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Pushing the river with boats

  1. Oct 17, 2011 #1
    There's a rather comical attempt being made in Bangkok to push the Chao Praya river out to sea faster using anchored/moored boats with the engines/props running.

    Here's a news clip:

    1,000 boats to push flood waters October 11

    BANGKOK: -- On October 11, a fleet of some 1,000 boats would help push water from the Chao Phraya River into the sea fast, Science and Technology Minister Plodprasop Suraswadi said Saturday.

    At the boat project launch held at Nonthaburi's Pakkred Pier with 40 boats attending, Plodprasop said that this voluntarybase project would help drain water three times faster, from two knots to six knots, and prevent floodwater from pouring into Bangkok City.

    He said the 1,000boat fleet would push water down to the sea on October 11, as to help lower flood in the Chao Phraya Riverside provinces such as Sing Buri, Angthong and Ayutthaya.


    -- The Nation 2011-10-08



    And a Video


    http://youtu.be/MLeywAoX19Q


    Is it just me or is this a silly idea?
     
  2. jcsd
  3. Oct 17, 2011 #2

    xts

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    It is not extremely crazy idea.

    I have no data about Chao Praya river, but it seems to be typical lowland large river.
    I may compare it with Wisła river near Warsaw. In a high water times the river carries 200 cubic metres of water per second, the slope in this region is about 1/2000 (10m per 20km the river makes in the city area). So in this area the gravitational power propelling the river is about 20MW. Your river seems to be wider, but slower - anyway, the order of magnitude is similar.
    If you add to this some additional power from boats which may be quite comparable to natural (1000 boats 50kW each make 50MW - 2.5 times more than natural propelling!), the river may really speed up significantly.

    Put numbers (discharge flow, slope, length to be cleaned) for your river to check if it makes sense.
     
    Last edited: Oct 17, 2011
  4. Oct 17, 2011 #3

    sophiecentaur

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    Think of the fuel bill with that lot running 24/7.
     
  5. Oct 17, 2011 #4
    it sounds really dumb.

    Props from boats spin very quickly which means lots of energy is wasted.

    And that energy is coming from gas via boat engines, which is inefficient again...
     
  6. Oct 17, 2011 #5

    DaveC426913

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    Just to stay on point, no one said anything about this being efficient. In a flood situation, energy efficiency is not exactly a priority. What is a priority is lowering the flood waters quickly to ameliorate the damage.

    The question at-hand is: would this technique be effective?
     
  7. Oct 17, 2011 #6
    Makes me think of the Halifax Harbor Ferry which uses a circulating paddle drive. I think it could push enough water to make a difference if you parked it in a river channel. I don't know about prop drives but I suspect that if you anchor a boat securely and just use the engine to push water you would dig a decent hole in the water. If it squats a 15 foot boat 1 foot when you run the motor full throttle at a standstill (seems reasonable) that would seem to indicate a respectable increased flow rate. I actually expect it might be more efficient than pumps and hoses which lose a whole lot of energy in pipe turbulence.
     
  8. Oct 18, 2011 #7
    Everything can happen in Thailand. Lol
     
  9. Oct 18, 2011 #8
    I can understand that after the boat initially slows down the flow and water is forced to go around it, that the prop can then increase the flow in a small area.

    But 20-30 meters down the river, this force is absorbed. I don't see any possible way that these props are pushing the entire river that is currently downstream any faster. The props only move a little water at and below the point of contact. The kilometers of river downstream from these boats is surely not being pushed any faster. If they just pushed more at the surface then we would see a tidal wave effect, with clearly isn't happening and if it did would cause more water to overflow over the banks and result in more flooding.
     
  10. Oct 18, 2011 #9

    xts

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    Polish firemen use small flat-hull motorboats equipped with special diffusor shaping the stream of water, which may be lowered to the propeller. They were used during floods a year ago not only to drain flooded areas and streets of flooded towns, but also to improve flow in channels, narrows and gaps under bridges. Single such boat pumps 0.5m3/s at high speed. I guess ordinary boat without such diffusor is less efficient, but on other hand may pump more water at lower speeds - which is better on large slow river.
    wjc7qf.jpg
     
    Last edited: Oct 18, 2011
  11. Oct 18, 2011 #10

    A.T.

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    Every boat has a maximum speed where the hull drag cancels the propeller thrust (let's ignore aerodynamic drag here). So if the river is already flowing at the maximal speed of the boat, an anchored boat pointed upstream with engine running will not transfer any net momentum to the water. And the river in the video is in fact flowing quite fast, but I think not as fast as a boat would go. And the boats would be parked in the river anyway, so compared to not running their motors there is a net transfer of momentum to the water.
     
  12. Oct 18, 2011 #11

    Low-Q

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    The answer will depend on the existing mass of water. Say the river is 100m wide, and 1m deep. The project will be less effective if the river is 10m deep.

    Who knows. Those "logs" with a V8 engine they use in these countries can for sure move lots of water...

