Instructions for designing a new Propeller

Discussion in 'Propulsion' started by Gokul, Jul 4, 2010.

  1. Gokul
    Joined: Jul 2010
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    Gokul Student

    Hi Everyone,

    I'm new to this forum as well the topic ship design. I would like to introduce myself as Gokul, an university student. Currently I started a project on designing a propeller for a yacht (YD 40)

    As I'm totally new to propeller design, it would of great help if somebody give me the instruction (steps to be followed) while designing a propeller..

    Initially I started with choosing the Engine and my engine was YANMAR that is 26kW @ 3000 RPM... and my gear ratio is 2.5..

    And I have all the specification of my boat YD 40..

    But I'm little bit confused with, how to start and where to start???

    Appreciate your time and help and for the valuable comments...
     
  2. jehardiman
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    jehardiman Senior Member

    Your cart is way before your horse.....

    First, decide how fast you want to go and the maximum propeller diameter.

    They you work up the most efficient wheel.

    Engines and gears are the last thing to check ...

    Do you want to design a wheel totaly from scratch? Or use an existing series?

    FWIW, I saw a student design team develop a propeller for thier senior project once. The wheels diameter ended up being 3m to work with the speed and engines they wanted...problem was, they drew a hull that only had 1m between the shaft CL and the hull....a professor caught that in the middle of thier presentation.
     
  3. Gokul
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    Gokul Student

    thanks jehardiman for your suggestions.....

    Okay... let me put this in another way....

    My main concern was about the material for the propeller...... Actually i'm working on the project, how does selection of material affects the propeller efficiency and how its related to designing stage of the propeller...

    In Precise, an optimum propeller as designed to deliver a certain speed.. so Whether this propeller deliver the same speed in any material( like one made in Iron and another made is Reinforced Polymer). If so what factor of the material is connected to design stage of the propeller..

    suggestions are very welcome!!!!!!!!!!!!!!!!!!!!!
     
  4. masalai
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    masalai masalai

    Buy one...
     
  5. Gokul
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    Gokul Student

    thanks Masalai !!!!
    but that is one such option..... I need some technical suggestions....
     
  6. masalai
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    masalai masalai

    I could not help it, Honest... Before the pontification got too anal and far above my level of interest, it seemed a little "off-the-wall-humour" was warranted....

    Even with modern day applications, most is based on "suck-it-and-see" also politely known as 'trial-and-error' methodologies... - as designing a propeller for a one off, is a HUGE task involving many fields related to the engineering/hydrodynamics and so on - and sometimes extending to the borders of witchcraft (economics) where they consistently try to prove 3 + 4 = 0 or less...

    Rick Willoughby is our resident expert on low velocity propeller efficiency - holding several records for his efforts in man or low energy powered propelled vessels...

    Yours is almost in the standard of, 'try this then try that', using a couple of rule of thumb formula guides... - where efficiency is not really critical... As a university level assignment, this could stretch out to 20 or so pages of carefully worded analysis derived from reading some 20 books where the content was so dry they would each make a good Martini if the books could be liquidised... Start with E=MCsquared and conclude with "thrust = .... "
     
  7. jehardiman
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    jehardiman Senior Member

    While it will not be a bad as Masalai projects, this is a rather narrow and technical field. Usually, for a vessel your size, you contract a propeller manufacturer and they suggest/design the wheel....

    That said, propeller material selection greatly influences propeller efficiency, and suitability, but not speed that much. Propeller materials are selected as much for thier stiffness and damage tolerance as for thier strength. Some propellers are expected to ingest fairly large and hard items (like whole aircraft...) and others can turn into pom-poms if it hits a fish. All that and size/thrust/rpm/balance is going to determine the root section which drives the hydro end of the design.

    To start out, I would suggest a good naval architecture book to familiarize yourself with the terms and concepts (such as PNA or Basic Ship Theory), then READ AND UNDERSTAND Lerbs' 1952 SNAME Paper "Moderately Loaded Propellers with a Finite Number of Blades and an Arbitrary Distribution of Circulation" (SNAME Transactions Vol 60, 1952). Then come back and we can delve in some real esoterica.
     
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  8. Paul Kotzebue

    Paul Kotzebue Previous Member

    If you start with E = MC^2 you'll never get there. Start with F = MA .....
     
  9. masalai
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    masalai masalai

    Now you both have surpassed my interest/knowledge by about a friendly mile or so... So I will leave you in the tender mercies of the "experts-descended-from-on-high" ... :D :D :eek:
     
  10. Gokul
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    Gokul Student

    thanks jehardiman!!!!!!!!!

    I used a program developed by MIT to design a propeller in mat-lab. Since I know the specification of my yacht and the engine, its relatively easy... I justed plugged the value in the Mat-lab and got a optimum propeller....

    while analyzing the Mat-lab code I couldn't find a relation between the material properties related to design stage of a propeller!!!!

    this made me to think, how can i include the material characterization while designing propeller......

    Fyi... the code developed by MIT is Opwnprop and its quite good. U can also try that...
     
  11. jehardiman
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    jehardiman Senior Member

    Based upon how you started this thread, I doubt that Openprop gave you a optimum propeller...and you are foolish to assume it did. I'm not trying to be harsh, but the most important thing in engineering is to know what you don't know...not hide your ignorance behind someone elses work. After you finished Lerbs' paper I was going to suggest you move on to Morgan, von Manen, and Kerwin's work. You don't know it, but a nasty little war was waged over propeller design in the '70's and 80's that still divides propeller code.

    So FWIW, I wouldn't use the MIT program blindly, I know it has issues, but then most propeller programs do based upon their development assumptions. You choose your poison when you use a program without understanding it because most code is based upon patently false assumptions that are twisted and warped my mathematics to approximate a physical process when viewed from the correct position. Openprop is just one example amoung many out there on the web. And unless you are willing to wade through all the theory and understand the limits of it's design you cannot say that the propeller it gave you is "optimum"...especially if you wish to change materials.
     

  12. FAST FRED
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    FAST FRED Senior Member

    To me propellers are not the problem , the pain in the butt to get a prop fitted or removed is the problem.

    The only solution I have read about was Rob White who on an Atkin ,Rescue Minor had a low power (15hp) inboard diesel, and used a shaft with a duplicate of an outboard splined setup.

    This allowed prop changes (hatch over prop) with the simple removal of a cotter pin , as on an outboard.

    An easy to change prop would be a REAL ADVANCE .

    For many boats a CPP is far to expensive , but most cruisers do have a spare prop , one for speed , one for long range efficiency and a rapid change over would be grand.

    FF
     
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