Increase efficiency in the cargo ship industry without a propellor

[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?

 

[jbriggs444]

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?

A high performance sailboat on a perfectly calm day can sail upstream on a flowing river. An anchored craft cannot. We've had threads on this sort of thing in years past.

 

[russ_watters]

LOL Your post about the Panama canal was an attempt to dismiss mine about water resistance making below water tractors / trains not viable...

It still looks to me like you are being purposely obtuse (because if you knew that, why didn't you respond to it the first time?). This isn't hard:
1. You claimed that underwater mules would create a lot of drag based on their surface area.
2. I pointed out that underwater mules would be tiny compared to the ship they pull.

(which I actually tried to support with some math BTW).

Ok, fine. Using that equation, what is the difference in [viscous] water resistance between one vehicle and another with 1/100th the surface area?

Note also: from your link, page 10, the viscous resistance is a fraction of the total drag on a surface ship. Maybe half or less.

 

[A.T.]

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.

See my post #27.

 

[sophiecentaur]

See my post #27.

What is the actual point of your idea / comment? On a windless day, even with a yacht with very low windage, your velocity over ground will still be backwards and greater than zero. How could that be better than dropping anchor, if you want to go upstream? Progress up tidal rivers used to involve dropping anchor during an ebb tide and lifting anchor during the flood. Wind achieved little more than allowing boats to steer better.

As a past 'Broads Sailor'* I know about tacking upstream but even that process requires some wind and it can be a largely pointless exercise.
*The Norfolk Broads is a system of recreational inland waterways in the UK.

 

[sophiecentaur]

A high performance sailboat on a perfectly calm day can sail upstream on a flowing river. An anchored craft cannot. We've had threads on this sort of thing in years past.

I don't get this. No wind (aka calm) means no force from the wind. Where would an upstream force come from? (Inshore eddies don't count in this.)
Sailing 'into the wind' (in the quoted thread) can only be achieved if there's actually some wind. It is a pretty marginal process and would seldom beat a significant water flow.

 

[A.T.]

On a windless day, even with a yacht with very low windage, your velocity over ground will still be backwards and greater than zero.

This is not true. Did you read my post #27:

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.

The key is not just low windage, but high lift to drag ratios in both: air and water.

 

[A.T.]

I don't get this. No wind (aka calm) means no force from the wind.

See post #24:

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

 

[sophiecentaur]

"so there is wind relative to the water." Yes I agree, however:
This is a 'theoretical' point, concerning specialist craft but does it have a lot of relevance to a real water transport situation? The thread title refers to Cargo Industry rather than some special designs of land yacht.
An anchor is very cheap and all craft (should) have one. The crew can be mostly stood down when anchored (apart from checking the oil in the lamps). 'Sailing' at a fraction of a kt with full crew would be hard work when, in a few hours, the current would be taking you upstream at 5kt or more.

 

[Arjan82]

Let's simplify the case to the basics:
Will a towed ship be more efficient than a propeller driven ship at equal speed compared to the shore? So disregard all other factors.

First, let's split the efficiency of the propeller in mechanical efficiency and hydrodynamical efficiency. Mechanical efficiency is the efficiency from energy content of the fuel to the torque + rpm on the propeller, the hydrodynamical efficiency is the efficiency from torque + rpm on the propeller to thrust and speed.

A hydrodynamical efficiency of 80% of an inland ship is actually rather high. Somewhere between 60-65% is not uncommon actually. This is because of the draught requirement forcing them to have smaller than optimal propellers, also the propeller is surrounded by tunnels and many rudders because maneuvering is very important for them (often two rudders per propeller, sometimes rudders in front of the propeller for backing performance).

Then there is another effect, called thrust deduction. The propeller is situated aft of the ship which means that it is generating a low pressure area on the ship aft body. This is an extra drag an can account for 10% or more extra required thrust. In other words: the towing force on a ship can be more than 10% lower than the thrust a propeller needs to provide.

Let's also assume that you can take off all appendages of the ship regarding propulsion: less rudder area (you need a bit to keep course) no tunnels, no shafts, tunnel thrusters etc. This reduces the resistance even further.

