Two crews are attempting to break the world record for crossing the English Channel in a dragon boat.
Prince William and Kate Middleton split up earlier this year
The Brotherhood and Sisterhood teams are paddling from Dover to Calais across more than 20 miles of open sea in aid of charity.
The current dragon boat crossing record stands at seven hours 45 minutes.
Prince William's on-off girlfriend, Kate Middleton, had been due to join the all-female Sisterhood team but pulled out due to security concerns.
Organisers said the concerns had related to media coverage of the event.
Beauty v Brawn
The 25-year-old had been persuaded to join the challenge by Sisterhood member Alicia Fox-Pitt, a school friend from Marlborough College, and had been due to be at the helm of the boat.
The Beauty v Brawn cross-Channel challenge is raising money for the Babes in Arms charity, which sponsors research into abnormalities in newborn babies, and the children's hospice charity, Chase.
The women are set to become the first all-female team to cross the Channel in a dragon boat.
The teams hope to make it to France in four or five hours.