The chain is made of harder (by a substantial enough margin to not wear through too quickly) metal than the ship. Also, the cross section of any given link is much thicker than any one piece of steel in the ship so it doesn't really need to cut efficiently, it just kind of crunches and tears the ship out of the way using the ship's own weight.

True, although

> “Approximately 25 hours into the cut, the cutting chain broke,” St. Simons Incident Response writes on its website.

It's a wear part. Yes, it will wear, but that's logical, just like a knife will get dull when cutting cheese. The impressive bit to me is that it lasted 25 hours. They then fixed that one link and went on cutting. Keep in mind that it still is a chain and that all it took was for one link to break under a load that was not exactly typical for a chain and after some abrasion of the link itself.

At these scales, and being able to use the hull's weight against itself, that chain is plenty "sharp" just by itself, I guess (sharpness being the ability to concentrate force on a comparatively small surface area). Adding teeth to the chain would probably not be worth it and/or they would just dull or break soon anyway.

Also using an alternative line an abrasive cable would not make sense either. If friction causes it to tear it would need complete replacing, which would be plenty $$$.

Instead, with a chain they just replace a couple of broken links and back on track. Which is apparently what they've done:

> Unfortunately, the chain actually broke during the cutting operation. “Approximately 25 hours into the cut, the cutting chain broke,” St. Simons Incident Response writes on its website. Luckily, there were no injuries and there was no damage to the equipment. The team simply fixed the chain’s broken link, inspected the other links for signs of fatigue, and continued on.

I'm guessing that another advantage of chain instead of cable is what happens if it breaks.

IIUC, it's much less dangerous to be around a breaking chain than a snapping cable.

A chain is never harder to repair than its weakest link, I guess.

>>> Adding teeth to the chain would probably not be worth it

When the Kursk was raised they first cut it in half using an abrasive chain/cable. In that instance it was pressed down from above the structure.

"A cutting chain made of abrasive covered bushes on a thick steel wire was pulled back and forth across the hull by the cylinders. Vertical forces were supplied by the controllable vertical position of the suction piles."

https://www.maritimejournal.com/news101/tugs,-towing-and-sal...

Yes, I think this is the right intuition. Given the size and weight of the hull, physics is operating at a different scale and “sharp” looks quite different from our daily experiences.

It reminds me of a Fermi estimate that my physics professor presented of how tall the tallest mountain on Earth or a generic planet of mass M could be. He started with the intuition that when the weight of a mountain overcomes the typical strength of chemical bonds, what we typically experience as “solid” rock will behave like a liquid and collapse.

>> albeit a very large one.

By maritime standards that chain is tiny, akin to a thin wire cutting through cheese.

https://www.reddit.com/r/interestingasfuck/comments/6wvvkj/a...

https://www.usni.org/magazines/naval-history-magazine/2017/d...

It is. Between being harder than the iron of the hull, and the links, it acts like a chainsaw. The teeth are not as ideally formed, but it doesn't really matter as ideally formed teeth are not really practical anyway because of size (they need to be huge so as to not load up with chips).

Various other stories refer to the chain as a "diamond-encrusted cutting chain" so perhaps there's more to the chain than what you can see in the pictures.

I'm guessing similar to the diamond blades and pads used to cut granite. The edges usually aren't sharp, the blades are usually pretty wide, 1/8" kerf, but they're encrusted with diamonds. The blades don't usually get dull, they just wear down until the diamonds are gone. The diamonds not the blade itself does the cutting. Once the diamonds are gone, the blade will actually start melting and stuff.

I'm thinking it's got abrasive material embedded in the outside of the chain links. I kind of doubt it's diamonds, because that wouldn't work too well for steel and other ferrous metals, but it might be CBN or similar.

It surprises me that the chain cuts so neatly - if you look at the images the cuts look rather clean and surgical. I would have expected it to be more of a crushing and jagged cut.

The gear used in these large scale engineering ops is absolutely remarkable, it makes me wish I had a better understanding of our physical world.