In the days of cannon, it took as long as twenty minutes to load and fire a gun. When a ship fired her guns in salute, she rendered herself powerless for the duration. By emptying their guns, the ship's crew showed shore batteries and forts that they were no threat. Over time, this gesture became a show of respect, with both shore and ship gun batteries firing volleys.
The habit of firing salutes became wasteful, with ships and shore batteries firing shots for hours on end. This was particularly expensive for ships, which had a limited space to store powder (which went bad quickly in the salt air). The British admiralty first dictated the policies now in place as a practical matter to save gunpowder. The rule was simple, for every volley fired by a ship in salute, a shore battery could return up to three shots. The regulations limited ships to a total of seven shots in salute, so the 21 gun-salute became the salute used to honour the only the most important dignitaries.
When president is honoured with 21 gun salute, most Navies provide gun salutes for naval officers on significant occasions, using the following protocol:
Admiral-17 guns
Vice Admiral-15 guns
Rear Admiral-13 guns
Rear Admiral-11 guns
All gun salutes are fired at five second intervals. Gun salutes will always total an odd number.
