Merchant ships salute each other by dipping the flag.

商船互相点旗致敬。

《新英汉大辞典》

However, their ships rarely fly the flag of the home countries, choosing instead "flags of convenience".

然而，他们的货轮却挂“方便旗”，而很少悬挂本国国旗。

youdao

Pirate ships often flew the flag of a friendly country as they sailed toward the ship they planned to rob.

海盗船在向意图抢劫的目标船只行进时，通常悬挂友好国家的旗帜。

youdao

Pirate ships often flew the flag of a friendly country as they sailed toward the ship they planned to rob.

VOA: special.2009.05.24

About twenty of these private ships flew the Confederate flag.

VOA: special.2009.09.10

So far this year there have been a total of 8 norovirus outbreaks on U.S.-flag cruise ships, compared with 15 for all of 2009.

FORBES: Magazine Article

The tribunal in Paris awarded the assets on the condition the shipping company operated under the French flag and Eurotunnel leased the ships to a workers' co-operative.

BBC: SeaFrance ferry

In the case of Carnival Cruise Lines, the owner of the Carnival Triumph that spent days in the Gulf of Mexico disabled after an engine fire, the company is incorporated in Panama, its offices are based in Miami and its ships fly under the Bahamian flag a matrix that is not unusual in the cruise line industry.

NPR: No Central Agency Oversees, Inspects Cruise Ships