Controlable Pitch Prop low rpm efficiency question

Discussion in 'Props' started by Red Dwarf, Sep 18, 2012.

  1. Red Dwarf
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    Red Dwarf Senior Member

    Here is a typical power curve from Yanmar. Note-I added the green line, see below. Obviously the prop is chosen to match max power/rpm. That really hurts a slow cruise type setup where the engine will run maybe 1200rpm, the efficiency is in the toilet at 1200rpm.

    Clearly a CPP setup is the way to go to help low rpm efficiency. My question is how much does the CPP help low rpm operation? Does anyone have before and after power curves for a standard prop vs CPP?

    I sketched a green line on the Yanmar power plot. Would the change to a CPP be a trend similar to along the green line?
     

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  2. TeddyDiver
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    TeddyDiver Gollywobbler

    Nope.. it would be the whole area under the engine power curve..
     
  3. Red Dwarf
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    Red Dwarf Senior Member

    Wow that is a huge improvement! Good news.
     
  4. TeddyDiver
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    TeddyDiver Gollywobbler

    Yes, it's possible to choose any engine loading between ~5ish% to 100% (5ish for frictional an other losses in the drive train) at any rpm from idle to max. Best way to judge the proper loading is with pyrometer (exhaust gas temp)..
     
  5. jonr
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    jonr Senior Member

    If you know that you only need cruise power, you can alternatively just put on a higher pitch prop.
     
  6. FAST FRED
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    FAST FRED Senior Member

    If you know that you only need cruise power, you can alternatively just put on a higher pitch prop.


    Pitch can easily get to inefficient due to cavitation.

    Diameter puts power in the water so a big prop with minor pitch is more efficient than a smaller prop with monster pitch.

    A CPP is the answer for a cruiser , tho the cost is not low.
     
  7. jonr
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    jonr Senior Member

    > Pitch can easily get to inefficient due to cavitation.

    Except not so likely when you are turning at slower RPMs with less HP (which is the case here). But yes, add diameter if you haven't maxed it out already.

    I also agree that as fuel prices rise, a CPP makes sense for more situations. You can get efficient cruise and top speed.
     
  8. Joakim
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    Joakim Senior Member

    Is that really possible? I don't know much about CPP propellers, but I would guess they are dimensioned to work at nominal pitch at full power or cruising speed. Thus diameter and nominal pitch would be about the same as a fixed propeller would have.

    When you look at the propeller curve vs. output curve at 1200 rpm, you notice that output is about 6 times more. You can't increase the power 6 times from "nominal" by increasing just pitch. Probably you can do 2-3 times, but is even that possible with good efficiency.
     
  9. gonzo
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    gonzo Senior Member

    It will overload the engine at higher RPMs. The green curve is not based on real propeller data. If you only need 3HP, then install a smaller engine, otherwise, you will get chamber temperatures and pressures beyond the engine parameters and cause damage.
     
  10. Petros
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    Petros Senior Member

    you can not tell efficiency by that curve, that is only a power out put curve. the difference between the upper and lower curves is the excess available power.

    it is not necessarily more effienct to run at max rpm or full throttle. it just depends on the internal dyanmics of the engine. Typically the maximum efficient occurs at max torque out put. Some diesels have max torque at max rpm, but not always. Gasoline engines never have max torque rpm occur at max hp output, same is true with most diesels. The torque vs. rpm curves should have the same shape curve as the BMEP (brake mean effective pressure, calculates average combustion chamber pressure at each rpm condition, the higher the mean pressure the better the efficiency). You want to match your cruise rpm (prop pitch and/or reduction drive ratio) so the engine rpm will occur at the peak torque rpm of the engine to get best efficiency.

    You do not have enough information from that curve to know if that occurs or not.
     
  11. TeddyDiver
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    TeddyDiver Gollywobbler

    Partly true but..
    Done right you don't size the CPP with the same principles as fixed prop. Bigger the better (if not bigger diameter then increased blade area, 4 bladed..) so with max rpm you run "reduced" pitch.
     
  12. Joakim
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    Joakim Senior Member

    We don't know much of the setup in question, but typically a Yanmar running well over 3000 rpm would have a gear ratio between 2 and 3. At that rpm "bigger the better" is not true at all, quite often it is possible to go for the optimum diameter (at that rpm, power and speed). Certainly you can choose higher nominal pitch for CPP, but how much can you adjust the pitch with good propeller efficiency. A CPP can have optimum efficiency only at nominal pitch. If you double the pitch, what would be the propulsion efficiency compare to nominal pitch?
     
  13. Red Dwarf
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    Red Dwarf Senior Member

    I am looking for a continuous 24/7 25 hp. This is for a catamaran so 50Hp total. I also want to take advantage of the speed potential so want max power per engine around 200-250Hp. I have plenty of room for a 20 inch prop.

    This 10:1 ratio of max to cruise power creates my dilemma. If I run the engine just off idle I can get the cruise power I need but prop rpm's will be very low, 400rpm. At max throttle prop rpms will be around 1300. This is for a engine that turns 2600 rpm max.

    If I use a standard prop the efficiency at cruise is pure crap and all my cruising will just waste fuel. I was hoping the CPP would overcome this and give me good cruise fuel efficiency.

    One note, in regards to fuel usage, cruise is way more important than top end. It will be at cruise 99% of the time.

    Another concern is oil pressure at the low rpm but I think I can overcome that with an auxiliary pump.

    There are other solutions to this situation. Run 2 different engines. Run a hybrid setup with a generator for the small electric motor. Both of those add a ton of cost and complications. But I am considering both.
     
  14. Petros
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    Petros Senior Member

    find an engine more suitable for your needs, sell the one you have. keep it simple.
     

  15. WestVanHan
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    WestVanHan Not a Senior Member

    RD have you contacted any CPP makers and asked for documentation/curves/info etc?

    FWIW,notice Teddy's avatar,and where he's from...and consider that the Scandinavian fishing fleets have been exclusively CPP for 50+ years.
     
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