The crew will be at sea for around three months

A boat made of 12,000 plastic bottles has set sail on a voyage from San Francisco to Sydney to spread awareness about pollution in the world's oceans.

Environmentalist and banking heir David De Rothschild and a crew set out on the appropriately named Plastiki catamaran.

Their 11,000-nautical mile journey will go past the Great Pacific Garbage Patch - a sea of waste about five times the size of the UK or twice that of Texas.

Four out of five plastic bottles end up in a landfill, according to the UN.

"It is time we beat waste and this is an out-of-sight, out-of-mind issue that needs to be addressed," Mr De Rothschild told the BBC earlier this month.

The 31-year-old adventurer, who has completed expeditions to both poles and various jungles, was already tweeting on Saturday, hours after the boat set sail on its three-month voyage.

"Travelling 2.0 Knots ummm! That's a lot of ocean ahead!" he said on his Twitter page. "Just saw our first bit marine debris - a plastic cup!"

Green credentials

The Plastiki takes recycling to a whole other level.

The 12,000 used water bottles are filled with carbon dioxide to make the vessel durable and buoyant.

The catamaran is powered by solar, wind and sea turbines.

An exercise bike will power the boat's laptops and there is also a composting bathroom and gardens to grow food.

Critics say the expedition only perpetuates the belief that it is acceptable to use plastic as long as people recycle it, rather than encouraging people to cut down on its use entirely.

They also point out that if the Plastiki were to break apart mid-journey, it would dump thousands of bottles directly into the ocean.