Variable DIAMETER ? Prop

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

  1. powerabout
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    powerabout Senior Member

    Fred
    I dont think the very high pitch at low speed woud cause it to cavitate as the blade loading would be very low for the low speed as we know you are only using very small power....
     
  2. apex1

    apex1 Guest

    Two very dangerous assumptions!

    First,

    you do NOT understand a CPP. (and quite obvious, you do not like to learn it)

    second

    Rick is the last one I would ask for anything related to real life propulsion.
    He might be pretty capable of operating programs, but thats it. He knows NOTHING!!!!!!!!!!!!!!!!!!!!!!!

    .........except the right theory




    So, go on with your ideas, and do not play this idiotic game any longer! We know you do´nt like CPP systems (and that you do not know them).
     
  3. FAST FRED
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    FAST FRED Senior Member

    We know you do´nt like CPP systems (and that you do not know them).


    I operated a heavy 55 ft motor sailor double ender in wood with a Hundstead CPP , no clutch for a season.

    While the unit was both efficient and easy to learn , it did have its own operating requirements. Mostly you had to speed up to about 750 RPM before shutdown , and set the pitch to neutral at that RPM.

    On start the engine would be advanced and monitored for the first 5 min of warmup to stay at that rpm, or the boat would move, dramatically as the prop was about 30 inches.Ant RPM change was motion!

    On a motorsailor it was a great concept , but I'm not thinking of a motorsailor , but a boat that will have a slow cruise regime , most of the time.

    Remember when the boat is at 20k THE WATER IMPINGING THE PROP IS ALSO 20k At 10k the water is also 10k so the amount of pitch from 20K will be far too much . Since the only (fixed diameter) way to load the engine is with pitch , i sure do worry the prop will simply boil the ocean.

    I would love a vessel with CPP , and 2 speed tranny, as I am used to aircraft where full prop controls are the norm, bur weather the cost of the unit would ever be returned in a cruising pleasure boat is a concern.

    FF
     
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  4. TeddyDiver
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    TeddyDiver Gollywobbler

    You make an assumption of equally high engine loading. In such broad rpm range operation you need a pyrometer to get engine loading inside the acceptable range. This way the real engine loading might be somehere btw 40 to 100%.. Anyway with a large prop enough you will never cavitate at low rpm even with prop blades at 90deg angle.. Some sidewalking might happen thou..
     
  5. FAST FRED
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    FAST FRED Senior Member

    "You make an assumption of equally high engine loading."

    The desire is to properly load the engine at minor RPM as with a "normal" prop load the engine can consume 2/3 more fuel per hp


    "a pyrometer to get engine loading inside the acceptable range"

    No problem a pyrometer is about $100 US .

    "Anyway with a large prop enough you will never cavitate at low rpm even with prop blades at 90deg angle.."

    Remember to gain efficiency the prop will be a 2 blade , of large diameter , so tip cavitation IS a concern

    FF
     
  6. TeddyDiver
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    TeddyDiver Gollywobbler

    Engine rpm at idle 750, lets say 2.7 reduction means about 270 prop rpm, 20"/50cm prop would be ~6.6m/s tip speed... Typical power curve for 70hp/3600 diesel says 28hp at 750, lets assume 60% eff means ~17hp.. Cant go further without assumpions of the boat restance too.. However I've never seen cavitation marks in a prop under these circumstances..
    Baeckmo! need some expert opinion here :D

    ps. Something related..
    http://atljsoft.com/HTML_Help/Calculating The Cavitation Number.htm
     
  7. FAST FRED
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    FAST FRED Senior Member

    The latest issue of Pro Boat Builder (free on line) has a good article on the efficiency topic.

    Seems the Gori prop with built in shift ability to "overdrive" could solve the problem at a lower cost than a CPP.

    True it could not be optimized as well for every condition and sea state as a CPP, but at $10k to $20K cheaper ,

    and that it can easily be retrofitted , or removed for the "next " boat,,,,,,,

    FF
     
  8. TeddyDiver
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    TeddyDiver Gollywobbler

    etc...

    Please FF, you can try whatever gizmos but you refuse to consider something what's reliable and well tested for decades..
    CPP:)
     
  9. FAST FRED
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    FAST FRED Senior Member

    Gori has been building props for decades,

    if we gave up on any innovation till proven by decades we would still be using silk and bamboo to build 777's.

    The question is to solve a problem ,,,, a standard prop is underloaded at most cruise speeds , and the engine is a huge inefficient fuel consumer at light loads.

    Variable pitch might help , but for us in the real word with recreational vessels , COST does count , unlike the commercials that may run 2000 hrs a year where a 30%-50% saving can be big bucks.

    FF
     
  10. TollyWally
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    TollyWally Senior Member

    "The question is to solve a problem ,,,, a standard prop is underloaded at most cruise speeds , and the engine is a huge inefficient fuel consumer at light loads."

    Thinking out loud here. One time tested solution to this problem, engineered in the field by pragmatic sportsmen is the kicker concept for trolling.

    Another point to ponder, at what point does it become acceptable to to sacrifice flank speed for a suitable level of cruising speed effiency. For me I'd happily make the trade off at 12-15 knots.
     

  11. TeddyDiver
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    TeddyDiver Gollywobbler

    "Variable pitch might help , but for us in the real word with recreational vessels , COST does count"
    Why you don't make a real comparison instead of guessing?
    http://www.kastenmarine.com/CPprops.htm

    "if we gave up on any innovation till proven by decades we would still be using silk and bamboo to build 777's"
    Of course there's nothing wrong with new innovations, but so far there's nothing better available.. btw bamboo and silk are great materials ;)
    BR Teddy
     
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