Increase efficiency in the cargo ship industry without a propellor

[MrNewton]

Not sure if this is the right subforum to place this question?

I stumbled on this idea, to make the cargo ship industry more efficient.

The propellor of a cargo ship (<100m long) is relative inefficient, around 80% ( i know, this is pretty efficient for an boat propellor), and the harder the current of a river flows, the harder the ships engine has to work to maintain the same speed relative to the shore. The idea is to use a tracked vehicle on the bottom of the river to pull the ship against the current of the river.
If the current is flowing with 5kmu against the direction of the ship, and the ship wants to move with 5kmu relative to the shore, it has to use power on its propellor as it is moving 10kmu without current.
With this tracked vehicle, the speed of the current becomes a much less important factor. If it wants to move 5kmu relative to the sure, it just does.
Offcourse the current here is still important because the load of the tracked increases if the current increases and the ship gets "heavier".

The idea is, because this vehicle moves relative to the non-moving bottom of the river, instead of the propellor which is moving relative to the flowing water, it will be much more efficient. This vehicle will be poweren by 2 electric motors with gear reduction (offcourse).It sounds like a ambitious project, and in my mind the theorie works, but what do you think the result will be in real life? Offcourse there will be a lot of factors that has to be calculated, like the materials of the bottom of the river, the length of the cable.

My question: Do you think that the concept can work? Or am i missing something important?

 

[A.T.]

Offcourse there will be a lot of factors that has to be calculated, like the materials of the bottom of the river, the length of the cable.

Since the aim is to increase efficiency, you should check the efficiency of tracked vehicles on land. Then consider how an unpredictable, varying riverbed would affect it.

 

[anorlunda]

The idea is, because this vehicle moves relative to the non-moving bottom of the river, instead of the propellor which is moving relative to the flowing water, it will be much more efficient.

That is not necessarily true. You would have to prove that.

 

[Halc]

This is how it was done in the early days with barges powered by mules. I think it might be better implemented as a track on the shore along which such a standard vehicle might operate, rather than designing a general purpose tractor that can deal with an unpredictable bed.
I've seen tugs pulling strings of over a dozen barges up the Rhine/Neckar and marveled at the power required to fight the high current that seemed ever-present. I'm currently near an estuary (which used to be named Mahicantuck [river that flows both ways] by the natives of the area) where, even 200 km inland, the current runs both ways making it little problem for underpowered ships. Unlike the German rivers, there are only a few narrow spots where the current is significant.

Another instance is the USS Constitution escaping a British squadron in July, 1812 by using such a technique in shallow seas when the wind was nearly nonexistent. In this case, the energy was human power, but much more efficient than rowing since the force was exerted against the sea bed instead of the water. Of course it also helped that they were shooting at each other, which hinders the pursuers and helps the pursued.

 

[nasu]

By "tracked vehicle" you mean a vehicle running on some underwater rails?

 

[russ_watters]

I stumbled on this idea, to make the cargo ship industry more efficient.

The propellor of a cargo ship (<100m long) is relative inefficient, around 80% ( i know, this is pretty efficient for an boat propellor), and the harder the current of a river flows, the harder the ships engine has to work to maintain the same speed relative to the shore. The idea is to use a tracked vehicle on the bottom of the river to pull the ship against the current of the river.
If the current is flowing with 5kmu against the direction of the ship, and the ship wants to move with 5kmu relative to the shore, it has to use power on its propellor as it is moving 10kmu without current.
With this tracked vehicle, the speed of the current becomes a much less important factor. If it wants to move 5kmu relative to the sure, it just does...

My question: Do you think that the concept can work? Or am i missing something important?

It isn't efficiency you are trying to improve per se, but rather you are trying to reduce the work requirement to move a ship. Like making a car more aerodynamic vs making the engine more efficient. So you need to ask yourself (calculate) if this method changes the work requirement. If it does, then maybe you have something here.

 

[Vanadium 50]

A railroad works on land because the rail reduces frictional forces - i.e. forces that oppose the direction of motion. Here those forces come from the water the ship is in. Those forces are the same whether the ship is under its own power or being towed by the riverbottom-tractor.

So what is the advantage here? Being quantitative is more convincing.

 

[Dale]

Those forces are the same whether the ship is under its own power or being towed by the riverbottom-tractor.

The forces are the same, but the distance is different. The track doesn’t move, so none of the energy goes into accelerating the track and it all goes into moving the boat. The water moves so a lot of the energy goes into accelerating the water rather than moving the boat.

