Scott Vernon wrote:
Ships of the line had 21 guns on each side. To salute, or to show
friendliness, a ship would fire off all 21 guns on the same side &
just before of passing another ship. Therefore not allowing enough
time to reload and fire upon the other ship.

The idea is right but ships of the line had well more than 21
guns on a side. The 1st rates would have 100+ guns (50+
a side, assuming chasers are counted on a side) and the
smallest ship of the line would be ~60. A frigate (like
John Paul Jones' Bonhomme Richard) would have about
that number, however (42 total.)

But the idea's right anyway -- they wouldn't necessarily be
able to fire the lower gun decks, cutting down the number,
they'd want to leave some guns free to cover misfires, etc.