Can 1,000 Boats Really Push the Chao Praya River Out to Sea Faster?

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In summary, this comical attempt to push the Chao Praya river out to sea faster using anchored/moored boats with the engines/props running is being attempted in Bangkok. The boats are likely to be inefficient and the technique is not likely to be effective.
  • #1
HomeyG
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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?
 
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  • #2
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.
 
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  • #3
Think of the fuel bill with that lot running 24/7.
 
  • #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...
 
  • #5
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?
 
  • #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.
 
  • #7
Everything can happen in Thailand. Lol
 
  • #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.
 
  • #9
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.
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  • #10
HomeyG said:
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.
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.
 
  • #11
DaveC426913 said:
The question at-hand is: would this technique be effective?
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
 
  • #12
A.T. said:
...compared to not running their motors there is a net transfer of momentum to the water.

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.
 
  • #13
@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
 
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  • #14
DaveC426913 said:
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.
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.
 
  • #15
A.T. said:
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.
No it wouldn't; the energy would go into pushing water up - which uses a lot of energy, but is useless for our purposes.
 
  • #16
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'.
 
  • #17
A.T. said:
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.

DaveC426913 said:
No it wouldn't; the energy would go into pushing water up - which uses a lot of energy, but is useless for our purposes.

I'm not talking about energy. I'm talking about momentum.
 
  • #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).
 
  • #19
A.T. said:
I'm not talking about energy. I'm talking about momentum.

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 won't 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.
 
  • #20
mrspeedybob said:
Propellers aren't really designed to move water, that is a side effect.
That doesn't change the fact that they do accelerate the water. There is no way around momentum conservation.

mrspeedybob said:
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.
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.

mrspeedybob said:
I suspect most of the momentum will be water moving around in circles.
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.
 
  • #21
I'm thinking the anchored boats will also slow the flow of the river. 40 small hulls?Subtract that from the process. What is the efficiency of a boat motor? I'm sure they all have differences. Friction from the hull, turbulent flow displacement of the water with the additional friction caused by that. Here's what I see. If you jump right before a falling elevator hits the ground you will reduce you velocity by the speed of your jump minus the acceleration of your mass pressing on the elevator floor. Wait, they're are the same. By anchoring or tethering, I can transfer some of that force into the shore but really? The difference isn't worth calculating and is probably impossible to calculate in the real world. Then again, never underestimate the power of positive thinking.
 
  • #22
brettjor said:
I'm thinking the anchored boats will also slow the flow of the river.
But they are anchored there anyway, so any acceleration from the motors is a gain compared to not running them. I addressed the points you make in post #10:
https://www.physicsforums.com/showpost.php?p=3565102&postcount=10
brettjor said:
The difference isn't worth calculating and is probably impossible to calculate in the real world.
But it is very easy to see if such an anchored boat accelerates or slows the flow. Just look in which direction it pulls the anchor ropes.
 
  • #23
brettjor said:
I'm thinking the anchored boats will also slow the flow of the river. 40 small hulls?Subtract that from the process. What is the efficiency of a boat motor? I'm sure they all have differences. Friction from the hull, turbulent flow displacement of the water with the additional friction caused by that. Here's what I see. If you jump right before a falling elevator hits the ground you will reduce you velocity by the speed of your jump minus the acceleration of your mass pressing on the elevator floor. Wait, they're are the same. By anchoring or tethering, I can transfer some of that force into the shore but really? The difference isn't worth calculating and is probably impossible to calculate in the real world. Then again, never underestimate the power of positive thinking.

By that argument, a boat couldn't be moved forward by its propeller. The friction against the hulls is no more relevant than all the friction and turbulence as the water flows against the bottom and sides of the river.


"transfer" of force with no movement is neither transfer of Energy nor transfer of Momentum You could just as easily have the propellors attached directly to the shore.
 
  • #24
Yup, I screwed that up. Of course the boats were already there. Of course they will accelerate the water. I let my incredulity get in the way. And of course, my incredulity got in the way of the obvious fact that this will make no difference. Oops, there it is again. ( I was thinking about the river, not the boats, again my obvious bad.)

I withdraw.
 
  • #25
A.T. said:
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.

Ummm...:uhh:
http://en.wikipedia.org/wiki/Angular_momentum
 
  • #26
And how would angular momentum help with making water flow down a river? Some would have to flow up at the same time. Rotation would not help at all - in fact, it would produce more turbulence - more energy loss.
 
  • #27
A.T. said:
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.

mrspeedybob said:

If not specified otherwise : "momentum" = "linear momentum"

And what is your point here anyway? The circular back flow around the propeller edges that you were referring to has zero total angular momentum, because it is symmetrical. And the circular back flow will not happen in a flowing river anyway. It can happen around static condition.The only angular momentum transferred to the water by the propeller is swirl. But that has nothing to do with "water going in circles".

The propeller is mainly adding linear downstream momentum to the water, and that momentum is conserved as a vector including the downstream direction. If some of the accelerated water should turn around due to some pressure differences within the water as you suggest, then its downstream momentum was simply passed to other water masses, that are further downstream. The only way that this added downstream momentum can leave the water is by interaction with ground fixed obstacles.
 
