The United States addresses this issue only to prohibit a foreign vessel from flying a U.S. flag to avoid inspection by U.S. Customs. There are, otherwise, no U.S. laws that require a foreign vessel to fly a courtesy flag while visiting this country, and this is not the subject of any international treaty.
Some countries may impose “civil penalties” for the violation of traditional flag etiquette, but this is more of a revenue-generating device than a strict legal sanction.
Notwithstanding the legal ambiguity, there is no “down side” to following proper flag etiquette. No particular legal rights or legal standing are granted to a host country when a visiting yacht or ship flies that country’s courtesy flag. The flags are used to communicate the status of a visiting vessel, which simplifies the work of the local officials. This, in turn, will make your stay in the country a lot more enjoyable.
The protocol for a particular country is simple to determine through a quick Internet search, but the basic “rules” are fairly consistent throughout the world. Upon entering a foreign port, the vessel should hoist the yellow “Q,” or “quarantine” signal flag until it clears customs.
After clearing customs, the vessel should replace the Q flag with the “courtesy ensign” for the host country, and keep it flying until departing territorial waters. The courtesy ensign for a particular country may or may not be the national flag — but, again, this information is available through the Internet.
The decision to fly a courtesy flag is a simple gesture that will ease a cruising boat’s interaction with foreign officials. It is not technically a legal requirement, but local officials may see things differently — and most of us would prefer to avoid the prospect of tracking down a local attorney in a foreign port.