The Duke and Duchess of Cambridge's day supporting sailor Sir Ben Ainslie and his crew in the America's Cup World Series was blown off course by bad weather.
Parts of William and Kate's itinerary had to cancelled and new venues found for other events after heavy rain and blustery conditions swept into Portsmouth.
Sir Ben, the most successful sailor in Olympic history, is due to skipper his team on the second day of the opening leg of the World Series but it was not clear if conditions would improve to allow the sailors to compete.
After being forced to return to dry land, Sir Ben said: "The English summer, you just can't count on it.
"Yesterday was the most amazing day of my sailing career. There were so many people out, everyone had a fantastic time.
"Today was a shame but we will get that sorted out and book the weather in for next year."
The royal couple had hoped to watch the first of two races from a boat but this was cancelled along with a tour of a festival site as the ground was waterlogged.
William and Kate left their children - Prince George and Princess Charlotte - in the care of their personal staff to travel to the south coast for the event.
The Duchess, who is a keen sailor, joined Sir Ben in June last year when he formally launched Britain's bid to win the America's Cup for the first time.
She is a committed supporter of the sailor's team BAR (Ben Ainslie Racing) and is also royal patron of the 1851 Trust, which works to inspire the next generation through sailing and the marine industry.
The America's Cup is international sport's oldest trophy, first staged in 1851 off the Isle of Wight, but it has not returned to British waters following that inaugural defeat.
Sir Ben, who has won four Olympic golds and a silver, was famously employed as a tactician during the last America's Cup race, helping the US Oracle team overturn an 8-1 deficit to beat New Zealand.
Points collected during the World Series determine who will challenge the US for the America's Cup which will be staged in Bermuda in 2017.
Sir Ben is off to a flying start after his crew clinched a win and a second place during the two races staged yesterday in front of thousands of spectators.