Looking every inch the experienced sailor she is, the Duchess won two races piloting an America's Cup sailing boat, leaving William trailing in her wake.
Kate could not hide her delight when she beat her husband and was seen punching the air on both occasions her sleek racing yacht finished first with her at the helm.
Back on dry land, the Duke joked "we were sabotaged" and when someone said the Duchess was looking pleased he replied: "I bet she is. Selfless husband. I wanted a quiet night."
The Duchess' past experience on the sea no doubt counted as during her gap year she crewed on Round the World Challenge boats in the Solent.
The couple are said to have a competitive streak and the victories were revenge for the Duchess who lost to the Duke in a dragon boat race during their tour of Canada in 2011.
Eric Haagh, 36, the skipper of the Duke's boat, conceded the Duchess had won both of the two three-mile races but that their yacht had developed problems.
A lock which attached the mainsail to the top of the mast had developed a problem and the sail was out of shape and their was also a fault with the spinnaker sail.
Mr Haagh said: "William was a bit nervous at the start. These boats are very powerful and he just wanted to get on the grinder (sail winch) to begin with.
"When we wiped out because of the problem with the sail we were near to some rocks and William said, 'That'll be a good photo'.
"William was a good sailor but Kate pushed us around into a bad position. She did very well.
"William was very competitive right from the start - he didn't want to lose. When Kate won both the races he just said, 'At least she'll be happy'."