could widely spaced in-line props reduce cavitation while

Discussion in 'Boat Design' started by Squidly-Diddly, May 17, 2010.

  1. Squidly-Diddly
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    Squidly-Diddly Senior Member

    boosting the ejected water stream to higher speed?

    I've seem prop planes with nose and tail mounted props, and I believe part of the reason was props were running out of steam at med-high subsonic speeds because they couldn't grab enough air and would only create a limited amount of vacuum, which was an even bigger problem at higher altitudes with lower pressure, so the idea was for the tail mounted prop to boost the pre-accelerated air to higher speed than a single prop could, and thus push the plane faster than any single prop plane, even one with multiple single prop engines. This was beyond the obvious streamlining of the inline set-up.

    Could this be used to push water faster without cavitation?

    I wonder how much efficiency is lost from the 2nd prop being fed "rough" air.
    [​IMG]
     
  2. u4ea32
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    u4ea32 Senior Member

    Well, kinda.

    Two props are more lightly loaded, therefore less pressure differential, therefore less likely to drop the water pressure to vapor pressure level, and therefore less likely to experience cavitation.

    The choice to configure the props as in the picture has to do with other things: its the way to have two separate powerplants, totally independent, and yet not pay the large penalty in wetted surface for putting the engines in pods on the wings.

    Actually, the airplane performs better on the aft engine than on the forward engine: its not the prop in dirty air, its getting the entire fuselage out of the dirty air of the forward prop.

    Also, the aft part of the fuselage, just like the aft part of a ship, is dragging the air along with it, so the prop is more efficient (its working in slower moving air), and the prop helps clear the air being dragged along.

    That's why vessels that are all about efficiency -- big commercial ships -- have one prop at the aft end.
     
  3. baeckmo
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    baeckmo Hydrodynamics

    Yes, tandem propellers in boat propulsion can be used to solve cavitation problems. Specifically when the shaft rotational speed has to be kept high for some reason. This trick was used by "The Honourable Charles Algernon Parsons" in 1897, to propel the then radical "Turbinia" to her top speed of 33 knots.

    His original design was cavitating heavily and lost thrust at about 16 knots, making for a disaster, but Parsons went back home, built a cavitation testing facility, got a grip on the "new" phenomenon and came up with a solution. He lowered the specific propeller loading and the tip speed by using triple screws on three shafts.

    Your question say "widely", are you referring to the airplane analogy? If so, David has given the correct picture, but if you are talking tandem props in general, there is reason to place them in close proximity and orient the blades so that an optimum velocity distribution occurs over the blade rows.
     
  4. jehardiman
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    jehardiman Senior Member

    There is a major problem with the concept though which trumps ever considering multiple props per shaft.

    The reason you don't see it is because the efficiency curve is very peaky and never as good as other arrangements. Think of this, the water entering the first wheel has the normal wake distribution. Leaving the first wheel it not only has added speed, it has a rotary component, swirl. The second wheel must then be designed to utilize this swirled wake (which reduces apparent AoA) at the same RPM as the first wheel. For the third wheel, the swirl would be even greater, etc. While it is possible to design a set of wheels that could have reasonable efficiencyat a single RPM, it is impossible to get good performance across the speed ranges because the swirl is not directly proportional to the increase in wake speed. There would need to be a different geometric pitch for each subsequent wheel for every RPM. Additionaly, you will always have less efficiency due to more blade area and tip losses. FWIW, Parsons didn't use any reduction gears on Tubinia. When he reduced the prop diameters to prevent cavitation, there was hardly any blade area so he just added props to up the blade area.

    [​IMG]

    For increasing the thrust of the propeller jet it is important to remember where the increase in energy went across the wheel. {start Hardiman's Unified Propulsion Theory} Remember the swirl? If you take the streamline del in the axial direction, you will find that there is no increase in axial velocity across the propeller disk, all increase in flow occurs before the disk. All the energy added by the wheel went into swirl. { end Hardiman's Unified Propulsion Theory} This energy, just like hull wake energy, is available to be extracted. You do this by taking the swirl out of the wake with a second counter-rotating wheel ( or a Grim Wheel). The nice thing about this is that the counter-rotation increases apparent AoA which gets rid of the whole non-linear swirl to advance ratio. If well done, the energy extracted from the swirl will excede the blade area and tip losses of the second wheel and give slightly more thrust. Additionaly, because more blade area is available split between two smaller diameter wheels, tip speed is reduced and cavitation is delayed. If you look carefully at the Dornier 335 pictured, you will see that the two props are actually a counter-rotating set, not a series set.
     