So, let's now also assume that the mechanical efficiency of the tower (mule?) is in the same order of magnitude of the engine of the propeller driven ship (why wouldn't it be?). I would also assume that the efficiency of torque + rpm on the wheels to the track is much higher than the hydrodynamic efficiency of a propeller.

So, this all points into one direction. Using a mule would indeed be significantly more efficient (in terms of dollars per mile) than a ship propeller, comparing at equal speed along the shore.

Should the mule be underwater or on the shore? Underwater has many practical issues, including it's own resistance that is not negligible anymore. But a mule on the shore is always towing with an angle, meaning a transversal force is applied on the ship which needs to be counteracted. This will cost efficiency. However, this is many orders of magnitude more practical than underwater.

So, why aren't we doing this? Probably because of the many practical issues and high costs surrounding building this track. Using the costs of building and maintaining this track you can make up for a lot of propeller propulsive efficiency.

 

[BWV]

It’s not clear to me that energy saving is the primary rationale for mules that exist in places like the Panama Canal, or whether other considerations like running propellers designed for deep ocean crossing in a narrow loch come into play. As I posted before, the current speeds in navigable rivers tend to be low ~ 1 knot for the Mississippi for example. Would the economics change, say for the navigable portion of the lower Congo where the river can average over 8 knots? However the Congo is also extraordinarily deep >700ft

if there is a significant disadvantage to a transverse force from the shore line, ISTM that it would still be easier and more efficient to construct some sort of surface - level towing system, pulling the boat upstream, perhaps from a series of bridges or pylons

Finally, any fixed towing system creates bottlenecks as the capacity will likely be lower than what the channel could support for propeller driven craft. Not an issue for single file locks in the Panama Canal, but likely a deal killer in a wide river like the Mississippi which can accommodate a high amount of traffic

 

[jbriggs444]

but likely a deal killer in a wide river like the Mississippi which can accommodate a high amount of traffic

I grew up in a city on the banks of the Mississippi. One shudders to contemplate the effort that would be involved to construct and maintain a tow path on low muddy banks and across tributaries in order to tow barges on a very wide river subject to floods and shifting sand bars.

 

[Arjan82]

The mules that exist in the Panama Canal are for careful manoeuvring. Ships are built to the size of the Panama Canal (bottle neck being the locks, which is where the mules are) called Panamax.

A ship that big has only 2ft space at each side (!) and 10ft at the bottom. This means that if the ship is going into the lock, most of the water in there must go out through these small gaps... This is really pushing a cork into a bottle... Because the tiny margins of error this is much easier to do with mules than with tugs, let alone it's own propulsion (not many ships of that size even moore on their own power anyway).

Furthermore, I don't see how current really changes the outcome for this discussion since any form of propulsion must move the ship through the water, whatever the speed of the water compared to the shore (unless you want to compare energy usage when not moving compared to the shore, in which case the mule, or anchor for that matter, is the clear winner).

 

[Arjan82]

This is how that looks by the way:

 

[sophiecentaur]

I imagine there could be some journeys where shore based traction could be provided. You could imagine overhead cables, not high above the barges and these could be mounted on piers on the navigation channel, well away from tidal, sloping banks. A number of cables, in parallel, could be run to take different convoys up and down a waterway. The piers would need to be pretty sturdy but they wouldn't all have to be driven. Something like a cable car system.
But any design would need to pay for itself (or the shareholders,)

 

[A.T.]

This is a 'theoretical' point,

This is what happens when an engineering question is posted in the physics subforum.

'Sailing' at a fraction of a kt with full crew would be hard work when, in a few hours, the current would be taking you upstream at 5kt or more.

The most efficient water boats achieve downwind VMG > 2x windspeed. So in a downstream current of 5kt, they could go upstream at 5kt relative to the shore on a calm day.

