Portal, who appeared in The King's Speech and is dating Kristin Scott Thomas' sister Sam, signed up to row 3,000 miles (4,828 kilometres) from the Canary Islands to Barbados with his friend James Cash.
They set off on their mission last month (Dec11), but their vessel was hit by huge waves over the Christmas (11) holidays and the pair's oars broke - leaving them drifting across the ocean.
Sam tells Britain's Daily Mail, "Their boat capsized... and could have sunk. The boat is designed to right itself and they were both in the cabin with the door shut when it turned over. Had they not been, the cabin would have filled with water.
"They are just sitting there in huge seas, stuck in a tiny cabin. The weather has been absolutely dreadful - lots of wind. And to capsize 1,000 miles out in the Atlantic is very scary. My sisters Kristin and Serena have been incredibly supportive. Bertie and James have been working towards this challenge for two years. Their courage is amazing."
Sam reveals a rescue ship is due to deliver the pair new oars by the weekend (07-08Jan12) so they can complete their challenge, which is to raise money for actress Natascha McElhone's late husband Dr Martin Kelly's health charity Facing The World.