Modern paddlewheels

Discussion in 'Boat Design' started by Guest, Dec 23, 2003.

  1. Jeremy Harris
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    Jeremy Harris Senior Member

    When I looked at paddle wheels for low power, shoal draft, use it came as a bit of a surprise to find that a slow turning, large diameter, feathered blade paddle wheel could be as near as dammit the same efficiency as a large diameter, high aspect ratio, slow turning propeller. Both can get into the 80% plus efficiency region, but both then have drawbacks when it comes to using them on a practical boat.

    Large diameter, high aspect ratio, slow turning propellers increase draft and are prone to weed fouling.

    Large diameter, slow turning paddles take up a lot of space and create windage.

    Reducing the diameter of either, and increasing rpm to get the same performance, results in lower efficiency, as efficiency relates fairly closely to blade loading (the higher the blade loading the lower the efficiency, as a rule).
     
  2. Nolen
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    Location: california

    Nolen New Member

    Hi all,

    Apologies for resurrecting this thread once again, however, I do have activity of relevance. We’ve completed recent testing and as Mr. Harris referenced above, blade loading is a forefront issue. At-the-moment, the test boat is back in the shop. The drive will receive slightly re-contoured blades and shroud repair.
    Our goal is a quick turn around for more testing in July. Enjoy the pictures and linked videos.

    Cheers
    Nolen

    P.S. Blade failure in Part2

    http://www.youtube.com/watch?v=pCePvbVwx9g
    http://www.youtube.com/watch?v=sH3o_NCpzXo
     

    Attached Files:

    Last edited: Jun 17, 2013
  3. michaeljc
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    michaeljc Senior Member

    I am interested to know how you worked out the thrust for the paddles Jeremy. What is the force on a 1 sq m of surface perpendicular to a water body moving at 1 m/s?

    Do hydro dam turbines have anything to teach us?

    M
     
  4. Jeremy Harris
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    Jeremy Harris Senior Member

    The work was originally done by Rick Willoughby on here some years ago, but I can't find the thread now.

    In essence, paddle wheel design is similar to prop design, in that you want to minimise slip. The paddle velocity along the major axis of the boat needs to be greater than the boat velocity by just enough to generate the required thrust.

    You have two choices to get the right paddle blade velocity, you can have a small diameter paddle wheel spinning fast or a large diameter paddle wheel spinning more slowly. Because you want to optimise thrust in the aft direction, and minimise lift as the paddle enters the water and down thrust as the paddle exits the water, the greater the diameter of the wheel the better (given that boat motion varies the immersion depth of the blades on a big wheel less than with a small wheel).

    Thrust can be estimated from the relative velocity of the paddle blade and the blade area, using a suitable figure for the blade drag coefficient (probably somewhere around 0.8 at a guess).

    Articulating, or feathering, the blades allows the thrust to always be in the right direction, and makes the wheel less sensitive to immersion depth.

    As with a propeller, moving a large mass of water slowly to gain the thrust needed is more efficient that moving a small mass of water quickly, which is another vote for having the largest diameter wheel you can. The analogy here is a bit like rowing, where moving short oars quickly tends to be less efficient than moving long oars more slowly.

    To answer the question about the 1m² blade moving at 1m/S relative to the water, then assuming the blade Cd to be about 0.8, then the force would be:

    F = 0.5 x rho x Cd x A x V²

    so F = 0.5 x 1000 x 0.8 x 1 x 1² = 400 N, or around 90lbsf for non-metric folk.

    True paddle wheel calculations have to take into account the total blade area immersed at any instant and calculate the vector sum of the forces that the paddles generate (unless it's a feathering wheel where the blades are always perpendicular to the direction of thrust).
     
  5. michaeljc
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    michaeljc Senior Member

    rho = ? : Mass of 1 cu m of water?

    A = ?
     
  6. Jeremy Harris
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    Jeremy Harris Senior Member

    Sorry, yes, rho is the standard meaning, density, in this case 1000kg/m³ (assuming fresh water), and A is the projected area of the blade, 1m².
     
  7. michaeljc
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    michaeljc Senior Member

    Ok: real life problem: A tug has to generate 15000 n of thrust at 1.7 m/s in 1.2m of water. Theatrically possible with paddles?

    Thanks for the help
     
  8. Jeremy Harris
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    Jeremy Harris Senior Member

    Quick, and very rough, back-of-fag-packet estimate:

    Assume 20% slip for high efficiency initially, so paddle velocity is 20% greater than boat velocity, giving a differential velocity of 0.34 m/S.

    Assume paddle immersion depth can be half water depth, so 0.6m (this could be greater, it depends where you put the paddles and how much you allow for boat roll, heave and pitch).

    Assume that the paddle wheel blades have a Cd of 0.8.