    Last edited: May 18, 2010
  5. Easy Rider
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    Easy Rider Senior Member

    David,
    If one had the right prop area in the first place why would anyone just add another prop on the same shaft and double the blade area? If your'e going to add another prop the logical thing to do would be to take the old prop off and install two props roughly half the blade area of the original single prop. Now you've got extremely high aspect ratio blades or a much smaller disc diameter neither of which will be ideal .. assuming the original was optimized. Counter rotating propellers very close together have some advantages that jehardiman may have explained but I don't understand his presentation. What is AoA? I do think that the largest disc diameter should be the most efficient. That means the power available is working on the largest span of water possible. Disc diameter loading I believe it's called. Most people think a single screw is more efficient than a twin and if it is so perhaps we have the reason here on the table. Do the counter rotating blades of a jet turbine overcome this and if so can the advantage be used for marine screw propulsion?
    Actually, the airplane performs better on the aft engine than on the forward engine: "its not the prop in dirty air, its getting the entire fuselage out of the dirty air of the forward prop." I think what your'e trying to say is that the aircraft flies better on the rear engine because the front engine isn't blowing the airplane backwards as all tractors do.

    Easy Rider
     
  6. jehardiman
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    jehardiman Senior Member

    Angle of Attack. Because the water rotation (swirl) is in the direction of the blade rotation, the AoA of the actual 2nd blade section is reduced. Do a vector diagram of inflow to a single blade section..
     
  7. Easy Rider
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    Easy Rider Senior Member

    OK I get it. Then w a fixed disc dia does the volvo twin prop thing actually deliver more thrust or is it just a torque thing? I would think the 2nd blade would require more pitch or faster speed.

    Easy
     
  8. jehardiman
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    jehardiman Senior Member

    In a counter rotating set, the aft wheel is usually , but not necassarily, slightly up pitched relative to the leading wheel and both turn at the same rate though opposite hand. The increase in pitch is dictated by blade loading through its effects on the swirl. The big thing a counter rotating set does is to reduce the blade loading of each wheel (when diameter is limited) which increases the overall efficiency of the wheels, this is more than the efficiency gained by recovering some swirl. If diameter is not limited, or there is not some other reason for counter rotation, it is better to have a single, larger, propeller.
     
  9. daiquiri
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    daiquiri Engineering and Design

    Hi, I am also trying to understand better this thing...
    If the counter-rotating props reduce disc loading and increase efficiency for a fixed diameter, then why do you say that it is better to have a single larger prop when diam. is not limited? The first phrase would imply that adding a second disc would increase the efficiency at any given diameter.
    Or is it a matter of mechanical complexity, weight or something else?
     
  10. jehardiman
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    jehardiman Senior Member

    You need to look at what you want to accomplish.

    In a broad generalization, for a given advance, thrust is proportional disk area and torque is proportional to blade speed^2 plus a fixed loss for each blade due to root and tip losses. If I increase my diameter of a single wheel to match the thrust of a CR set, I need to increase the diameter by sqrt(2) but reduce my turns by 1/sqrt(2) which means that my losses only increase by a factor of sqrt(2) not 2, and my fixed losses decrease because I have fewer blades.

    Of course blade loading, blade area ratio, aspect ratio, cavitation, and number of blades are all woven into that. Carried to the extreme, the most efficient wheel would be a single bladed wheel as large as possible turning as slowly as possible.
     
  11. mark775

    mark775 Guest

    "actually a counter-rotating set" - for sure?
    As far as swirl and the different RPM difficulties, why not, in this instance, just reduce it to a non-issue with radial fins in front of each prop?
     
  12. jehardiman
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    jehardiman Senior Member

    The Do 335 had counter-rotating props, RH forward, LH tail.

    The use of stators is inefficient because they cause more drag than they make back in thrust and are commonly used to manipulate inflow in situations where the application of available power, rather than absolute efficiency, drives the design of the propulsor.
     