 

[jbriggs444]

The forces are the same, but the distance is different. The track doesn’t move, so none of the energy goes into accelerating the track and it all goes into moving the boat. The water moves so a lot of the energy goes into accelerating the water rather than moving the boat.

The scheme would have greatest benefit for an under-powered craft going upstream on a flowing river where the ground speed is a small fraction of the speed relative to the flowing water. For a craft moving at 1 meter per second ground speed upstream against a 2 meter per second downstream flow, that offers [on paper at least] a 3 to 1 efficiency boost.

 

[BWV]

Cargo ships don’t spend a meaningful amount of time in rivers, you mean barges?

 

[Vanadium 50]

Barges are a good way to think about it. The question then becomes about the efficiency of tugboat vs. tractor.

It's not clear to me how the tractor is even powered. Batteries? Not for long. An IC engine? Where does the air come from? (It's also not clear why it needs to be underwater and not on a riverbank)

 

[Dale]

It's also not clear why it needs to be underwater and not on a riverbank

I agree with this. To me this is just a standard canal barge towpath:

https://en.wikipedia.org/wiki/Towpath

I am not sure why this method fell out of favor, but it clearly did quite some time ago.

 

[BWV]

I agree with this. To me this is just a standard canal barge towpath:

https://en.wikipedia.org/wiki/Towpath

I am not sure why this method fell out of favor, but it clearly did quite some time ago.

Hard to see that working on the Mississippi, and a standard arrangement is one tugboat to 15 barges:

Coal-carrying barges move in tows of fifteen to forty barges, pulled by a single towboat of 2,000 to 10,000 hp. A "jumbo"-size barge carries 1,800 tons (1,633 tonnes) of coal, so a large tow can move 72,000 tons (65,304 tonnes) of coal, as much as five unit trains. These large volumes result in significant economies of scale. Barge rates can run (on a cost-per-mile or cost-per-kilometer basis) a quarter or less of rail rates.

https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/coal-transportation-and-storage#:~:text=Coal%2Dcarrying%20barges%20move%20in,much%20as%20five%20unit%20trains.

 

[Ibix]

I am not sure why this method fell out of favor, but it clearly did quite some time ago.

You have to maintain the towpath and the terrain isn't always suitable. When you're using muscle power you have no choice, but a steam boat can go where the river goes with no infrastructure maintenance costs. That's not to say we shouldn't reexamine it, at least in some cases.

As an aside, I remember my Dad pointing out a roving bridge when I was a boy, and that little flash of understanding of how you can make a simple design decision and save everybody's time.

 

[DaveC426913]

If the current is flowing with 5kmu against the direction of the ship, and the ship wants to move with 5kmu relative to the shore, it has to use power on its propellor as it is moving 10kmu without current.
With this tracked vehicle, the speed of the current becomes a much less important factor. If it wants to move 5kmu relative to the sure, it just does.

In addition to what others have said, note that "speed relative to shore" is a meaningless metric. It makes no difference to the ship what the land around it is doing; the ship only knows its journey's total distance. Going up-river simply makes the total distance a bit longer.
(Akin to a plane in a head wind - it doesn't matter what its speed is relative to the ground - it merely makes for a longer journey).

That being said, there are two things that mitigate this:
1] If the current is fast enough, it can dramatically slow or even stop underpowered vessels from transiting the river.
2] Due to the proximity of land there is an opportunity to save onboard fuel by transferring effort to a shore-bound mechanical mechanism.

So your idea isn't invalid, but your metric for assessing efficiency/success is incorrect.

 

[anorlunda]

The water moves so a lot of the energy goes into accelerating the water rather than moving the boat.

Therein the 80% efficiency of a marine propeller according to the OP (which I have not verified.)

That says nothing about the efficiency of an underwater bulldozer, gear reduction, and tow line.

1] If the current is fast enough, it can dramatically slow or even stop underpowered vessels from transiting the river.

That's the key point. My boat been stopped and even driven backward by an opposing current, so the power efficiency goes through zero into the negative region. But a tractor could also be stopped and driven backward.

But that's not enough to presume that the tractor would be more efficient. There are lots of sources of inefficiently there also.

The key parameter when fighting opposing currents is transit time. The more time you spend in that state, the more the energy losses. A warship able to do 40 knots, might be more efficient in opposing currents than a rowboat. The key is speed, not propulsion method.

So to get through with minimum energy, I would use the propeller, plus an underwater tractor, plus towpath tractors, plus sails, plus any other form of propulsion that would get me through it faster.

