# Challenge to naval architects and marine engineers

Discussion in 'Props' started by sandhammaren05, Feb 8, 2022.

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### sandhammaren05Senior Member

-Prop shops hammer blades and cut blade area in the search for more speed or acceleration, or both. 'Or not' refers to measuring a blade straight out of the box from a factory before a prop guy has changed it.
-Camber that can be fit approximately by a formula (using splines, e.g.). A simple example would be parabolic camber, the standard example in wing theory. We used parabolic camber to design our CNC prop. In design, we 'cut' the blade shape from a raked helicoid and then add camber. This is using CAD. The CAD program then is used to print out a 3D model of the design.
-A pitch gauge, as it moves up and down along the propshaft axis, traces out a radial arc on the prop blade-the gauge moves up and down with
the distance from axis to blade held constant (by a metal bar). Prop shops measure 'pitch' (meaning progressive pitch) along such an arc. Only a helicoid
has constant pitch. Progressive pitch (variation in pitch along the arc) is due to camber, the deviation of the surface from a helicoid, and varies along the arc increasing from leading to trailing edge.

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### gonzoSenior Member

• I am not sure of your use of "systematic" to describe a curve of best fit.
• A pitch gauge measures a standard pitch . That is a straight line form leading to trailing edge. You are describing camber, which is a different thing.

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### sandhammaren05Senior Member

Gonzo, you need to visit a prop shop. No pitch gauge made today measures pitch, meaning pitch from LE to TE. They only measure pitch along a small chord.
Best fit is not important, let me clarify: the slope of the camber gives the progressive pitch, it's mainly the slope that we care about. We do need a formula for CAD is we want to design a new prop.

What pitch gauges measure is progressive pitch along a small chord. The point of this discussion is that progressive pitch and camber are exactly mathematically the same. As Bodo pointed out, it's merely a difference of terminology.

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### gonzoSenior Member

Terminology matters.
A curve of best fit is how you generate an equation for the curve that can be entered in CAD. Excel will do that for you.
The slope of the curve is the derivative of the equation.
Progressive pitch (which you have not defined) in not exactly mathematically or otherwise to camber. Camber can be a circular arc, and it can be regressive.
You need to be careful about definition. Otherwise, only you know what you mean.

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### sandhammaren05Senior Member

To understand progressive pitch you don't need the curve, only the slope along the arc.
We don't use curves from existing props so best fit is irrelevant. We use certain mathematical formulae in CAD.
Best fit would be of interest if one would want to quantify what manufacturers are doing, and what a prop shop has done.
We design our own props from scratch.
Progressive pitch and camber are identical. That's for you to work out. I've already done it. You have to figure out how to relate camber, which is seat along a radial arc, to progressive pitch, which is measured along a radial arc.

I'm not giving away the answer in advance.

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### gonzoSenior Member

I don't think you understand what a curve of best fit is. It is a mathematical representation of a curve that fits a set of points. For something like a propeller blade, a set of piece functions are most likely to be more accurate. If you have the slope at certain locations, then integrate and get the equation of the curve.
Camber may have a varying slope, is that what you call "progressive pitch"? The pitch on a propeller changes proportionally to its distance from the center, which is unrelated to the camber. There may also be changes in camber. Unless you define your terms, nobody except you knows what you are talking about.

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### HeimfriedSenior Member

If you prefer to look at it this way, just transform the coordinates in my sketch by turning the axes CCW by the angle alpha as shown here:

This way the camber will be identical with the z coordinate and the tangens of the local slope ("pitch") starts with a negative value (intervals from right to left),
but is nevertheless increasing, "progressive pitch".
Looking at the second interval the slope is (cb1 - cb2) / (u'1 - u'2) = tan(alpha2) ; or (equation multiplied by r): (cb1 - cb2) / (phi1 - phi2) = r * tan(alpha2) ;
(cb1 - cb2) = (phi1 - phi2) * r * tan(alpha2)

In general
(cb(i) - cb(i+1)) = (phi(i) - phi(i+1)) * r + tan(alpha(i+1))

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### gonzoSenior Member

Are you breaking the camber into short circular sections? The camber is a complex curve. That is why I think using a curve of best fit is the best way to generate a function that can be entered into CADD. He is still confusing camber with pitch though. It makes it difficult to follow his reasoning. The pitch must change as a function of radius length, since the radial velocity changes. However, the slope of the camber is not the same as the pitch of the blade.

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### sandhammaren05Senior Member

You obviously have no knowledge of mathematics, hydrodynamics, or propellers. In short, you have no idea what you think you're talking about. End of conversation.

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