Flag etiquette has been passed down to us by generations of mariners. It is often totally lacking or incorrectly displayed on pleasure yachts, in particular charter yachts. We are not totally fanatical about flag etiquette but we like to observe at least the minimum requirements of this age old tradition and basic rules.
The appropriate time to fly the ensign is from sunrise to sunset, except when racing. However, whenever a boat is taken into international or foreign waters, the 50-star U.S. ensign is the proper flag to fly and the yacht ensign cannot to be displayed. In other words, if you own a US boat in the British Virgin Islands, you should not fly the Ensign, but the National Flag.
Boats today fly the ensign from the stern, which provides the best visibility or at the lower third of your mizzen sail (generally from the mizzen's topping lift). When flown from the stern, it should be on a staff (pole) that is sufficiently long and angled, and that is offset to one side (traditionally the starboard side), so the flag flies clear of engine exhaust and rigging.
If you are preparing to clear customs while returning from foreign waters, the "Q" flag should be flown on your starboard spreader, below the country's courtesy flag. Once formalities are taken care of, immediately remove the "Q" flag.
All charter boats should carry the national flags of neighboring islands as well as the yellow "Q" flag in case charterers want to visit those islands. As a side note, some authorities are not amused at all if you fly their courtesy flag using an old, raggy flag. Some will even fine you for disrespect. So check your flags before setting off to another country to ensure the flags look good enough to display!
It may be flown day and night. Many people opt to fly the burgee lower in the rig hoisted to the end of the lowest starboard spreader on a thin flag halyard. While purists rail this practice, it is an accepted adaptation of another tradition which is that the starboard rigging is a position of honor. Note that when you visit a foreign port, that's where you fly the host country's flag. Besides being reasonable, flying the burgee in the starboard rigging is such a widespread custom that to try to end it would be close to impossible.
A private signal is a small, custom-designed and custom-made flag that carries symbols for/about the owner of the flag and can basically be anything. The signal may be flown day or night, but is not displayed when another skipper is in command. (The rule is: the private signal and burgee follow the sailor, not the boat.)
On a multi-masted boat, the private signal is flown at the head of the aftermost mast. On a sloop, the private signal may be flown from the starboard rigging, either below the burgee or alone.
Lastly, it is also a common courtesy to fly the national flag(s) of your guest(s) on board, if they have a different nationality than the ensign is showing. This flag/flags should be flown on the port spreader lower than the courtesy flag as a sign of respect for the host country.
Although flags come in standardized sizes, there are guidelines to help you selecting the proper size for your boat.
The size of a nautical flag is determined by the size of the boat that flies it. Flags are more often too small than too large. So in the rules below, round upward to the nearest larger standard size.
The flag at the stern of your boat: U.S. ensign or national flag should be about one inch for each foot of overall length. For example, on a 40ft. boat, the ensign should be 40 in. i.e. about 3.5 ft.
Other flags, such as club burgees, private signals and courtesy flags for use on sailboats, should be approximately 1/2 inch for each foot of the highest mast above the water. For example, on a 30ft. boat, with 50ft. between the masthead and the water, the burgee should be about 25 in. i.e. about 2 ft. The shape and proportions of pennants and burgees will be prescribed by the organization which they relate to.
Fly the ensign from morning (8:00 a.m.) to evening (sunset) whether the boat is at rest, under sail, or under power. The exception to this rule is: The ensign is not flown on a boat in a race. The absence of the ensign signals to other boats that you are racing.
To prevent wear and tear, the flag may not be flown when out of sight of other vessels or when nobody is aboard. The flag is flown while entering or leaving a port even at night. For purists: in the morning, the ensign is hoisted rapidly before other flags. In the evening, it is lowered slowly and with ceremony after other flags come down.