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Chained to the real world

If you're used to trek on flat lands with rivers here and there, you might have seen these boats as replacement to bridges :

A chained ferryboat

A chained ferryboat, by Ludovic G. (resized photo, original picture on wikimedia commons)

In case you've never met these, don't worry, I wrote this section to explain how it works :).

These nice-looking ferryboats work with 2 long chains on each side that are each bound to one of the river banks. Their length is about the same as the distance to cross, and the chains will dive underwater if they're not fully stretched or kept on land/boat.

In order to cross the river, you first need to get the boat to your side (if it isn't already) by pulling the chain part standing on the land. When done, jump on the ferry and pull the other chain on the boat (the one linked to the other side you want to reach) and therefore pull yourself with the boat over the river. In other words, you reduce the chain's length, effectively reducing the distance between you and the ground.

Now these things are quite (read really) safe to maneuver around as you can't flip the boat or move it outside the metallic chains's range, and it is a relatively simple way to cross rivers without bridges. Plus, it's really fun to pick up the wet chains on the boat ^^.

The question

So far I've only seen these chained ferryboats for small distances (<25m), as one of its main weakness is that it's slow, especially if you need to get multiple groups on it (you double the distance as you push back the boat to its starting point). But I like them, and am thinking to improve on its effective range (or range at all if it is painfully ineffective) to use for some specific regions of my world.

So here's my relatively simple question. How far could we scale this kind of boat to move around? Could we cross a big lake with this? A sea? A whole ocean?!

It's important for me to keep the basic concept about chained ferryboats : One or more chains join the lands together, and a moving platform uses these chains to move safely around, without the common risk of being led astray with people having no experience in sailing. I'd like to avoid using external engines (e.g.: Propellers which don't interact directly with the chain) as much as possible.

However, I'd like to see how far we can push this up, so I'm willing to let go some of the things which are really cumbersome on increased distances, including the manually powered chain pulling (obviously), number of chains, and how all of it is bound to the vehicle. Speaking of vehicles, You can alter the boat shape and size as needed, though if you can keep the overall rectangle shape, I'd be happier.

You can use all modern technologies, and leave out costs unless it exceeds the yearly revenues of many countries (the very concept of building up a chain across a sea doesn't seem to be cheap, anyway). Consider the body of water to cross to be anything like one of Earth, including any of its related constraints : Currents, landscape at sea-floor (in case your chain limps on seaground), and so on.

So how far can we scale up the chained ferryboats reachable distances?

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    $\begingroup$ If you have a better word for this kind of boat than "chained ferryboat", I'll take it gladly. I couldn't find a sure way to tell it in my native tongue, so in English :p... $\endgroup$
    – Tortliena
    Oct 19 '21 at 20:39
  • $\begingroup$ In The Fellowhip of the Ring Frodo, Sam., Merry, and Pippin cross the Brandywine river to Buckland on what appears to be a cable ferry as well as I can remember. So apparently Tolkien thought such a ferry would work at distances of a hundred meters or more. $\endgroup$ Oct 19 '21 at 21:55
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A Really Long Way

Chain ferry and cable ferry refer to the same basic set up. If you don't mind swapping a motor for arm power, the ferries can get really big and the distances quite long.

The longest one appears to be in BC and is over a mile long (even longer in kms!) -- the Baynes Sound Connector.

Cables have been laid across oceans, but realistically, currents, storms and surges will mostly likely break any chain or cable that is pulled relatively tightly between opposite sides of a wide ocean.

I'd suggest that a cable ferry could probably be constructed to cross a wide but not too deep lake, like Pontchartrain which is about 25 miles wide and no more than about 10 fathoms deep. There are already bridges, so the ferry would not be an economic rival, though it might attract seasonal tourists.

Additional information: this article has lots of interesting facts & numbers, mostly about horrific emergency conditions and maximal loads.

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While I like @elemtilas' answer, there's some practicalities you should think about

If the "chain" weighed absolutely nothing and the boat was stable enough to withstand oceanic storms and contained enough food for the entire journey. Yup, you could cross an ocean.

But that'll never happen.