  13. mark775

    mark775 Guest

    There is a lot happening in the pic of that boat...maybe my eyes are tricking me but it looks like RH port, LH stbd., three props on each shaft to me.
     
  14. jehardiman
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    jehardiman Senior Member

    The picture I was refering to for counter rotation in the last paragraph of my first post is the Dornier Do 335 aircraft posted by Squidly-Diddly. RH forward, LH tail, counter rotating to cancel torque.

    The picture I posted was of Turbina, which has 3 shafts, each shaft with 3 props. She has the normal "outboard turing" arrangement (i.e. the direction of rotation nearest the hull is in the outboard direction), the stbd and center shafts are RH wheels, the port shaft is LH wheels. Here is a website with much better shots of the wheels, the wheels are 18x24.

    http://rides.webshots.com/album/551810472FDLUHb

    [​IMG]
     

  15. baeckmo
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    baeckmo Hydrodynamics

    To get a grip on this balance, it is useful to go back to basic momentum theory, regarding the propeller disc a "black box" that imparts a velocity increase to the incoming flow. In a free flow (neglecting hull influence) the propulsion efficiency (etaprop) is the product of the jet (or momentum) efficiency (etajet) and the hydromechanical efficiency (etapump)of the pumping mechanism that creates the velocity increase.

    For a given thrust and disc area (Ad) we have a thrust coefficient Ct = Thrust/(density/2*Va^2*Ad). From Ct we get etaj = 2/(1+(Ct+1)^0.5).

    Since we also have etaj = 2*Va/(Va+Vj); the required jet velocity (Vj) can be calculated for the given design problem. Regardless of what is inside the black box, the jet efficiency and the velocity increase is fixed from fixing thrust and diameter.

    Now to the hydromechanical mechanism inside the black box. This is in fact a pump, that creates a pressure increase equal to (Vj^2-Va^2)*(density/2); in N/m2. In 1756, Leonhard Euler published the theory on pressure difference in pumps and turbines, it is still valid....... It states that, for a pump runner with axial inflow, the head increase dH = U2*dCu/g; where H = (pressure/(2*g)); U2 = runner peripheral speed at discharge; dCu = change in peripheral composant of discharge flow.

    This implies that no matter what is inside the black box, it has to deliver the same sum of dCu, provided diameter, thrust and rpm are fixed. If we have one heavily loaded propeller or two in tandem (same rotation) is equal, the swirl rate out of the box is the same, giving the same loss (yeah, I will come back to counterrotation later!). Thus for a fixed diameter propulsor, the jet velocity (thereby the jet efficiency) and the rotation of the outlet flow is fixed.

    If we have been forced, by any design constraints, to use a restricted diameter, we also have been forced to increase the pressure increase of the runner. This is where the simple tandem configuration actually gives us possibilities to improve the hydromechanical efficiency ("pump efficiency")! With a single impeller ("propeller, runner, screw,.....) the pump efficiency generally drops with increased pressure load. Cavitation has a similar effect.

    If we split the power between two tandem impellers, the detrimental effects of cavitation and pressure loading are reduced, i.e. the hydromechanical efficiency is improved. The positive effect of using the front propeller as an inducer, suppressing cavitation on the second stage is so strong, that the tip speed of the "combo" may be increased, or the pressure loading of stage two can be increased while keeping its hydromechanical efficiency high. We havea situation where 1+1>2.

    Now to the counterrotating two stage unit. Keeping the dia fixed (=design constraint), the velocity increase for a given thrust is exactly the same as for a single, or tandem unit, and so is the jet efficiency. Again, the difference lies in the pump efficiency. The dCu from the front impeller is normally ~50 to 60 % of the total increase. But in the CR case, the second stage is adding its share of dCu in the opposite direction, leaving only ~0 to 10 % of residual swirl in the discharge. This is where the CR earns its improved pump efficiency.

    But in this case, the preswirl coming into the second rotor is detrimental to its cavitation performance, and the pulsating inflow due to the blade-to-blade velocity profile leaving the first stage is giving a varying incidence angle to the second rotor. All in all, this means that the total benefit of the added mechanical complexity is not as high as expected. In fact, a single prop, selected free from diameter constraint may very well have a better overall efficiency. As I said in my first note here, two-stage impellers are best used in special cases; the tandem on a common shaft is sadly overlooked as a means to solve cavitation problems.
     
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