Although the sails are mostly decorative, in a moderate breeze they actually do some good.
*When you go:
-You can generally book a cruise a day or two in advance out of Hanoi. All the tour operators have offices in the central tourist area, or Old Town of Hanoi. There are also numerous private tour operators throughout the city.
-The two day, one night cruise is the most popular way to see Halong Bay. A second night will cost you about 40% more. We opted for the Bhaya 2day/1night cruise ($159 promotional price) which included the three hour shuttle to and from Hanoi.
-Check the weather reports for a "good weather day". It can be cold, foggy, and gloomy even during the mild high tourist season.
-Pack light for the overnight stay. It could get chilly at night so bring a sweater or light jacket.
-If you want to kayak or swim (probably not recommended as the water is pretty murkey) bring appropiate clothing.
*Halong Bay time: