A plan to build a ship tunnel
newatlas.com
newatlas.com
Built in 1679, 165m long: https://en.wikipedia.org/wiki/Malpas_Tunnel
Built in 1832, 3333m long: https://en.wikipedia.org/wiki/Canal_de_Bourgogne
Built in 1927, 7120m long: https://en.wikipedia.org/wiki/Rove_Tunnel
The first two are canal tunnels not at sea level, but the last one is connected to the Mediterranean at sea level.
TFA describes the Norwegian tunnel as "the world's first ship tunnel of any significant size", but "significant" changes as engineering advances, and Harecastle was certainly significant at the time - contemporaries thought it an unearthly achievement. Scepticism was such that it was nicknamed an "air castle" (after "castles in the air"). It was the absolute limit of what was believed possible with tunnelling technology through such rock: the 7ft width defined the size of locks and bridges for the rest of the Grand Trunk, or Trent & Mersey, Canal. In turn, this defined the size of the English "narrowboat", the dominant inland craft for cargo carrying and, since the 1960s, pleasure boating. (In modern terms this would be called "boat" rather than "ship", but in contemporary documents it was less clear, and it seems likely that the size was chosen as a half-ship - half the width of the Mersey Flat sailing vessels, on the river with which the canal connected.)
The original Harecastle Tunnel is now closed due to subsidence, but the 1820s replacement is still operational, and the rest of the Trent & Mersey Canal is fully navigable and very enjoyable: I moored a boat on it for six years.
(For anyone interested in finding out about Britain's 2,500-mile network of historic canals, here's the charity that runs them: https://canalrivertrust.org.uk/ )
The general definition is a boat is small enough for a ship to carry it. Narrowboat even uses the term boat.
(Incidentally, the "general definition" you cite is one used by the US Naval Institute, but not universally recognised. As the former editor of a monthly news-stand boating magazine it's certainly not one I've ever used. In the UK, for example, you can legally define your small boat as a ship by signing up to the Small Ships Register.)
There are a lot of canal tunnels in the UK.
https://en.wikipedia.org/wiki/Naval_museum_complex_Balaklava
Which is why the UK has its missile subs based so close to a major city!
See: The Silent Deep by James Jinks and Peter Hennessy
https://en.wikipedia.org/wiki/Paw_Paw_Tunnel
They actually built this during the days of black powder.
How do the physics of "crushing" a boat by raising water level in a confined space work?
> But a storm surge would be unpredictable, right?
> How do the physics of "crushing" a boat by
> raising water level in a confined space work?
If the ship is stuck against the ceiling of a confined space the pressure it's excreting on the ceiling is going to be the equivalent to the displacement of that "extra" water.
1. Leaving aside the full calculation which takes the difference in the weight of air & water into account for the purposes of this explanation.
The photos also show a mountain going straight into the ocean, which means raising the ship with a lock wouldn't save you any money, since you'd need an unbroken sequence of locks going all the way to the top of the mountain.