Re: Evinrude e-tec or Yamaha 4 Stroke
When a prop varies in "pitch" from hub to blade tip, how do you measure "theoretical pitch" and is this different from "actual pitch"?
Have you learned nothing here Backfire?
<br /><br />The difference between "theoretical pitch" and "actual pitch" is
slip. Each application or condition the prop is under will result in different slip, or "actual pitch". Theoretical pitch (what is stamped on the prop) will equal actual pitch only under perfect zero slip conditions (impossible). 6-10% slip is common. <br /><br />Again, pitch is merely the distance the prop moves forward in one revolution with zero slip.<br /><br />You can have constant, progressive, regressive, variable and adjustable pitches. It doesn't matter. A 15 pitch prop (whatever way that prop is designed) can only move through the water a maximum of 15" per revolution (under
perfect zero slip conditions). <br /><br />You're confusing this "pitch" (inches of forward movement) with "pitch angle" (angle of the pressure face along the pitch line with respect to the plane of rotation measured in degrees).<br /><br />The type of angle of the blade, from hub root to tip, doesn't matter. For example a 2-bladed 15 pitch prop will have a steeper "pitch angle" because it only has two blades to grab water for 15" of movement. A 4-bladed prop will have less "pitch angle" because it has 4 blades to grab water for 15" of movement. Different blade angles, but both still have a 15 pitch.<br /><br />FWIW, the relationship between "pitch" (inches) and "pitch angle" (degrees) is given by: <br /><br />Tan A = pitch / 2P r where A = pitch angle, r = radius, and P = 3.14.<br /><br />
If the pitch progression, different at each point of measurement is averaged,is this "assigned pitch"?
Not sure what you mean by "assigned pitch". Are you talking about pitch angle again?
"Pitch line" is used as a reference for pitch angle, and it is measured as an average along the blade.<br /><br />Regardless, the pitch stamped on the propeller is theoretical pitch, or the distance the prop moves forward in one revolution under perfect, zero slip conditions.<br /> <br />
Knowing that blade contact area is subject to a wide variety of differing load forces-soft water, hard water, etc...,can a certain formula be used to weight the most effective area?
Commonly accepted formulas (that were derived long ago) have been used by the industry for years. They are standardized. I'll spare their explanation.<br /><br />
Since todays manufacturers employ mostly new "modern" prop technology, is there a standard by which all props are rated/measured?
Today's prop technology is not new. And yes, there are standardized methods and calculations for measuring propellers. <br /><br />
do the mfgs stamp a figure on their props that is their assessment devoid of any benchmark or standard?
I hope not! <br /><br />
The fact is that there is a great difference in performance between props of the same pitch. I call 1-4+ mph a great difference concidering the hp needed to produce results like this. If two devices are the "same" but produce different results, what accounting procedure do we subscribe to?
You still don't get it. The difference in performance of two different 15 pitch props could be a result of anything. Rake, cup, skew, camber, diameter, number of blades, thickness, material, etc. But all are an attempt to achieve zero slip and a perfect 15 inches per revolution. Different combinations work for different applications.<br /><br />Again (and I feel like I'm repeating myself over and over!), you can not make a 15 pitch prop do any better than 15" of forward movement for each revolution. And it appears by looking at the data posted, that two E-Tech engines are doing this. I say they are exaggerating.
