Princess Anne met 300 Sea Cadets and officially named their new training ship TS Royalist in West India Dock on Friday.
She replaces the flagship TS Royalist - also named after HRH the Princess Royal - which was retired on November 20 after more than four decades of service.
At the formal ceremony held in Marsh Wall, there were speeches from Martin J Coles, CEO at Marine Society and Sea Cadets and chairman of the Flagship Appeal Committee, Robert Woods CBE.
Princess Anne named and commissioned the ship - which will support 900 young people on a week-long sea voyage every year - and then sealed it all by smashing a bottle of champagne onto it.
Petty officer, cadet Gareth, 16, from Northern Ireland, has sailed on the previous flagship several times and was awarded the Cadet of the Year in 2014.
He said: “It was the best week of my life, a massive adventure where I got better at navigation and problem-solving and stronger on team working and more focussed as I developed my self-discipline. It’s an amazing experience; it’s changed my life completely and made me a better more confident person.”
Sea Cadets Captain Phil Russell RN, said: “We have been eagerly anticipating the arrival of our new state of the art flagship. The ship will help develop valuable life skills through a challenging but fun experience for thousands of Sea Cadets in the years to come. ”
The former flagship gave 43 years of service taking 30,000 cadets to sea in her lifetime.