Propeller calculations - HELP!

Discussion in 'Props' started by sailor0000, Dec 27, 2012.

  1. sailor0000
    Joined: Dec 2012
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    sailor0000 Junior Member

    I'm trying to determine the right propeller for the wooden power cruiser I'm restoring and modifying. I used the Crouch method and the Bp - δ method in Dave Gerr's Propeller Handbook, and the Vicprop online calculator. I'm getting vastly different results. I'm lost! Can anybody help, does anybody know why this happens? Which method should I trust, which prop should I use?

    Here's the particulars of the boat and the engine: Displacement 10,000lb, LWL 32', BLW 8.5', Hd (molded hull depth) 1.2', Cb 0.48, DL 136, 25 shaft hp @ 2500 engine rpm, 2.94:1 reduction, 850 shaft rpm.

    And here's the results for a 3-bladed prop, type B, dar 50%:
    Crouch method: diameter 21.04", pitch 22.06"
    Bp - δ method: diameter 22.58", pitch 13.77"
    Vicprop calculator: diameter 21.2", pitch 17.9"

    I can live with the diameter results, I'm aware this isn't exact science, but what do I do about the pitch??? I checked and double-checked everything, couldn't find an error. Can anybody tell me what I'm doing wrong, or point me in the right direction, please? Thanks...
     
  2. MikeJohns
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    MikeJohns Senior Member

    I'd say the 21 by 22 will be just a bit over what your engine can produce at wide open throttle. But close, maybe an inch less pitch. But what speed were you using?
    Its good to run through with the extant data from before the refit. How fast did it go at what RPM and power WOT and what was the prop?
    If you have that data then you can validate the model and then go on with some confidence to look at the new setup.
     
  3. sailor0000
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    sailor0000 Junior Member

    Dave Gerr's book and Vicprop agree on the top speed: 8.2 kt.

    Unfortunately, I don't have any data. My father-in-law designed and built the boat himself, but he passed away in 1998, actually before we married. Also, the engine has been changed from a big gas V8 to the small diesel I'd like to keep and make work. So really, I don't know anything about previous performance.
     
  4. BertKu
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    BertKu Senior Member

    I am getting different information from the propcalc program from Castle Marine.
    Pitch ratio 0.71 slip ratio 0.38 , 3 bladed 21.04 Inch Pitch 14.99 but only 6.5 knots.
    Castle marine makes props. They allow you to download their programme, but you cannot ask questions about the program. Except maybe if you order a prop from them.
    If you cannot get it to work (If you have win7 per example) , give me a private mail, I will try to transfer the program to you.
    Bert
     
  5. BertKu
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    BertKu Senior Member

  6. sailor0000
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    sailor0000 Junior Member

    Thanks very much, Bert! I'll try and download the program tomorrow.

    Gerold
     
  7. Submarine Tom

    Submarine Tom Previous Member

    Sailorzeroes,

    Gerold,

    Reread Gerr's introduction and opening Chapters.

    Many formulas are too general and inaccurate.

    Some prop manufactures can be quite helpful in choosing and designing props.

    You are in a tough place but persevere.
     
  8. Landlubber
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    Landlubber Senior Member

    Before you get too involved in calcs, go measure the space available for the prop, allow 10% tip clearence, then do the pitch calc to suit the diameter.
    You want the biggest prop you can get in the hole, reduction gearbox it to suit the diameter.
     
  9. sailor0000
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    sailor0000 Junior Member

    Thanks. Actually, I have room for a huge prop, 25" or so. The engine is a Solé diesel, Mitsubishi based. I'm willing to change the transmission, the highest ratio I found is a PRM 120 with 2.94:1. So diameter will be 21" or 22", that seems pretty clear. What drives me crazy is this: Gerr says to use the Bp - δ method if you want really precise results. And it gives me a pitch which clearly seems way to low! An apparent slip of only 15%, that can't be right, can it?
     
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  10. BertKu
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    BertKu Senior Member

    My apology Gerold. I thought you were finished with your thread having a second prop calculation received from me. I will remove my question and start a new thread.
    Soooooorrrryyyy.
    Bert.
     
  11. sailor0000
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    sailor0000 Junior Member

    Not a problem! I don't see it as "MY" thread. I'm new here, it's your forum, it's for everybody, right? I'm grateful for all the help and comments I'm getting, I'm not going to be particular about anything. So don't worry...

    Gerold
     
  12. BertKu
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    BertKu Senior Member

    Thanks Gerold. The reason why I added a request at the end of your thread (I thought it was the end) MikeJohns has responded to your thread and then he is automatically linked and get any reply etc into his email box at home. Should I have given MikeJohns a private e-mail, sometimes people only open it a few days or sometimes weeks thereafter. Bert.
     
  13. MikeJohns
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    MikeJohns Senior Member

    Here's a good effort you might want to have a look at:

    http://www.electricboats.co.uk/surfprop/index.html

    Playing with a spreadsheet like this should give you a feel for factors such as slip which as you surmised is not sensible at some of the values you have for pitch.
     
  14. sailor0000
    Joined: Dec 2012
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    sailor0000 Junior Member

    Thanks. This looks like a good resource. I'll check it out.

    Gerold
     

  15. MikeJohns
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    MikeJohns Senior Member

    Bert
    A propeller blade has two sides, termed a Back and a Face the face is like the underside of an aircraft wing and the Back is the curved portion. A prop blade bent from a flat sheet is no different in concept from a single surface aircraft wing. So the face is cancave and the back is convex.

    In practice however you are going to get a lot of cavitation and edge erosion because the shape is far from ideal. You would get a better result with a thicker section rectangular blade shape with the edges tapered and bent slightly with the leading edge down and the trailing edge up. The thicker section would also give a better root stiffness and strength. But it looks like you are having fun.

    [Added]
    Just thought I'd better add that the prop "Back" faces forward .
     
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