"The weather is great -- I've got a swell of one meter and I'm moving at two or three kilometers an hour," Jean-Jacques Savin told AFP by telephone after setting off from El Hierro in Spain's Canary Islands.
"For the time being my capsule is behaving very, very well and I've got favorable winds forecast until Sunday."
Savin had worked on his vessel for months in the small shipyard of Ares on France's southwest coast.
Measuring three meters (10 feet) long and 2.10 meters across, it is made from resin-coated plywood, heavily reinforced to resist waves and potential attacks by orca whales.
Inside the capsule, which weighs 450 kilograms (990 pounds) when empty, is a six-square-meter living space which includes a kitchen, sleeping bunk and storage.
A porthole in the floor allows Savin to look at passing fish.
A former military parachutist who served in Africa, Savin has also worked as a pilot and a national park ranger.
He has stowed away a block of foie gras and a bottle of Sauternes white wine for New Year's Eve, along with a bottle of red Saint-Emilion for his 72nd birthday on January 14.
Savin hopes currents will carry him naturally to the Caribbean without the need for a sail or oars -- "maybe Barbados, although I'd really like it to be a French island like Martinique or Guadaloupe," he quipped.
"That would be easier for the paperwork and for bringing the barrel back."
Along the way, Savin will be dropping markers for the JCOMMOPS international marine observatory to help its oceanographers study the currents.
And he himself will be the subject of a study on the effects of solitude in close confinement.
Even the wine onboard will be studied: He is carrying a Bordeaux to be compared afterwards with one kept on land to determine the effects of months spent tossed on the waves.
Savin has a budget of 60,000 euros ($68,000) for his expedition, covered in part by barrelmakers and a crowdfunding campaign.