    Rearranging the force equation gives:-

    A (blade effective area) = F / (0.5 x rho x Cd x V²)

    therefore A = 15000 / (0.5 x 1000 x 0.8 x 0.34²) = 324m²

    This isn't really practical, as the total effective blade width (not the actual blade width as more than one blade may be immersed at the same time) would be 540m.

    You could accept that efficiency isn't that important when looking at bollard pull, which is the stance taken when designing tug props. In this case, the slip ratio could be allowed to go as high as, say, 70%, maybe more. Redoing the rough calcs for 70% slip gives a relative paddle to boat velocity of 3.97 m/S. This reduces the effective blade area right down to 2.37m², giving an effective blade width for 0.6m immersion depth of 3.96m, a far more practical proposition.

    So, the answer seems to be that, if you are prepared to accept a loss of efficiency at this low speed, high bollard pull, condition, then yes, a paddle wheel would do the job.

    There's anecdotal evidence that steam paddle tugs were fairly highly regarded in the past because they could deliver a high bollard pull, which supports the above very rough and crude estimate.
     
  9. michaeljc
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    michaeljc Senior Member

    Mmm - but this tug delivers cargo over long distances. Fuel consumption is an issue. What would the percentage loss of thrust be between 0 and 1.7 m/s hull speed?
     
  10. michaeljc
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    michaeljc Senior Member

    Feathering paddles have moving mechanical components. Messy.

    We see forms of fixed hydrofoils being used. This is to minimise the effects of changing of angles of attack, I believe. How effective are these?
     
  11. Jeremy Harris
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    Jeremy Harris Senior Member

    Let's pick an intermediate slip condition as the above rough estimates gave two wildly different paddle sizes. Using the assumption that blades with a total effective width of around 6m, immersed to a depth of 0.6m as before, might be acceptable (which might end up as side wheels around 2m wide or so each in practice), then to deliver 15,000 N bollard pull at zero boat velocity would need a paddle velocity, V of about:

    V = SQ RT (F / (0.5 x rho x Cd x A)) = 3.23 m/S (where A = 6m x 0.6m)

    The paddle slip ratio would be 100% under this condition.

    As the boat accelerates the thrust drops, so at 1.7m/S the differential velocity between the paddle and boat speed = 3.23 - 1.7 = 1.53m/S

    The thrust produced with a 6m effective width blade, immersed to 0.6m, with a relative velocity of 1.53m/S would be:-

    F = 0.5 x 1000 x 0.8 x (6 x 0.6) x 1.53² = 3370 N

    So, the percentage drop in thrust from the zero velocity bollard pull condition to the 1.7m/S boat speed condition is about 88%.

    This only applies to this set of conditions, though, which were randomly picked. The process of design would be iterative, in order to work through each of the trade-offs between efficiency, blade width, wheel diameter and wheel rpm.
     
  12. michaeljc
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    michaeljc Senior Member

    This is interesting. To utilise HP effectively the engine needs to increase RPM as hull velocity increases. Maximum torque demand is going to be at 0 rpm (I think). This suits steam as it has maximum torque at 0 RPM but for diesels it may be problematic.

    Something has to slip or the motor will stall i.e. if its' gear ratios are set up for most efficiency at 1.7 m/s.

    In short: a screw prop set up to deliver most thrust at 1.7 m/s will readily slip at 0 hull speed, but a paddle wont. If I am right in this then a motor would have to have a torque converter.

    The bottom line is that paddles deserve further consideration. I would love to do some tank testing with a scale model.
     
  13. michaeljc
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    michaeljc Senior Member

    On the other hand if the clutch was engaged at very low RPM then increased gradually the diesel may cope. I don't really know.
     
  14. Jeremy Harris
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    Jeremy Harris Senior Member

    Yes, paddles were a good match for steam, both in terms of speed and torque.

    A paddle will still slip at zero boat velocity, but will produce a lot of static thrust, and absorb a fair bit of torque, as it does so, whereas a prop might well stall and produce relatively little thrust under that condition (depending on the blade pitch and loading).

    If you could come up with an efficient variable speed drive to the paddle wheels then I think many of the potential problems can be overcome. There remain the physical limitations, like the size of the wheels, their susceptibility to damage from floating debris and the windage from the paddle housings, but these may be offset by the advantages of having high low speed thrust and shoal draft.

    One project boat I'd like to build one day is a small, efficient, low speed, electric paddle boat for inland waterway use. I have too many other things on the go to look at it at the moment, but maybe one day.
     

  15. michaeljc
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    michaeljc Senior Member

    How do you think paddles on a track mechanism running in a tunnel fore to aft may work? Clearly there will be some disturbance between paddles but who knows? The Americans have a light semi- amphibious swamp craft that uses this system. This reduces the wheel diameter and increases surface area running perpendicular to thrust direction.
     
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