 

[sophiecentaur]

The most efficient water boats

We're talking about water transport. Efficient hulls are not too good when they need to carry a big load. What you are suggesting is worse than just marginal, as soon as you bring practicalities into the situation. Slow water transport, in the form of a canal network, doesn't fill the bill these days. If you can't carry stuff significantly faster than the 4mph average that canal transport was limited to then it's a non starter. The tugs used on fast moving rivers (like the Rhine) allow short enough journey times for the goods in the barges. Excessive bufferage with fleets of barge flotillas isn't acceptable so speed counts.

 

[sophiecentaur]

The most efficient water boats achieve downwind VMG > 2x windspeed. So in a downstream current of 5kt, they could go upstream at 5kt relative to the shore on a calm day.

etc. etc.
But wouldn't that imply a speed through the water of more than 5kt? What sort of Power would be needed to exceed (double?) that for a loaded barge? I mentioned land yachts in my initial response to sailing upwind. Land yachts have very little drag and so do 'very efficient water boats', which do little more than support a mast and a helmsman.

Actually, this avenue of thought would be a non starter in the case of most inland waters. Even a modest sailing boat mast wouldn't clear bridges on most rivers. I once looked into sailing up the Thames with an air draft of just 10m. Not worth trying except at low tide. Bridges are already established and were chosen to suit existing air drafts.

 

[jbriggs444]

But wouldn't that imply a speed through the water of more than 5kt?

Yes. 10 kts water-relative velocity made good. Which means a water speed even higher than that. So?

 

[sophiecentaur]

Yes. 10 kts water-relative velocity made good. Which means a water speed even higher than that. So?

So what about the drag of the hull? The speed against the wind is not an absolute. Moving a cargo through the water uses massive engines. Upwind experiments are all done with small hulls. I can’t see any equivalence to cargo transport without acres of sail. And that is not possible on rivers with bridges.

 

[jbriggs444]

So what about the drag of the hull? The speed against the wind is not an absolute. Moving a cargo through the water uses massive engines. Upwind experiments are all done with small hulls. I can’t see any equivalence to cargo transport without acres of sail. And that is not possible on rivers with bridges.

The design should be such as to minimize hull drag, yes. The speed against the wind will not be absolute, yes. It will, in particular, depend on wind speed. And likely surface conditions. Yes, I do not see any sweet spot where this is economical for cargo transport. In general, sail fell out of favor for cargo transport long ago.

 

[A.T.]

In general, sail fell out of favor for cargo transport long ago.

Of course. My mention of modern sail boat capabilities was merely a physically interesting side note in reply to the historical account about Cartier sailing up the Saint Lawrence. Not a realistic proposal for cargo shipping.

There is a classic puzzle related to this, which I just posted here:
https://www.physicsforums.com/threa...classical-physics-puzzles.999164/post-6456536

 

[Baluncore]

If the tracked tractor runs on the river bed. Will it be approved by the environment department, or will it stir up too much mud while killing fish.

Track ground pressure will be less underwater due to buoyancy of the tractor. The tractor would need to carry a dense ballast.

A tow line from the tractor to the barge will rise diagonally. As the pull becomes greater, or the water deeper, the tractor will tend to be lifted off the bottom. (A farm tractor hitch or plough is designed so that the drawn load will pull the rear drive wheels down onto the ground, to increase traction).

A light weight underwater, tracked pusher could to be mounted on a rigid arm that hangs below, and rearwards, from near the front of the barge. That way the diagonal compressive force applied by the tracks will increase the track ground pressure, and lift the barge, helpfully reducing the displacement of the hull.

 

[hmmm27]

Unless the canal is actually a submerged concrete road, an underwater tractor will just trash everything, not only in the vicinity, but downstream for a long ways. Even if you're warped enough to think that's a decent tradeoff, it will also stop working after a couple trips, when the tractor starts bottoming out in the ruts.

What about a submerged rail ? Or, overhead, like a trolley bus.

 

[sophiecentaur]

Overhead gets my vote. Underwater would be a nightmare. But overhead would look unsightly.

 

[anorlunda]

There are two ways to interpret the word tracked.

One is tracked like a bulldozer or a army tank.

The other is tracked like a railroad with underwater rails.

Underwater rails would quickly sink in the silt and in my opinion are a nonstarter.