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How is that? It was at surface pressure and then rolled over.

If the booyancy of the trapped air was a significant factor in the boat not sinking, then the air would have been at pressures over 1atm.

Put a barometer in an empty, upside-down cup. Force the upside down cup 1/2 way down into the water, trapping the air inside.

The barometer will be at the sea pressure of the bottom of the cup.

Makes sense if the glass is perfectly vertical, air can’t escape and force is applied from the top. But if you capsize a glass by slightly turning it on its side such that most of it sinks and the air escapes by water displacement you would get some compression on the remaining air but its unclear how much.

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