The Tall Ships Festival is the biggest event ever held in Belfast, organisers said.
Half a million people are expected to visit the city's docks during the next four days.
Around 50 vessels from 15 different countries will be moored on the River Lagan ahead of the start of the sailing training race to Norway, Sweden and Denmark.
Chairman of Belfast Tall Ships 2015 Gerard O'Hare said: "It is a cast of thousands that helps create this piece of theatre over four days for Belfast.
"It is the biggest event I believe there has ever been in Northern Ireland.
"For the next four days we will be the biggest show in the country."
The Brazilian flag of the Cisne Branco naval vessel from the South American giant flew in the wind as cadets made preparations for Sunday's departure.
The name means the White Swan and crowds queued, eager for a tour as the crew scaled the masts checking the rigging.
Ships from Ecuador, Portugal, Spain, France, Norway, Sweden and Denmark are competing in the race.
Another ship taking part is the Dutch bark Europa.
Official programme available HERE
Its captain Klaas Gaastra said the vessel could take up to 45 young trainees from all over the world.
"It is to get them out of their normal society for a little without internet.
"We have quite an international crew of trainees, different backgrounds, different religions, different cultures.
"It is really nice for young people that they see that they still can do a lot of things if they work as a team and get to know each other on another basis then in their own country and their own surroundings, it makes a big difference."
The Europa is known as the Ocean Wanderer - as the vessel is one of the most travelled of the Tall Ships fleet, having circumnavigated the globe.
It recreates the spirit of the late 1800s when square riggers like Europa routinely sailed the waves carrying freight or the "greyhounds of the sea" clippers raced around the Capes bringing goods to Europe.
Bjorn Baaga, 16, from Holland, is spending a second summer training on board the Europa.
He said they had sailed to Belfast from the Azores.
Duties included pulling in ropes, cleaning, steering, climbing the mast - as well as socialising.
He added: "It is very social on ship, it is very nice to meet friends, from Denmark and France and Germany."
He said sleeping onboard was not a problem, adding: " It is very nice sleeping, you are like a baby in a crib."
There are two main sites, on the River Lagan beside the Titanic Quarter and at Pollock Dock near Duncrue Street.
The race begins on Sunday.