will travel in one turn.

For every inch of prop pitch reduction you would gain about 200 RPM.
Last edited by donald branscom; 08-19-2007 at 07:23 PM.

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Originally Posted by donald branscom
For every inch of prop pitch reduction you would gain about 200 RPM.
Lower pitch does in fact result in higher rpm. 200 rpm per inch is good for some combilation of engine and prop but it certainly won't apply equally to a 2000 rpm diesel and a 6000 rpm Yamaha. Not trying to be confrontational but the generalization is a bit broad in my mind.

5. One thing rarely talked about props is what effect prop diameter has on rpms.
I noticed with in stock configurations, that diameter is proportionate to pitch,at least on smaller outboards. The more pitch added,the diameter lessens. It seems somewhat counter productive in my eyes when looking for an in between. I started out with a 10x16p on a 45 hp. Revs too high. Went to a 10x17 and still maxes out the r's. The dealer told me 11x13 and still. . .
If they would just tell me what effect that diameter has,I could zero it in better. As it is,my hand on the stick is the revlimiter and the performance just is as it is. Fast. They don't make a prop with a 17 pitch greater than 10"diameter for my engine. I can't even imagine what a rocket it would be with a prop that actually got the r's near red line.
On outboard props,the number is usually on the outside of the hub or part of the inner hub that is hidden on the backside when installed. Diameter is the first number.

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Part of the problem with outboards is there is such a huge variety of hull shape and weights on which a given engine can be installed. To get the optimum prop for each condition would require a massive inventory. I suspect most forum members don't fit the generic profile for outboard buyers. Not many Bayliners. Further, most folks don't appreciate how to trim an engine much less select a prop. Selection is better than it was 30 years ago, but as you point out there are still missing pieces.

7. Originally Posted by Capm Ed
As mentioned in another thread, I am looking for another prop. I have found several that might work. My question is - Is it possible to visually determine the pitch of a prop? If so, how?

Thanks again.

Ed Everett
The answer to your question is no. An expert might be able to make an educated guess but you need to take measurements and do a little math to get the prop pitch. In fact, two props of different diameters might look just alike in shape but the larger diameter one will have the greater pitch.

To find the pitch of a prop, lay it down on a flat table. Select a point about half way out on the length of one blade. Measure the distance of that point from the center of the drive shaft hole. Call this dimension A. Now draw a line across the chord of the prop perpindicular to the blade axis. Take a small carpenters square and determine the length of the horizontal projection of the blade chord on the table by marking a point under each end of the chord line. Call this dimension B.

With a ruler find the vertical rake of the blade at the marked chord by measuring the difference between the height of each edge of the blade at the chord line off the table. Call this dimension C. Now multiply A X 2 PI to get the circumference of the blade at the marked chord.

Divide the circumference by B and multiply by C. The result is the pitch of the prop. That is, as the prop rotates through one revolution, the prop advances by C/B times the rake of the blade.

Notice that as the radius A increases, the rake C decreases and the pitch stays the same and the forward progress of the prop through the water is equal for all parts of the blades. Of course there is always some slip but that is another issue.

A little involved but there is no substitute in the average home shop. I recently made a propeller for a home built aircraft that looked at first glance to be much less aggressive than most boat props. However, the true pitch of this 76" diameter prop is a whopping 45.

This is a little long but I lack the skill to make it shorter. If you already know this, maybe it will help someone else.
Last edited by Tom Lathrop; 08-20-2007 at 10:06 AM.

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Another way to look at the pitch/diameter problem is this: a 12x12 prop will advance 12 inches through a 12 inch diameter column of water(everything being ideal). A 13x12 prop will advance 12 inches through a 13 inch column of water. The volume of water involved is 17 percent greater with the 13inch diameter versus the 12. But if the pitch only is changed you get only an 8 percent change in the volume of water column. Within some narrow variables one inch of diameter is approximately equal to 2 inches of pitch. Blade area ratios also play an important role.

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Hi Folks,

Thanks to all for getting back to me regarding the new prop for my boat. I think I found one. I won't know til I get to the boat which may be a coupla days. I understand how diameter and pitch affect the prop. My problem is making it fit into an aperture that was designed for a 9" prop and a gas engine.

Ed Everett

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