Propeller Theory - upstream

Discussion in 'Props' started by kach22i, Apr 4, 2006.

  1. kach22i
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    kach22i Architect

    Would there be any design differences between a propeller, and a propeller designed to run up against a current?

    The current I am assuming is upstream and you are going right into it.

    I know water does not compress, so it will not be exactly like an aircraft propeller or fan (turbofan). However there is a difference between velocity and static pressure for airscrews. Is there such a measurement for watercraft?

    Do water propellers have similar properties to contend with (and be measured by) such as "velocity and static pressure"?

    Is there a ratio of volume to velocity for a waterscrew which is the idea model?
     
  2. DMacPherson
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    DMacPherson Senior Member

    Think like the propeller...

    Let's get a little Zen with this. Seriously - think like the propeller.

    A propeller does not know anything about the water velocity except what is in its immediate local region. For most vessels, the existence of a hull will slow the water to the propeller. In some planing craft, the water is actually accelerated a bit. Shallow water can also change the velocity to the propeller. And finally, a current - either against or with - will also change the local velocity at the propeller.

    The speed that the propeller "sees" is the "speed of advance" (typically noted as VA). The difference between this and boat speed (V) is the wake fraction (w), or VA = V * (1-w).

    To simplify the problem with current, we typically just add (or deduct) the current from the boat speed to get an effective V. This is not always exactly right, however, as the existence of the boat (and its viscous boundary layer and blockage to the propeller) can alter this somewhat. Also, this may change with propeller type. For example, a ducted propeller will be affected differently than an open propeller. Having said that, there is probably more "noise" in the calculation than the imprecision of using the simplifed assumption of just adding (or deducting) the current velocity to the boat speed.

    Regards,

    Don MacPherson
    HydroComp, Inc.
     
  3. kach22i
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    kach22i Architect

    Aircraft speed - headwind speed = Ground speed

    Boat speed - head current = Water speed

    That is the simple part, no major confusion with that.

    Let's have two situations.

    1. The boat (or aircraft) rapidly decelerates (for any reason you want to imagine). At some point the propeller blades act as drag sails, perhaps because the engine stalls. Forgot where I was going with this, hold the thought.

    2. Lets say I have an electric motor which can instantly spin a propeller. I'm just sitting in the water and let her rip. The propeller for a moment will be straved for supply water (ducted case even more so) as it cavitates, right? Therefore you can withhold/strave a propeller (water or airscrew) from it's supply medium, right? Can you force feed or over supply a medium (water or air) and reduce or increase propeller efficiency by design?

    Note: I have never thought about the boat hull slowing down the boundary layer of water before - cool.

    Example 3:
    This is one of the hardest concepts to get across. I run into this every summer with my wife. She puts the fan an inch from the wall and complains that we need a new fan. I pull the fan a foot or more from the wall and the breeze increases (and you can hear the difference). Therefore the rate the supply air enters the drive fan makes a difference.

    Once we had a fan in the upstairs window on and a gust of air pushed the fan blades faster (was blowing towards the inside) which transfered the gust to the interior of the house - papers went flying. The sound was quite remarkable as the electric motor was being pushed and not pulling the fan blades.

    Positive and negative pockets of feed air directly effect propeller/fan thrust and performance. What is this phenomena called?
     
  4. DMacPherson
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    DMacPherson Senior Member

    In reply...

    1. I'm holding the thought.

    2. It sounds like you're using aircraft jet analogies, where a compressor changes local air velocity into the thrust-making rotational elements. On a boat, I suppose that you can think of this as going from a propeller to a waterjet. In certain speed regimes, you are inproving efficiency by altering the water velocity. As water is "incompressible", you really cannot get the same effects as a propulsor in air, so its not about changing (or starving) the medium, but rather about optimizing the propulsor to whatever flow you have. This leads to...

    3. By placing the fan near the wall, you changed its ability to create airflow into the blades. Fans and propellers all accelerate the medium to some extent into the propeller/fan disk (called the induced velocity). When the fan was near the wall, it required more of the energy to pull air to the fan, reducing the actual induced velocity to the fan blades. This made the blades over-pitched relative to the new lower local air velocity right at the blades. This probably tresulted in a reduction in rpm as the electric motor torque was unable to properly spin the blades in the slower air - resulting in an even lower thrust though the blades.

    The phenomenon that you describe is reflected in what is called the four quandrants of propeller performance - where each quadrant is related to velocity and rpm. For example, the principal first quadrant is forward velocity and forward rpm. Backing up with both velocity and rpm in reverse is the fourth quadrant. A crash back stop is forward velocity (the boat is still traveling forward) with reverse rpm - the third quadrant. Your fan example, with reverse velocity and forward rpm is the second quadrant.

    Regards,

    Don MacPherson
    HydroComp, Inc.
     
  5. kach22i
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    kach22i Architect

    Thanks Don (DMacPherson), I now have some terms and concepts to research.

    I do have an interest in boats, but I relate it to hovercraft because that's what I've owned for the past 15 years - gets me out on the water. It's more of an adventure craft than a water craft.

