You wouldn't need many ships. The volume of a single shipping container is enough for 1300 tons of gold. Of course gold is extremely heavy, you'd need a specially built container to support that much gold, but the point is the storage space of any ocean going ship can comfortably carry the entire gold reserve of an entire country.

If you actually loaded gold into shipping containers and obeyed the weight restrictions, all the gold in the world would take up less than 8400 TEU, which would be considered a mid size cargo ship.

> I imagine a convoy of small but heavily armored ships

It’s typically flown [1].

[1] https://www.independent.co.uk/news/world/americas/us-politic...

This actually happened in 1965. De Gaulle, the president of France at the time, sent the French Navy over to America and took their reserve gold back home. This prompted other countries to do the same and by 1971 America had switched from a gold standard to a fiat currency.

https://en.wikipedia.org/wiki/Exorbitant_privilege