  • #28
Why do they need the whole boat? Why don't they just take the engine out? If you hold it down, it doesn't need to float.
 
  • #29
In my opinion, even if the force was perfect and they had some sort of diffuser setup, it wouldn't matter.

The increase in flow is only near the surface in a small area. There's no way they can "push" 20-30km of river that is downstream of these boats. Any acceleration is likely absorbed/mitigated by the resistance of the water. The economic phrase "pushing on a string" comes to mind.

This is a big wide meandering river, no where close to being uniform or straight. As I see it, the flow is a function of the channel shape and geometry, Roughness of channel, Sediment load, the overall volume-discharge of the stream, and elevation drop-gradient, which for the last dozen kilometers is quite minimal. I fail to see how a few boats 20-30k upstream are going to push this mass of water into the gulf any faster.

Sure you can accelerate the water directly behind the props, but how much real effect is this having on draining the river? I think if they measured flows even just a kilometer below the boats with the engines on, and then off, there would be no measurable difference.
 
  • #30
HomeyG said:
There's no way they can "push" 20-30km of river that is downstream of these boats.
They don't try to do that. They want to accelerate the flow in the bottlenecks of the city to reduce the water level in the city.
 
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  • #31
My understanding is that a bridge pier may slow down the water directly behind it, but doesn't slow down the whole river. After 30 meters or so, the effect is completely wiped out and the overall flow is not effected.

Why would a prop be any different? It's not. After a handful of meters the acceleration is completely absorbed as if it didn't happen. What possible good is it to increase the surface flow for such a small area?

Seems to me that they are lucky it doesn't work. If it did, if would be creating more surface waves and more water overflowing the banks. Remember this is not a uniform pipe, this is a meandering river with turns, changes in depth, width, and elevation.

Where are these bottlenecks? The whole area is in danger not just a few 30 meter spots along the river.
 
  • #32
jetwaterluffy said:
Why do they need the whole boat? Why don't they just take the engine out? If you hold it down, it doesn't need to float.

Because the boats are already there. The reason this is being experimented with is because it's quite easy to do with little additional cost.
 
  • #33
HomeyG said:
My understanding is that a bridge pier may slow down the water directly behind it, but doesn't slow down the whole river. After 30 meters or so, the effect is completely wiped out and the overall flow is not effected.
That's not true. Bridge pier does not slow flow - just contrary, water flows faster under the bridge than on open wider part. But it introduces significant resistance to the flow, effecting with damming of the water. You may better see it on smaller rivers flowing more quickly than Chao Phraya: the water levels before and after the bridge often differ by 20cm or more - which is equivalent to average level difference on over hundred meters along the river.

Boats (or other high flow pumps) installed in such bottleneck places may help to reduce (unload) this damming, thus making the slope higher on open area, which speed the whole river.

Such effect is even more important on very flat (low slope) rivers like Chao Phraya - here every 1cm saved on damming under a bridge increases then the natural gravity push for the river by 10% on next 2km.
Where are these bottlenecks? The whole area is in danger not just a few 30 meter spots along the river.
Bottlenecks are points where the flow encounters in a short area strong resistance, stronger, than average on hundreds meters, causing damming of water - even very small, like few mm, may be worth to reduce on such flat river.
 
  • #34
Thanks for the explanation XTS.

I still have a hard time believing boats are making any difference, but I appreciate you trying to explain it to me.
 
  • #35
Well - I am not sure how effective it may be on so large river like Chao Praya.
But it was used successfully on 100 times smaller (and much faster) rivers using just single pumping boats.
So I can't say if they may improve the flow by 1% or by 10% using that 1000 boats. But for some people even 1% improvement may mean that their houses stay dry - I saw TV this morning showing the water flooding streets of Bangkok already.
 

1. Can 1,000 boats really push the Chao Praya River out to sea faster?

It is possible for 1,000 boats to have an impact on the flow of the Chao Praya River, but it is unlikely that they would be able to push the entire river out to sea faster. The force of the boats would have to be significantly greater than the natural flow of the river, which is unlikely with only 1,000 boats.

2. How would 1,000 boats be able to push a river out to sea?

The force of the boats would create a disturbance in the water, causing the river to flow in a different direction. This could potentially lead to a faster flow towards the sea, but it would not be a direct pushing action.

3. Is there any evidence that 1,000 boats have ever pushed a river out to sea faster?

There is no scientific evidence or documented instances of 1,000 boats pushing a river out to sea faster. This is likely due to the fact that it is not a practical or feasible method for altering the flow of a river.

4. What is the Chao Praya River's normal flow rate?

The Chao Praya River has a variable flow rate, depending on the season and rainfall. On average, it has a flow rate of about 3,000 cubic meters per second.

5. Are there any potential negative impacts of using 1,000 boats to push a river out to sea faster?

It is possible that the use of 1,000 boats to push a river out to sea could have negative impacts on the river's ecosystem and the surrounding environment. The disturbance caused by the boats could disrupt the natural flow and habitats of aquatic plants and animals. It could also lead to pollution from the boats' engines and potential accidents or collisions.

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