Doubt: 0.7R Pitch and Pitch/Diameter ratio

Discussion in 'Props' started by Samuel Neves Jocas, Mar 12, 2020.

  1. Samuel Neves Jocas
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    Samuel Neves Jocas Junior Member

    Dear members,

    I have a huge doubt regarding the pitch determination in my project. I read in some materials that the pitch, by default, is determined at 70% of the thruster radius (0.7R). I also read that this pitch is also called Face Pitch or Nominal Pitch. I'm trying to design/generate the propeller for my project, but I have some doubts regarding this:


    1. Is the P/D ratio always given at 0.7R?
    2. If not, what can I do to find out how many percent of the radius is P/D?
    3. Is there a relationship between P/D and the Face/Nominal Pitch? If so, which one?

    I saw in some places that the pitch can be determined by a formula similar to:

    P = 2πRtan (Φ)​

    However, as I do not have the propellant created, it is impossible for me to predict the value of tan (Φ), and this method becomes useless to me. So, can anyone tell me how I can determine the pitch at 70% of the propeller radius? (If possible, from the P/D ratio).

    Sincerely,
    Samuel Neves Jocas.
     
  2. baeckmo
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    baeckmo Hydrodynamics

    You have to understand the difference between pitch and pitch ratio. In general, the pitch is defined as the forward travel of the helical surface when rotating one turn. Thus, the pitch has nominally the same value at all radii. The nominal pitch ratio is the ratio between pitch and tip diameter. Consequently, the pitch ratio will of course vary from hub to tip.

    In a specific propeller design, however, the pitch may vary slightly over the radius, for instance when the propeller is adapted to a special inflow condition. The pitch at the 0.7 radius is normally representing the actual "working pitch" of the specific propeller, so this is where the nominal pitch is measured. But it is still referring to the diameter when we talk about nominal pitch ratio.

    So, to check your understanding: if you have a propeller which has a pitch ratio of 0.8, the pitch is 80% of the diameter, what is the pitch ratio at 70% of the diameter?
     
  3. gonzo
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    gonzo Senior Member

    Is the project to design a propeller or to specify a propeller for a boat?
     
  4. Samuel Neves Jocas
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    Samuel Neves Jocas Junior Member

    I already knew this difference, I expressed myself badly to elaborate my question, sorry. However, I realized that the pitch at 0.7R is just a representation. But another question arose: why is the "working pitch" at 70% of the propeller diameter?

    Answering, it's P / D = 0.7. Thanks for the help.
    It is to select the ideal propellant for a vessel of a project that I participate (a competition team). We decided to take a leap this year in the propulsive sector of the project, and we are tracing lines of research in this direction. Just to be clear, this is a model of port tug. Thanks for attention.
     
  5. baeckmo
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    baeckmo Hydrodynamics

    Sorry, your answer is wrong, indicating that it is still unclear to you. The local pitch ratio ( at a given radius) is a measure of the blade angle at that location. With a pitch ratio of 0.8 in the tip region, the blade chord angle there is arctg(0.8/pi), ie ~14.3 degrees. To get the blade chord angle for this propeller at the 0.7 radius then: the reference diameter is 0.7, giving a local pitch ratio of 0.8/0.7 (~1.143) and a blade chord angle of arctg(0.8/0.7/pi), or ~20 degrees.

    It represents the average working condition of the propeller disc. Half the prop area is inside this radius and half is outside; the squared average of the peripheral speed (representing the major energy transfer) is found there as well. The value 0.7 is actually a rounding-off of the square root of 1/2.

    I recommend that you read the chapters on resistance and propulsion in "Principles of Naval Architecture", published by the Society of Naval Architects and Marine Engineers". In my corner of the world, it has been the standard litterature for generations of professionals, and has been edited in several steps. Better start with "the real stuff", than some of the light-chewed texts if you have a serious approach.
     
  6. gonzo
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    gonzo Senior Member

    He said they are specifying a propeller, not designing it. I think that he needs to find out the basic parameters from the boat: target speed, hull resistance, power requirements, maximum diameter, to start getting a range of values for pitch.
     
  7. DMacPherson
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    DMacPherson Senior Member

    While I am certain that the other posts were written with the best of intentions, they could be a bit confusing. Let me see if I can provide a different explanation for you. First you must appreciate that there is no one single pitch figure for any propeller - but there are meaningful figures that can be used for engineering purposes.

    First is to understand that pitch is a figure representing the geometry of a helix. Just like a machine screw, pitch is the axial distance traveled in one rotation. Unlike machines screws, however, there are multiple definitions of where the helix can be measured on the propeller blade.

    Let's start with any section, say at the 70% of the radius that you asked about. In the scientific side of propeller design, we position the helix between points on the nose and tail of the foil section (that is wrapped with the other foil sections to make up the propeller blade). However, the traditional method is to position the helix on the propeller's pressure face (known as face pitch). Same foil and 3D geometry, but different pitch values because the helix reference is different.

    So now let's consider how pitch varies at different radial positions. While many traditional propellers have a constant pitch distribution (typically using the face pitch definition), contemporary propellers typically have variation in pitch from root to tip. As was mentioned in an earlier post, this is to align the helix with the incoming velocities at each radius. Pitch times rotation (RPM) equals velocity. Consider the shaft and hub upstream of a propeller. They will tend to slow the water (lower velocity into the inner radii) and the design pitch near the hub is reduced. For example, check out the reference 4-blade BSeries pitch distribution, and you will see that it is reduced to 8/10ths of the pitch at the outer radii as a means to account for lower water velocities near the hub. The same then happens for all of the other radii for any variations in their inflow velocities. (We call this "wake-adapted" propeller design.) Hopefully, this can illustrate why contemporary propellers have a "variable pitch" (not to be confused with "controllable pitch").