    Vidar
     
  13. Oct 18, 2011 #12

    DaveC426913

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    Agreed, there is definitely a transfer of momentum to the water. But that does not necessarily result in a faster drainage rate of the river.

    Imagine a worst case, where the higher volume of water is simply piling on top of the slower moving water up 50m downstream of the boats, and backflowing. The energy from the boats is going into nothing more than creating a standing wave.

    I'm not saying that is what's happening, I'm saying that showing an increased flow rate locally is not enough to call this a successful proof of concept.
     
  14. Oct 18, 2011 #13

    xts

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    @HomeyG:
    as you've posted Thai TV clip, I assume you speak Thai.
    Could you find some basic data about Chao Praya (average discharge flow, main stream velocity, average width and depth, level difference and length in the endangered city area)? I couldn't find it in English services, but maybe those are available in Thai from local hydrology office, or just from Thai wiki.
    In English wiki I only found average discharge of 720m3/s and very small slope: 25m/370km = 1/15,000.

    @Dave: there are always some bottlenecks blocking the flow. In case of Polish floods last year, those were mostly gaps under bridges: 18th century heavy stone constructions, built where the river is narrowest. The level difference at the bridge was often of the order of 20cm. So anchoring a boat-pump under a bridge, was not creating of standing wave, but rather unloading such a wave created naturally.

    Of course - those floods took place on much smaller (but quicker) rivers than in Thailand - that's why I asked Homeyg for river data to compare.
    I still believe that a 1000 boats smartly located in bottlenecks in river estuary, could have a measurable contribution pumping its waters to gulf of Thailand
     
    Last edited: Oct 18, 2011
  15. Oct 18, 2011 #14

    A.T.

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    To flow back or even to stop, the accelerated water would have to transfer its momentum to that slower water 50m downstream, which would be accelerated itself.
     
  16. Oct 18, 2011 #15

    DaveC426913

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    No it wouldn't; the energy would go into pushing water up - which uses a lot of energy, but is useless for our purposes.
     
  17. Oct 18, 2011 #16

    sophiecentaur

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    Is there a lot of point in taking this discussion much further without some knowledge of the profile of the river bed and cross section downstream? Detail is everything in a situation like this. This has an electrical analogue somewhere (haha) but we'd really need to know the details of all resistances, capacitances and inductances as well as the emf and internal resistance of our 'extra power source', before we could have a clue about the change in current produced.
    There could be instances in which a bit of a helping hand would make a huge difference and instances where you would be 'pushing water uphill with a rake'.
     
  18. Oct 18, 2011 #17

    A.T.

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    I'm not talking about energy. I'm talking about momentum.
     
  19. Oct 18, 2011 #18
    I cannot read Thai, but I've asked a few people to help me and have done a little searching/translating on the web.

    This info is from an English page on the lsu.edu domain:

    http://www.geol.lsu.edu/WDD/ASIAN/Chao Phraya/chao_pharya.htm


    "Annual average river discharge is 883 cu m/sec and the discharge range is 2,838 cu m/sec."

    "The depth of the river ranges from 5 to 20 m and the width ranges from 200 to 1,200 m."


    This next one is a more current picture from a Thai website translation although I'm sure the levels are actually higher now. The year 2554 is 2011.

    "The Department also reported the amount of water in the river that flows through the critical point, as at 1 October 2554 that the water flowing through the measuring point on the Chao Phraya Nakhon Sawan 4344 cubic meters / second."

    It's a bad translation but good enough to understand the data (4344).
     
  20. Oct 18, 2011 #19
    Propellers aren't really designed to move water, that is a side effect. Propellers are designed to create a pressure difference between the forward side of the blade and the rearward side. It is this pressure difference that causes thrust. In the case of a stationary propeller there will be a high pressure region behind the propeller and a low pressure region in front of it. The water will take whatever path it can to flow out of the high pressure region and into the low pressure region. Part of this flow will be from upstream into the low pressure region and downstream from the high pressure region. Part will be from the high pressure region around the edges of the propeller and back into the low pressure region. The question is how much of the propellers energy will move the river and how much will just push the water around in circles. I suspect most of the momentum will be water moving around in circles.

    That being said, it wont be 100% ineffective. Flooding is orders of magnitude more expensive then operating boat engines so by all means, crank 'em up and let 'em rip.
     
  21. Oct 19, 2011 #20

    A.T.

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    That doesn't change the fact that they do accelerate the water. There is no way around momentum conservation.

    This is the same pointless cause-argument known from the endless discussions about "How do wings generate lift?". Is it because they generate a pressure difference, or is it because they accelerate the air downwards? Answer: Both happens and nature doesn't care what humans accept as an intuitive "cause". And neither does physics, it just describes the relationships between those quantities.

    Momentum cannot "move around in circles". It is a vector and it is conserved as a vector. Apparently you are confusing momentum with kinetic energy, just like Dave.
     
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