In estuaries, where the current reverses, the canonical tactic is to anchor when current opposes, and to make way only when current follows. Cartier was able to sail against fierce currents up the Saint Lawrence to the site of Montreal in the year `1535.

 

[A.T.]

In estuaries, where the current reverses, the canonical tactic is to anchor when current opposes, and to make way only when current follows. Cartier was able to sail against fierce currents up the Saint Lawrence to the site of Montreal in the year `1535.

A modern hi-performance yacht could sail against the current on a windless day.

 

[berkeman]

A modern hi-performance yacht could sail against the current on a windless day.

Sorry, how does a boat sail on a windless day?

 

[BWV]

The key parameter when fighting opposing currents is transit time. The more time you spend in that state, the more the energy losses. A warship able to do 40 knots, might be more efficient in opposing currents than a rowboat. The key is speed, not propulsion method.

So to get through with minimum energy, I would use the propeller, plus an underwater tractor, plus towpath tractors, plus sails, plus any other form of propulsion that would get me through it faster.

water resistance increases proportional to the square of velocity, how does increasing speed help? Cant believe I am lecturing someone on basic physics here.
If you are going up river all you care about is $ / mile (which equates roughly to energy spent).

 

[berkeman]

If you are going up river all you care about is $ / mile (which equates roughly to energy spent).

There are other costs too, which ratio with transit time. Like paying the crew for example. I don't know what the final equation would look like, but going real slow to save fuel may not give the optimum cost/mile...

 

[BWV]

actually, doesn't the whole issue of water vs air resistance make the original idea just dumb, relative to towing from the shore? Powering some sort of submarine tractor through the water is going to take way more energy than using a propeller.

 

[BWV]

There are other costs too, which ratio with transit time. Like paying the crew for example. I don't know what the final equation would look like, but going real slow to save fuel may not give the optimum cost/mile...

Sure, there is some tradeoff, but are we talking a brief passage of an ocean-going vessel through an estuary to reach port, or upstream barge traffic on a river? How fast can this underwater tractor conceivably travel? ignoring modern subs with supercavitation and nuclear power, WW2 subs traveled maybe 8 knots underwater, a reasonable upper limit for our tractor. How fast are these currents? The Mississippi flows at about 1.2 MPH - how big of deal is a speed reduction of about a single knot?

 

[russ_watters]

water resistance increases proportional to the square of velocity, how does increasing speed help? Cant believe I am lecturing someone on basic physics here.
If you are going up river all you care about is $ / mile (which equates roughly to energy spent).

Please slow your roll.

On a glassy lake, the optimal speed is arbitrarily low, but not on a river.

Take the limiting condition: zero speed or arbitrarily low speed. Resistance is zero/close to zero. But it's a river, so the ship is moving backwards. Or if the propelled speed matches the river's speed, the ship sits still and the energy requirement is infinite. There's a sweet spot here; a speed at which the energy requirement is minimized. What that speed is would depend on the ship and speed of the river.

actually, doesn't the whole issue of water vs air resistance make the original idea just dumb, relative to towing from the shore?

The OP apparently didn't think of towing from shore. Yes, towing from shore would be more efficient than an underwater tow system. But not necessarily substantially different if the towing device is small.

Powering some sort of submarine tractor through the water is going to take way more energy than using a propeller.

No way. We can make guesses about efficiency, but the key issue here is the actual propulsive energy required is lower for a tractor than a propeller. The force is the same, but the tractor travels a shorter distance (in the ground frame) than the ship does (in the river frame). This was discussed above.

 

[A.T.]

A modern hi-performance yacht could sail against the current on a windless day.

Sorry, how does a boat sail on a windless day?

There is no wind relative to the ground. But the boat is in a current, so there is wind relative to the water.

 

[russ_watters]

There is no wind relative to the ground. But the boat is in a current, so there is wind relative to the water.

The way I first read it, I thought you meant up-river. You don't, right? Still moving backwards with respect to the shore, just slower than the speed of the river?

 

[BWV]

Please slow your roll.

On a glassy lake, the optimal speed is arbitrarily low, but not on a river.

Take the limiting condition: zero speed or arbitrarily low speed. Resistance is zero/close to zero. But it's a river, so the ship is moving backwards. Or if the propelled speed matches the river's speed, the ship sits still and the energy requirement is infinite. There's a sweet spot here; a speed at which the energy requirement is minimized. What that speed is would depend on the ship and speed of the river.