    So what is a meaning full reference pitch? It all depends on the purpose for the value. As a catalog bookkeeping figure, it really doesn't make much difference. Use the face pitch at the 70% radius if you want. However, if you want a meaningful value to use for thrust and torque calculations, then a suitable "mean effective pitch" value is quite important. In this case, using the pitch at the 70% radius could be inappropriate. For a consistent figure, you have to use a mean effective propeller pitch based on what we call a "chord-radius" integration. Put simply, we weight the contribution of a section's pitch based on its radius (where the outer radii are traveling faster due to the longer arm length) and its chord length (where small chords, such as at the tip, do not contribute as much as wider sections).

    Finally, pitch can be represented in a few different ways. First is the pitch itself; that distance the helix advances in one rotation. Second, we typically turn this into a non-dimensional figure by dividing it by the propeller's diameter. This allows us to compare geometries for propellers of different sizes. Third, we can consider the "pitch angle". As the radius changes, the angle of the helix needs to change to match a particular pitch. This radial variation in pitch angle is what gives a propeller blade its "twist", with higher pitch angles into the root and lower angles at the tip - all for the same pitch.

    I hope this helps a bit.
     
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  8. gonzo
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    gonzo Senior Member

    The shape of the blade also affects the pitch. For example, cupping modifies the effective pitch even though only the trailing edge is changed. However, all this is a moot point since you are not designing a propeller, but specifying it. You need to use the common methods and formulas and look up the appropriate available propeller in a catalog. Further, the propeller specification is the result of the parameters I mentioned previously: target speed, hull resistance, power requirements, maximum diameter possible. The pitch will be whatever it needs to be.
     
  9. jehardiman
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    jehardiman Senior Member

    Ok, there are many ways to go about this, so first question is...
    Are you trying to design a new propeller form from the begining?
    or
    Are you trying to select pitch, diameter, rpm, etc from a known propeller series? Yes, the propeller curves will work in model (i.e. greater than ~100mm dia) scale.
    If you are trying for something less that 100-150mm dia, there are other considerations.
     
  10. gonzo
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    gonzo Senior Member

    He said it is to specify a propeller.
     
  11. Samuel Neves Jocas
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    Samuel Neves Jocas Junior Member

    I appreciate all the answers, and I will try to answer each one.

    Ah, I really got it wrong, sorry. There is still a small doubt. So, for this case, would I use the formula I presented initially? It would be something like:
    P = 2πRtan(~20)​
    Is this inclination the same as the angle of attack of the thruster section, but at 0.7R?

    I appreciate the explanation. I have the book and I will definitely check it, thanks for the guidance!

    We are working this way. We use the curve diagram of the Kaplan 4-70 naval propeller model. From it, we set up a spreadsheet to calculate the torque and thrust values at which the engine speed curve intersects that of the propellant for different P/D ratios. I am attaching an image "DiagramaMotor.png", so that you can have an idea.
    On the vertical axis there is the operating speed (RPM). The horizontal axis has the torque (Nm). In addition, "Um Prop." means "One Propeller" and "Dois Prop." means "Two Propellers".
     

    Attached Files:

    Last edited: Mar 16, 2020
  12. Samuel Neves Jocas
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    Samuel Neves Jocas Junior Member

    Second option, sir. And yes, our model uses a maximum diameter of 79.0 mm, which is the parameter determined by the competition. In the case of a port tug, we decided to use this value, following the reasoning that the largest possible diameter will be ideal for our case. May I know what the considerations are then?

    Thank you very much for the resolution, sir. This was very aggregating and interesting, clarifying some doubts besides those I had. I appreciate the attention.
     
  13. gonzo
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    gonzo Senior Member

    If it is a port tug, then bollard pull is one of the most important parameters. The propeller will have to be specified with a much higher slip than a high speed boat.
     
  14. Samuel Neves Jocas
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    Samuel Neves Jocas Junior Member

    You are correct. However, some tests of the competition, interestingly, also demand that there is a kind of bollard pull "in reverse". In this sense, we decided to use that our propellers will have rake of 0 degrees and we will use the Kort No. 37 duct, due to its more symmetrical geometry, to cover the bollard pull tests "in reverse".
     

  15. jehardiman
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    jehardiman Senior Member

    First, the root shapes and pitch reduction in "normal full sized" propellers is based upon material strength in relationship to blade root bending and physical size constraints related to the shaft/hub interface. A real world propeller this small is not subject to those constraints, but other ones. You can basically ignore the "full sized" shape of a Ka propeller inside the 0.5r radius, there are far better shapes and pitch distribution to use. With a prop this small at the RPM you wish to use, cavitation is going to be a problem, especially due to the very small submergence depth issue. 2500 rpm is 261.8 rad/sec, so the tip of a 79mm wheel is traveling 10.3 m/sec. Additionally, ducted propellers require very small tip clearances to achieve maximum thrust, however these small gaps between the prop tip and the duct will almost guarantee sudden onset of full cavitation at a lower rpm than open water.
    So, is the requirement of this competition to have a "100% true to scale" propulsor (knowing that small scale propeller testing is accomplished in pressurized tanks). or to make the highest thrust propulsor with a diameter of 79mm?
    Additionally, must the propulsor shaft reverse to be "in reverse", or can you have an azimuthing propulsor?
    As I said before, there are many ways to approach this problem depending on the real constraints.
     
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