  • The weight of the total length of chain must be less than the flotation capability of the boat when fully loaded. If this isn't true, the chain will simply drag the boat underwater. This issue is actually complicated by the fact that the weight of the chain on either side of the pulley mechanism must be light enough that it doesn't bind the pulley mechanism, or nobody (not even a motor) will pull the boat. This is pretty important. The stronger the pulling mechanism (aka, big honking motor) the longer the chain can be due to the mechanism's ability to overcome the tension force on the pulleys.

  • The ratio of the length of the chain to the perpendicular stability of the boat must be low enough that when the boat is midway through its journey, it won't be capsized by the tension of the chain. If that didn't make sense, think of it this way. Your chain isn't rigid, which means the boat will drift with wind and current away from the ideal "center line" of the chain. The further that drift, the more stable (aka "wider") the boat must be. If this still doesn't make sense, think about why boats with outriggers have outriggers.

  • Finally, the longer the journey, the more weather is a factor. A boat strong enough to withstand the weather might also be strong enough to be a wind anchor, resulting in pulling the chain off its mount. That mount must be strong enough to withstand the total weight of the chain and any force applied to the chain. If the boat is midway through the journey and a hurricane happens, those chain mounts had better be made of titanium.

  • Finally, finally, note that the heavier the chain (aka, the longer the distance of the journey), the more the chain must be centered on the boat. Maybe that's obvious, but I've seen too many movies where the rope/chain was mounted along the side of the boat. The further from the center line of the boat, the more unstable the whole set-up will be and the shorter your distance will be.

  • OK, finally, finally, finally... You absolutely, positively do not want a distance so great that the chain must be allowed to sit on the bottom of the ocean/lake/river.... Why? Because heavy chains sink into lovely, soft soil. You'd never be able to haul the chain up to run it through the pulleys.

Food for thought.

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  • $\begingroup$ That's some of the concerns I had indeed, though I didn't have the exact keys and keywords. About the position of a single rope on the side, wouldn't it just mean that you'd need to lower a lot the speed to avoid overturning/flipping over as forces are not centered and rotation is therefore applied? I'm not sure I understood the link between applying a force outside the center axis and the length of the rope. $\endgroup$
    – Tortliena
    Oct 20 '21 at 10:43
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    $\begingroup$ @Tortliena You're correct that the further off-center the chain/rope/cable is, and the less stable the boat is, the slower it must go. But remember that the heavier the rope is, the longer it can be if it is centered on the boat. Kinda look at it as if yours was the finger of God. Place your finger on the boat and push down. What does the boat want to do? The weight of the rope burdens the boat in the same way. Note that I left off the material of the rope/chain/cable. Rope stretches a lot, which would limit its value. It also absorbs water, while not heavier than metal, that's also weight. $\endgroup$ Oct 20 '21 at 14:52
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    $\begingroup$ Got it. To reduce this effect we would need to reduce the chain's weight, or... Increase its buoyancy to avoid drown-flipping the ship (but it's basically the same idea). Given how easily the small ferryboat chains drop into the water, I doubt they'd work for much bigger structure. $\endgroup$
    – Tortliena
    Oct 20 '21 at 15:57
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Across the oceans, if you spend enough money.

A chain thousands of miles long is of course infeasible, but between enough islands is possible. There are island chains between most major continents, so you could have a business which linked cable ferries between islands, and make artificial islands if there weren't enough.

It wouldn't be very economical, but maybe magic or sci fi technology could make it feasible. The heavy chains and storms make super long chains infeasible, but with enough money and artificial island building you could do it.

They are building an island for 50 million. and since you basically just need stable ish anchor points between natural island chains, you can probably do it for that price. At 5000 times that price, you can build an island chain every kilometer for a tidy 250 billion. The chains will be cheap by comparison.

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  • $\begingroup$ Good point raising up chains of chains. However, I'm not entirely sure you need to go up to building whole islands for that; Anchored structures like very big buoys or oil platforms would probably give the same result, but without that high of a cost. After all, if such things can't withstand the chains, neither will the boat. Now the issue is to easily transfer from one chain to the others... $\endgroup$
    – Tortliena
    Oct 23 '21 at 0:43
  • $\begingroup$ Deep water oil platforms can cost hundreds of millions of dollars, so this is a lowball cost for the floating island. $\endgroup$
    – Nepene Nep
    Oct 23 '21 at 14:05

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