The great controlable pitch prop debate.

Discussion in 'Props' started by Mik the stick, Dec 12, 2012.

  1. Mik the stick
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    Mik the stick Senior Member

    I read as much as I could and stopped reading about page six. I am a newcomer on this sight having much I want to learn about boats. BUT I DO KNOW ABOUT PROPELLERS.

    My knowledge is about aircraft but a prop is a prop. Aircraft can have fixed, controlable or Constant speed prop. Forget constant speed for the moment.

    For aircraft and boats controllable pitch props work the same. Early WW2 aircraft had controllable 2 pitch props fine pitch for take off/landing allowed revs and therefore acceleration to build up quickly. Once airbourne a change to coarse pitch allowed maximum speed. I'm not certain but I think props can be had which are infinitely variable.

    A constant speed prop works on load If you are cruising at say 150mph the prop will be in coarse pitch if you suddenly have to accelerate cause there is a bandit on your tail you slam the throttle forward and the hydraulics in the mechanism senses the load and reduces the pitch so you accelerate faster. when you are up to speed the prop automatically goes to the best pitch. the same situation with a fixed pitch you get very little acceleration. Its like pulling onto a motorway at 30mph in top gear, traffic will be up your backside in no time..

    THERE IS NO ENGINEERING REASON YOU CANT HAVE CONSTANT SPEED PROPS ON A BOAT BUT IT WILL COST MUCH MORE TO BUY AND MAINTAIN.

    I know much less about boats, I assume a CPP is infinitely variable and reversible. And the way to use one is this If your engine performs as in the table below. With the prop in fine pitch you choose to cruise at 1200rpm you will accelerate quickly to perhaps 2.5kts, when the pitch is adjusted slowly more course the rpm will stay constant the boat will get faster at perhaps 4.8kts the engine speed will start to drop because the engine cannot maintain 1200rpm under the load. Make the pitch slightly finer and you will cruise at 1200rpmand 4.7kts. If you cruise at 1600rpm in fine pitch your wasting fuel and probably doing less than 4kts.


    Available Power
    1200 RPM 15 SHP 4kts

    1400 RPM 24 SHP 4.5kts

    1600 RPM 35 SHP 5kts


    I'm not trying to teach granny to suck eggs but I think that just about covers the subject
    mik the stick.
     
  2. El_Guero

    El_Guero Previous Member

  3. TeddyDiver
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    TeddyDiver Gollywobbler

    You might know props but as you said yourself, not boat's, and that's where your reasoning fails. The big difference is resistance...
     
  4. michael pierzga
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    michael pierzga Senior Member

    I used one a few years ago on a delivery from Den Helder to Hamburg. I wasn't too impressed.


    Might be worth study. Perhaps its new and improved.

    http://www.autoprop.com/
     
  5. Mik the stick
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    Mik the stick Senior Member

    Constant speed is not infinitely variable what I was trying to describe is a prop which lets say varies 25 degrees in pitch. you can adjust it in motion to give you any pitch within the 25 degree range. in the case of an aircraft they can be ground adjustable or it can be done with a lever from the cockpit.
    Constant speed uses hydraulics to sense the engines load has changed and automatically adjusts the prop. the expense in buying such a prop is considerable and maintainance is not cheap.
     
  6. Mik the stick
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    Mik the stick Senior Member

    I don't understand how resistance makes my reasoning fail. Dare I say a prop has no brain it does not know if it is in water or air.:confused:
     
  7. jonr
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    jonr Senior Member

    Sure, you can get slightly better performance and economy with variable pitch. The questions are "is it worth it" and "would variable gearing be better".
     
  8. TeddyDiver
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    TeddyDiver Gollywobbler

    The resistance curve of a boat and an aeroplane differ very much. With an aeroplane you need high thrust during takeoffs, low speed and high resistance due greater angle of attack. At altitude the speed increases and resistance drops..
    Can your brain figure out what happens to boat resistance if cruising speed increases ;)
     
  9. Mik the stick
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    Mik the stick Senior Member

    OK I get it. Resistance goes up. Aircraft are different the air gets thin and the aircraft has less resistance to overcome.
     
  10. WestVanHan
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    WestVanHan Not a Senior Member

    This has been beat to death many times.... and this is the umpteenth time this page has been posted...

    http://www.kastenmarine.com/CPprops.htm

    Prices are old, price ratios may still be the same
     
  11. Submarine Tom

    Submarine Tom Previous Member

    What, there's a debate?
     
  12. johneck
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    johneck Senior Member

    The Kastenmarine paper greatly oversells the CPP. Typically a FPP is slightly more efficient than a CPP due to a smaller, better shaped hub and better blade root section shapes. The CPP also does not maintain high efficiency over much of its range of adjustment. When the blade is pitched, it is rotated about its axis. A 5 degree rotation has a much larger effect at the tip, where the pitch angle is only 10-15 deg, then it does at the root where the pitch angle is 45 deg. It is also very possible to pitch the blade into an orientation where there is a cavitration problem.
    CPP's are very good for vessels that have engines that don't reverse (gas turbines) or have very weak torque curves. They can also be used to fine tune the power vs RPM curve to improve fuel efficiency.
    There is no way they are cheaper than a similar FP system. I don't know how he got numbers to support that. In my experience, the difference is usually about a factor of 2, though maybe if you compare manufacture in China vs manufacture in Michigan he might be right.
     
  13. El_Guero

    El_Guero Previous Member

    Methinks, it is variable within the range it is designed for.

    Expensive? Of, course.

    But, turboprops are what they are.

    Wayne
     
  14. TeddyDiver
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    TeddyDiver Gollywobbler

    A lot of complete BS, thou a couple points right too. Sorry...
    Take a look at previous threads of CPP
    BR Teddy
     

  15. Mik the stick
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    Mik the stick Senior Member

    I did not mean to start another great debate. For me it is all very simple.

    1. Any kind of pitch control will cost more money.
    2. CPP is more complex therefore must be more prone to breakdown or damage.
    3. If you are going to use CPP it must have an advantage to offset 1 and 2.

    An example might be CPP on an offshore racing power boat. You would get that extra knot that a winner needs, and the owners/sponsers who want the winning publicity don't mind the extra cash because they have already spent considerable money.

    Personally I think the reversing is the best feature of CPP. If your boat does 12 knots max, and is designed to cruise at 9 knots your prop should be efficient at that speed and most owners will spend most of their time at 9 knots.

    From what I've read on this forum I think a reversing gearbox and fixed pitch is usually going to be the best solution.
     
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