How to start design of propeller?

Discussion in 'Props' started by Solarboat, Apr 22, 2020.

  1. Solarboat
    Joined: Apr 2020
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    Location: Netherlands

    Solarboat Junior Member

    Hey,
    I am looking for a propeller for our solar boat.
    But I don't know where to start calculating.

    I hope one of you can help me out.
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome to the Forum Solar.

    You will need to provide a lot more information before anybody on here can try to help you.
    Has the boat been built, or is it still in the design stage?
    Is it a monohull or catamaran?
    Can you post any photos of it, or copies of the drawings for the boat?
    Have you calculated the amount of power that you will need for your desired speed?
    What is the maximum diameter that the propeller can be?

    Edit to add this note -
    Have you got a copy of Dave Gerr's Propeller Handbook?
    https://www.amazon.com/Propeller-Handbook-Installing-Understanding-Propellers/dp/0071381767
    If not, it would be well worthwhile purchasing a copy - this appears to be the best work on the subject.
     
    Last edited: Apr 22, 2020
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  3. Solarboat
    Joined: Apr 2020
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    Solarboat Junior Member

    Thank you for your response.

    I have much more information.
    The boat has already been built. It's a Monohull.
    I have a resistance diagram.
    the propeller + -200mm
    I added a photo.

    p.s. I have a copy of the book.
     

    Attached Files:

  4. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Solarboat,

    Welcome to the forum.
    Post some links to this race please.
    These boats are highly developed, is there no one there on your end for props?
    Have you towed your boat at various speeds to determine the drag curve?
     
  5. Solarboat
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    Solarboat Junior Member

  6. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    There are only 3 inputs that you need to design a propeller.

    1. Speed of water - in way of the prop
    2. The power delivered to the prop
    3. The shaft rpm

    The key one is #1, the speed of water into the prop... or speed of advance as it is called.
    This is affected by the shape of the hull and the shaft angle. Since these influence the flow of water into the prop. So, do you have a GA of the boat?
     
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  7. Solarboat
    Joined: Apr 2020
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    Solarboat Junior Member

    I think you mean the maximum speed by #1. In that case it is 7.2m/s
    The power delivered to the prop is 6.2hp
    The shaft rpm is 2200rpm.
    Shaft angle is horizontal.

    And what do you mean by GA of the boat?
     
  8. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    No, he means speed of the water. The shape of the hull influences the speed of the water at the propeller. It is different than the speed of the vessel over the bottom.
     
  9. missinginaction
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    missinginaction Senior Member

    This is a fairly complex engineering problem. Not insurmountable but not something you are likely to have solved for you. If you're an amateur I'd strongly recommend a book by a respected naval architect here in the US. The book is called "The Propeller Handbook" and is written by Dave Gerr. It's been out of print for awhile but you should be able to find a copy.

    There are many interrelated variables involved in selecting an optimum propeller. If you have a good mathematics background and are somewhat familiar with fractional exponents you'll be able to work through the mathematics and get yourself a good approximation of what you need. Gerr provides a good explanation of the formulas but you do need to input your own data and there is quite a bit of data involved. I needed to study this book for quite a few hours before I was comfortable with the math.

    A couple of months ago I started a thread because I was unhappy with the performance of my boat with the original existing propeller and wanted to make an improvement. I'll post a link to that thread. It might give you an idea of what's involved.

    The mystery of a proper prop and terrible performance. https://www.boatdesign.net/threads/the-mystery-of-a-proper-prop-and-terrible-performance.63352/

    https://www.amazon.com/Propeller-Ha...=The propeller handbook&qid=1587654194&sr=8-1

    Good luck on your project!
    MIA
     
  10. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    I guess it depends if you just doing it for fun to paddle around with it, or if you want to race and really optimize. For example besides all that you might also want to optimize the motor and gears and the propeller for maximum efficiency. Someone might be able to suggest a good propeller based on the numbers you've given but it's not going to be optimal.

    Nice boat and welcome to the forum!
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    GA = General Arrangement.

    It is a plan and profile view of the design.
    This way we can see what if anything may affect the flow of water into the prop and roughly what correction factors to apply from wake reduction factors.
     
  12. Solarboat
    Joined: Apr 2020
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    Location: Netherlands

    Solarboat Junior Member

    I am now reading the book that you recommend to me.
    but I still have a question, how do I find out what the velocity of water is.
     
  13. gonzo
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    gonzo Senior Member

    That is a very complex hydrodynamics problem to get an exact value. You won't find a short answer to it. It depends on the shape of the hull, speed, waves, trim, appendages, etc. There are approximations using wake factor, which are in pages 68-70 of the Propeller Handbook. However, these are for hulls of conservative, normal shape.
     
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  14. Solarboat
    Joined: Apr 2020
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    Location: Netherlands

    Solarboat Junior Member

    And then another question.
    In many formulas they ask for the diameter (D).
    But is this the maximum diameter that the propeller can be?
     

  15. missinginaction
    Joined: Aug 2007
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    missinginaction Senior Member

    Solarboat,

    You are reading the Propeller Handbook by Dave Gerr, correct? It's going to be impossible for someone here to walk you through all the engineering involved. That book was very helpful to me but I needed to study it for quite some time. I also needed to use my sons scientific calculator to solve the formulas in the book that use fractional exponents.

    You might be able to hire a naval architect to run the calculations for you but you will still need to determine things like displacement, waterline length, beam waterline and the power that you have available at the prop, prop RPM and on and on it goes.

    I found it helpful to read that book before bed and "sleep on it".

    Briefly, the diameter of the prop that you would derive from the formulas would be the optimum for your application based on the data. You can then adjust the diameter and pitch to fit the space you have available. You also need to take blade loading into account. As for the velocity of water, are you talking about the speed of advance? I don't have the book in front of me but from memory you have to understand that as your boat moves through the water it "drags" a relatively small amount of water along with it. Say your boat is moving through the water at exactly 10 knots. Because of the bit of water being pulled along by the boats hull, the speed that the propeller actually "sees" is slightly less than 10 knots. That phenomenom factors into other formulas that determine optimum propeller size.

    Keep reading.......over and over until it starts to make sense....
     
    Last edited: May 13, 2020
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