Propeller performance vs. shaft inclination

Discussion in 'Props' started by daiquiri, Dec 9, 2009.

  1. daiquiri
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    daiquiri Engineering and Design

    Hello,

    I've decided to open this thread after having read the results of several experimental studies on azipod drives (the only ones that are being sistematically tested for different prop inflow angles) which report the maximum propulsive efficiency at inflow angles greater than 0° (mostly around 5°). In other words, when the propeler axis is not aligned with the flow. It goes pretty much against what I know about both aircraft and boat propellers, where an axially-aligned prop is generally considered the most efficient one. Several references about aeronautical props indicate the same secant law of decrease of prop efficiency with yaw angle: Eff,yaw = Eff,0 - 0.6 (sec(yaw_angle) -1), for angles in radiants.

    Yet the reports I've read do appear consistant with results of quasy-steady models for inclined props, like Gutsche's model, which state that both Kt and Kq can increase with shaft inclination. These models justify the increase in coefficients with the fact that the actual advance ratio J (the one calculated by using the free-stream velocity component perpendicular to the disc area) decreases with shaft angle. So, theoretically, a situation can be conjectured where Kt's and Kq's increase can produce an increased efficiency since it is given by (J Kt)/(2 pi Kq).

    Now, if those results about azipods could be extended to general fixed-shaft propellers, one could infer that there might be some shaft inclination (say, 5 degrees?) which would be beneficial to prop's efficiency - though we know it would increase prop-induced vibrations, noise and loads. I realize that this conclusion would go against a common knowledge and practice which has been discussed on this forum many times... But thinking about the variety of propeller geometries (rake, skew, pitch distributions, hub shapes etc.), why couldn't some of them benefit from shaft inclination? (thinking out loudly)

    It should also be said that quasi-steady models based on blade element theory (such as Gutsche's) are valid for non-cavitating propellers only, whilst real-world props will nearly always have to work with a certain degree of cavitation. Knowing that cavitation always degrades prop's efficiency, the actual usefulness of those theories for common applications might be questionable.

    So I am kindly asking you to help me here in resolving this puzzle, either by providing your personal experience or research in this field, or by pointing out towards other papers or books which could help confirm or confute the above thoughts. Which, I repeat, are my conjectures based on few papers found about azipods. I have 5 beefy books on propellers (both aeronautical and naval) in my library, but none of them goes too deep into this issue, as if the field of props in oblique flows was not sufficiently interesting (or known?) to the authors, so I'm relying mostly on few research papers available through internet.

    Some of reports cited above can be found here:
    http://ittc.sname.org/proc18/Session on High-Speed Marine Vehicle.pdf (pages 170-171)
    http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=8894975&article=0
    http://www.freewebs.com/ejname/j42/JNAME421.pdf

    Thanks!

    S.
     
  2. Joakim
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    Joakim Senior Member

    Those were interesting papers. Note that there was a huge different in the angle behaviour between pusher and puller configurations. In pusher configuration 0 angle had clearly better efficiency than 5 degrees.

    Also note that at 30 degrees the side force coefficients are easily bigger than the thrust force coefficient, thus the thrust vector angle is not even close to the shaft angle. Actually even at 10 degree the side force can be bigger than longitudal force.

    So what is the vertical force from an inclined shaft and how does it depend on the rotation direction of the propeller?
     
  3. baeckmo
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    baeckmo Hydrodynamics

    Nice job, D! However, there are in fact some indications from propeller tests that show similar trends, ie an increase in efficiency up to a certain shaft inclination. I am going to have a few days off until next wednesday; then I will see if I can find the appropriate papers.

    Also within reasonable limits, there are relative increases seen in cavitating operation with inclined shafts. Here part of the explanation is that we have a hysteresis effect in cavity development; with the periodically changing angle of attack in an inclined propeller, the net performance reduction may be lower than the reduction with a constant level of cavitation.

    And Joakim, the force line is practically always diverting from shaft direction with an inclined shaft; but since the flow field is influenced by the hull structure as well, there will be a directional change even if the shaft is parallell with flow and hull. This is what you have in the thread on inclided shaft.