    The starving for feed air/water comes into many of my paper designs because I'm always trying to make things sleek, stealthy and quiet. This oftens means feeding propellers and fans through remote duct inlets.

    One thing I stumbled across was a sort of an airbox, a plenum for fresh combustion feed air for a gas turbine on the Navy's LCAC's (large hovercraft).

    The volume of air in the box and it's distance is designed to seperate out the corrosive salt from the air (hovercraft make a lot of spray sometimes). This is supposed to make the turbine fan blades last longer.

    If anyone has information on how to calculate an air plenum feed chamber, please post me a link.

    I think some of the submerged sponson craft (SWATH) use an right angle feed to propellers and waterjets. I've wondered if these systems can work well at speed without starving for feed water. They at least to not get any induced velocity benefits, right?
     
  6. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    in cavitation tunnel the screw model is fixed and the water is running against...
     
  7. kach22i
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    kach22i Architect

    I looked that term up, looks to be similar to a wind tunnel but for water.

    [​IMG]
    My curiosity again is with the cavitation tunnel or wind tunnel that undersizing it could adversely affect the testing. How would you size such a thing? I suppose there are may factors, but it could come down to "no such thing as too big", just cost being the limiting factor.
     
  8. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    according to my own experience, the figures recorded using cavitation tunnel are enough realistic even if sometimes the prop. model are small.

    Don't forget that hydrodynamic is a not an exact science...
     
  9. jehardiman
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    jehardiman Senior Member

    A couple of thoughts.

    a) Looking at the standard J-Kt-Kq curves can offer insight. If Va >> n*pitch then Kt-Kq-eta are negative...i.e. the prop is a brake and power can be extracted. If Va < 0 then Kt-Kq are positive and greater than Kt-Kq|J=0 and eta positive (an even function). This is when the prop is having to reverse the inflow and can cause over curret/torque events.

    b) with inflows perpendicular to prop axis, thrust may still be developed, but not axial to the shaft (the gyroscope effect). For ducted props, the work needed to turn the flow may excede the power available to the prop, see work by Cmdr Poole, USNA on energy methods, and some unpublished energy theory developments for HPV's.

    c) in developing plenum chambers, the idea is cause areas of low loss/high momemtum with reversal to cause kenetic seperation of the water. Get out any good machinery design guide for cyclone seperators or oil/water bath air cleaners/scrubbers.
     
  10. kach22i
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    kach22i Architect

    I'm not having much luck on Google looking this up. Can you please provide a link or some good search terms to use?

    Was his work mostly about submarines?
     
  11. jehardiman
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    jehardiman Senior Member

    Try "Dr Patrick Poole". The paper you want was in an MTS publication back in '93 I think, I'll have to dig out my copy get the exact title. The original paper was for the ISRs but is applicable to all props.
     
  12. kach22i
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    kach22i Architect

    So let's say I have a tail heavy (typical planning hull boat balance) motor boat running at full speed. I attempt a sharp right angle turn, and the boat begins to slide across the water sideways. The propeller cannot under these conditions help me power out of the turn?

    To help illustrate this example; I am pretending for a moment that the rudder has broken off. I am also pretending that the rotational direction of the propeller, and the turn direction of the turn (right or left) are the worst possible combinations.

    I should note that the description above for a motor boat is the typical situation for a nearly friction free hovecraft attempting to bank a turn. Sometimes all you hear is the howling of the fan blades trying to draw air into the duct.
     
  13. jehardiman
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    jehardiman Senior Member

    At most planing boat speeds, side velocity due to a turn is negligable compared to ahead velocity. While a jet boat can achieve significant side velocity in a turn , it is unlikely that a propeller driven craft can. But Yes, if you somehow managed to break a conventional powerboat into a sideslip without flipping it, then the prop will provide significantly less thrust due to inflow AOA effects. I mentioned it because it is a common proplem in auzimuthing pods. Turn the pod at right angles while going fast and you don't get the response you expect
     
  14. kach22i
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    kach22i Architect

    I had to look up Azimuth Pods, found some interesting stuff.

    I thought a squid attacked a propeller when I first saw this image.
    http://www.naval-technology.com/contractors/propulsion/alstom/alstom5.html
    [​IMG]

    Looks like some serious money is being invested in this technology.

    http://www.sspa.se/research/optipod/
    If parasitic drag of a plenum water supply tunnel to a propeller were not so great, would it be more attractive and more immune to sudden changes in boat direction than a pod? The propeller would have to suck the tube dry before getting trouble, right? Would a properly sized supply feed plenum tube provide a smoother transition to right angle direction changes? Would this just send the design problem forward to the opening lip of the water inlet?
     

  15. kach22i
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    kach22i Architect

    How come water jets, which feed supply inlet water from a hole in the bottom of the ship hull, seem to work so well? Is the impeller of a water jet set up to maximize water intake “draw power” over all other factors and considerations?

    The centrifugal lift fans of hovercraft are similar to water jet impellers in that the thrust is at right angles to the axis. Is this where the similarities end? Do these so called less efficient than axial propeller fans actually providing a wider performance envelope and useful power band range?

    Lots of question there, sorry………..getting off topic is a bad habit of mine.
     
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