The OP apparently didn't think of towing from shore. Yes, towing from shore would be more efficient than an underwater tow system. But not necessarily substantially different if the towing device is small.

No way. We can make guesses about efficiency, but the key issue here is the actual propulsive energy required is lower for a tractor than a propeller. The force is the same, but the tractor travels a shorter distance (in the ground frame) than the ship does (in the river frame). This was discussed above.

Would be nice is someone could quantify this, I don't think the lower water resistance of a surface ship with a propeller vs and underwater tractor can be ignored even at the slow speeds discussed here. In reality, the answer is really it won't work because if it was viable, someone would have dun it already given the economic incentives

 

[A.T.]

The way I first read it, I thought you meant up-river. You don't, right? Still moving backwards with respect to the shore, just slower than the speed of the river?

No, I do mean moving up-river with respect to the shore on a windless day. Most sail boats cannot do this. But hi-performance racing yachts achieve downwind velocity made good greater than windspeed.

 

[BWV]

The viscous resistance of a ship (R) is 1/2CρV^2S,
where: C = coefficient of viscous resistance ρ = water density (lb-s2 /ft4 ) V = velocity (ft/s) S = wetted surface area of the underwater hull (ft2 )

so it scales proportionally to the underwater surface area regardless of speed- the trade off therefore is the increase in S from the tractor vs any gain in efficiency due to the shorter surface distance of the tractor. Tugboats have shallow drafts (as of course barges do) minimizing the contact with waterhttps://www.usna.edu/NAOE/_files/documents/Courses/EN400/02.07 Chapter 7.pdf

 

[russ_watters]

Would be nice is someone could quantify this, I don't think the lower water resistance of a surface ship with a propeller vs and underwater tractor can be ignored even at the slow speeds discussed here.

I feel like it should be self-evident. If someone really wanted to do this, I'd think they could install a track, similar to how ships are pulled through the Panama Canal. The tractors are tiny compared to a ship, and could be made quite hydrodynamic. Drag on the tug itself would be miniscule.

In reality, the answer is really it won't work because if it was viable, someone would have dun it already given the economic incentives

"won't work" and 'not viable' are totally separate issues. In reality, it would work if someone chose to do it.

 

[BWV]

I feel like it should be self-evident. If someone really wanted to do this, I'd think they could install a track, similar to how ships are pulled through the Panama Canal. The tractors are tiny compared to a ship, and could be made quite hydrodynamic. Drag on the tug itself would be miniscule.

The towing mules in the Panama canal are underwater? Have only seen above-ground trains

 

[russ_watters]

The towing mules in the Panama canal are underwater? Have only seen above-ground trains

Now you're being intentionally obtuse. Please stop.

 

[BWV]

Now you're being intentionally obtuse. Please stop.

Not trying to be, you just pick and choose what you want to pick at. Does the Panama canal have underwater towing vehicles or not? I thought that was the subject here. No one is arguing that above-ground tows are not useful

 

[russ_watters]

Not trying to be, you just pick and choose what you want to pick at. Does the Panama canal have underwater towing vehicles or not? I thought that was the subject here. No one is arguing that above-ground tows are not useful

I made my point very clear; it is you who chose to ignore it. Here it is again:

The tractors are tiny compared to a ship, and could be made quite hydrodynamic. Drag on the tug itself would be miniscule.

I didn't say they are currently underwater - that's a ridiculous thing for you to take from my post. I said they are small.

 

[BWV]

I made my point very clear; it is you who chose to ignore it. Here it is again:
The tractors are tiny compared to a ship, and could be made quite hydrodynamic. Drag on the tug itself would be miniscule.
I didn't say they are currently underwater - that's a ridiculous thing for you to take from my post. I said they are small.

LOL Your post about the Panama canal was an attempt to dismiss mine about water resistance making below water tractors / trains not viable (which I actually tried to support with some math BTW). The fact that the Panama Canal uses above ground trains is hardly support for the viability of underwater ones

 

[sophiecentaur]

I agree with this. To me this is just a standard canal barge towpath:

https://en.wikipedia.org/wiki/Towpath

I am not sure why this method fell out of favor, but it clearly did quite some time ago.

You have to maintain the towpath and the terrain isn't always suitable.

Fine in a non tidal river or canal but not workable in an estuary.

A modern hi-performance yacht could sail against the current on a windless day.

But that would achieve nothing compared with just anchoring to stop yourself going backwards. In any case, the current would always be accelerating any hull until it reaches the speed of the current. How, in any way, would that be a good thing?