    Before comparing results from azimuth tests with inclined shaft thrust, please observe that azimuth tests refer thrust to shaft line direction, not to flow direction.
     
    Last edited: Dec 9, 2009
  4. Joakim
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    Joakim Senior Member

    I knew that the force line is not exactly in the shaft direction, but in this report the difference is very big. How much of it is caused by the drive unit? Is it turning the force vector very efficiently like a rudder or is this phenomenom caused mainly by the propeller itself?

    At least the second paper reported the forces in the direction of the flow and perpendicular to it, not in the shaft line direction.
     
  5. daiquiri
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    daiquiri Engineering and Design

    ...which indicates the huge importance of the pod geometry and configuration, imho. In puller configuration the prop was seeing the undisturbed water inflow, while in pusher configuration it was working partially immersed in drive hub and leg's boundary layer. So the the mean water velocity (and therefore the effective J) seen by the prop was smaller for the latter case. Furthermore, the drive leg was probably somewhat straightening (or channelizing) the water flow towards disc for small azimuth angles, thus partially canceling the differences in water inflow angles for small azimuths.
    Good observations, imho. I've just made a quick Excel sheet with analysis of hydrodynamic coefficients at 8 different points placed around the 0.7D circumference of a prop disc (no drive unit leg). After calculating Cl (and hence Cx, Cy, Cz) at each point (for increasing azimuthal angles from 0° to 45°) and then averaging the coefficients over the disc area to obtain Kx, Ky, Kz for the prop, the numbers still don't add up to such a huge lateral force.

    BTW, that excel sheet is a way to calculate vertical forces, which you have asked about. It is essentially a blade element model taken at just few points around a fixed radius station - for quick qualitative analysis. There are moments due to these forces, which need to be considered too. But it is too late here for me to continue, I'll do it tomorrow and will try to see how big is the influence of pitching and yawing moments on reaction forces measured at the root of the unit, where dynamometers were placed.
     
  6. elaheh
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    elaheh Junior Member

    hi
    could you please introduce me an article about shaft inclination and it's hydrodynamic effects in high speed crafts?
     
  7. johneck
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    johneck Senior Member

    There is an effect due to the pod strut even in the puller configuration. It is not as great, but still changes the inflow. I suspect that the reason for the peak efficiency occuring at a non-zero inflow angle is the non-uniform inflow. However, I have been known to be wrong once in a while so it will be interesting to see what Baeckmo comes up with regarding open water propeller tests.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Could it be that more shaft angle is surprisingly efficient because the prop is working in water that is less affected by the drag of the hull pulling water along, than a prop tucked up closer to the bottom ?
     
  9. mydauphin
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    mydauphin Senior Member

    I am too tired to think about this thread right now. But my two cents for know.

    The angle of attack of propeller vs shaft angle is to my experience very important. This keeps the water from hitting prop dead on. Instead water goes into blade at an angle in a high speed craft with an angled shaft. Also water is not like air, water does not compress, in a airplane prop the air is compressed behind prop.

    In slower speed vessels horizontal shafts are better. They come out of hull bulge and water flow around them is minimized.

    I think proponents of pods have always promoted drag savings of not having shaft, but no one talks about the drag caused by the pods. Notice all high speed vessels including surface drives are using angled shafts.

    Now go plug this into your computers...
     
  10. yipster
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    yipster designer

    cant find some articles i looked for in a jiffy and although trust should be perpendicular to the blade
    also consider that blade rake on a surface prop changes direction of thrust
    many factors involved in propeller performance me thinks
     

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  11. philSweet
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    philSweet Senior Member

    Okay, after sending Daiquiri a few PMs, I realized this was a really old thread. Sorry D.

    two different scenarios here.

    With a vertical strut supporting a pod (second ref doc) -- I think the efficiency effect in yaw for a pusher is all strut and no prop. The strut induces a counterrotating inflow condition. We already know that is a good thing. The efficiency probably would have been even higher if the prop hadn't been rotated, just the strut. I wasn't totally clear about the gearing/bearing arrangement in the pod, either. It is possible that the gearset is sensitive to sideloads. If the torquing drive force also produces a sideload on the bearing, yawing one way increases bearing loads, yawing the other way decreases them.


    The situation in pitch is different.
     
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