Modelling a propeller

Discussion in 'Props' started by Roflhat, Sep 24, 2015.

  1. Roflhat
    Joined: Sep 2015
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    Roflhat Thundercat - 70.10mph

    Regarding material, does anyone have any experience with Aluminium Bronze/NiBrAl? Seems a lot of companies prefer to offer this for casting as opposed to stainless. The strength and hardness figures seem comparable to those of stainless steel, and the propeller would be 200 grams lighter in NiBrAl - 2kg instead of 2.2kg.
    Also on the Rolla site as well as others, it seems propellers for larger applications use NiBrAl.
     
  2. Joakim
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    Joakim Senior Member

    In the 70's and early 80's it was quite common to have bronze cleavers. I still have one made by Radice. Then stainless steel replaced them totally. I don't know what alloys and possible treatments they use, but stainless steel can have very high yield stress and it does have about the double modulus of elasticity. Thus a stronger and stiffer (or thinner) propeller can be made out of stainless steel.
     
  3. Roflhat
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    Roflhat Thundercat - 70.10mph

    That's what I had decided looking at HTB1, which is brass. But NiBrAl is aluminium bronze, for example AB2, which appears to have superior qualities to stainless in every way?
    Made a quick table
    [​IMG]
     
  4. Joakim
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    Joakim Senior Member

    You can make stainless steel (not 316) with up to 1800 MPa Yield Strenght. E.g. bicycle spokes and sail boat stays are well over 1000 MPa. Your table doesn't show modulus of elasticity, which is about 200 GPa for stainless steel and bornze is only about 100 GPa. So the stiffness is halved.

    Basic 316 is quite "soft", but you can get over 1000 MPa cold working it.
     
  5. Roflhat
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    Roflhat Thundercat - 70.10mph

    Yes you are right, the range of yield and tensile strengths for stainless was very large. But as far as I know props are made from 316, which is around the value in the table?
    You are quite correct about the Modulus of Elasticity, stainless is often over 190 from what I've read, whereas AB2 is closer to 120.
    Even with this variance, do you think the difference in performance between the two is substantial?
    Interesting that companies such as Rolla make propellers in both NiBrAl and stainless, depending on the application.
    http://rolla-propellers.ch/Rolla/surface_piercing_propellers_ogg_mp4.html
     
  6. Joakim
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    Joakim Senior Member

    I don't know what alloys stainless steel propellers are made of and how are they cold worked, if any. As I said even 316 can be cold worked to about 1000 MPa YS. I'm not an expert on cleavers, but I think blade thickness is an important factor and the material used determines the minimun. At least in the 80's it was not that uncommon to loose a blade from a cleaver. Also from what I have seen stainless steel cleavers have thinner blades than bronze ones.
     

  7. Roflhat
    Joined: Sep 2015
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    Roflhat Thundercat - 70.10mph

    What you say definitely seems to be the general concensus from what I've read online. The design is now fixed, but as it has relatively thick blades anyway either material should work fine. Seems Nibral is easier to cast, I'm struggling to find companies willing to cast a model in stainless.
     
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