Variable DIAMETER ? Prop

Discussion in 'Props' started by FAST FRED, Mar 2, 2010.

  1. powerabout
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    powerabout Senior Member

    Fred
    Yes a diesel and a boats prop curve are opposite. Diesels have the right torque curve ( rise) for a truck but not a boat.
    I would have thought the CPP is the best way to load the engine assuming you have an exhaust temp gauge or on an electronic engine it can tell you what the load is so you dont overload it.
    This would allow you as you say to get max thrust at the max torque setting which is usually max bsfc as well.
    I find it hard to believe you could wind in enough pitch that the prop 'stalls' and hence not load the engine?
    I could only imagine that if the diameter was half of what was required.. maybe?


    You are right when you say incorrect use of a cpp can overload the engine so that might scare the manufacturer as it could damage the engine just like the wrong fixed prop.

    The multispeed trans could be intersting as it could be programmed to load the engine for best efficiancy but change to a lower gear when the load gets too high?
    I wonder how the economics of CPP versus multispeed trans goes?
    Weismann make 6 plus speed boxes
     
  2. apex1

    apex1 Guest

    That is wrong Fred! How often must we repeat that?:?: :?: :?:

    You do´nt stall the prop when the CPP is choosen correct.

    And you can load the engine close to optimum from a tad above idle to full rpm.

    We had all the drama in exhausting depth on the CPP thread.
    For which reason ever you obviously do´nt like CPP´s do´nt you?

    Regards
    Richard
     
  3. sailor2
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    sailor2 Senior Member

    When you load the engine more at idle with CPP, the engine produces more power, and FF requested less power, not more.
    If an engine produces 105 hp at 2100rpm, and torque would be kept the same at 500rpm, the power will then be 25 hp, and he wanted 10 hp or less at cruise.

    That means the problem is the engine, not gear reduction ratio, or pitch or diam. Even changing those all couldn't possibly solve his request.
    What he needs for vast changes in power is another smaller engine !

    From post #8 by Fast Fred :
    Your problem is not the prop or gear ratio.
    It's the engine !
    If you take an engine designed to produce 140 hp at 2100 rpm, and want to load it at 500rpm to keep it efficient, it'll produce far too much power (over 30hp) than the 10 hp you want in slow cruise.
    That is the property of the engine itself, no chance in gear or in prop can possibly change that. No matter what.
     
  4. jonr
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    jonr Senior Member

    Agreed, not to mention that at 500 rpm, the engine won't be efficient (in terms of BSFC).
     
  5. gonzo
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    gonzo Senior Member

    A variable diameter prop would have to either retract into a huge hub or parts of the blades be hollow so half of it goes inside.
     
  6. gonzo
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    gonzo Senior Member

    Ailerons?
     
  7. Boston

    Boston Previous Member

    how about a series of vanes that pivot out from a central shaft each one being a bit longer than the previous and both adding to the pitch and length of the blades

    more vanes extended more diameter and more pitch
    few
    less

    a lot like the folding props but
    different
     
  8. apex1

    apex1 Guest

    let me say, I´m less and less amused about this sort of comments.:mad:
     
  9. FAST FRED
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    FAST FRED Senior Member

    So far the answer seems to be a choice,

    Either a CPP with a 2 speed transmission to allow for the really different load requirements,

    OR the simpler choice of machining the shaft , so simple hand tool prop changes (NO HUB PULLER REQUIRED) , as done on outboards.

    I have seen spline drives on surface piercing props , but their cost is space program.

    A 2 speed ZF would be cheaper than a couple of surface piercing props.

    But the chance of simple outboard props , changed in the comfort of the cock pit via a small well is very attractive.

    Custom diameter and pitch seems to be the only hassle .

    FF
     
  10. apex1

    apex1 Guest

    No comment....................except:

    you do not comprehend.
     
  11. WestVanHan
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    WestVanHan Not a Senior Member

    For Fast Fred's boat:what about dual 70hp engines,each with cpp?

    When FF wants to make time,they both run-with cpp pitched properly.
    When he wants to slow cruise,turn one off and feather the blades,and he gets his mileage.
    Maybe not down to 10 hp,but closer anyways.

    Or a would a 2 into 1 transmission work?
    -one engine at 30 hp (other turned off and declutched) to run at 10hp when he wants to go slow
    -and the other one a 160 hp (30 hp off) for when he wants to make time.
     
  12. powerabout
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    powerabout Senior Member

    I guess we need to know what speed range a CPP can handle?
     
  13. TeddyDiver
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    TeddyDiver Gollywobbler

    From reverse to stop to full ahead.. without steps.. without quessing..
     
  14. FAST FRED
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    FAST FRED Senior Member

    I think there is no way a CPP is going to be able to properly load an engine at VERY LOW RPM , with out having the pitch cranked up to the point where the prop will cavitate .

    Perhaps Rick could answer this question ,

    Can the pitch be changed enough to properly absorb 120HP at 2100rpm

    and 20 HP at 1100- 1200RPM with ant fixed geared tranny and any fixed prop diameter?

    Estimated boat speed 18K- 20K at high cruise , 10K to 12K at low cruise

    The point is Properly load EG to 80% of the engines rated power at that low rpm.

    If the prop diameter were selected for the low rpm (a large diameter) cruise regime , and the prop could be cranked to almost no pitch for the part time high speed operation . I'm not sure it would work, but it could.

    I have seen "LST" power packages for sale gov rebuilt for about $6000.00 as a complete package.

    These are 2, DD I 6-71 coupled to a common tranny & shaft.

    The USN used them in great numbers as one engine could be pulled off line , while running , with a simple lever.

    To re-engage the package does have to be stopped and the lever moved ..

    On a far larger boat than I am contemplating (say a 40 -60 ft trawler) , one engine could simply be replaced with a 2-71 or 3-71 (same bell housing ).

    Then the 6-71 could operate when 180HP worth fuel burn (12-14GPH) is needed and the 2-71 when 40 -60 hp will do the task .

    The 2 stroke DD are most efficient at 30 hp per cylinder , but will still be OK at 20 hp per cylinder.

    Since a 2 speed tranny and CPP is both complex and expensive , the concept of having a very fast and easy to change prop , and changing SYSTEM , to optimize the cruise regime seems an interesting concept work on.



    FF
     

  15. jonr
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    jonr Senior Member

    To some extent you can get the effect of trimming up by dumping some exhaust before the prop.
     
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