About 30 boats left Hawaii over the weekend in an ambitious effort to map the size and distribution of plastic garbage over 1.4 million square miles of the Pacific Ocean between Hawaii and the West Coast.

The mapping project, called the Mega Expedition, is the first step of a larger effort to raise about $60 million to build a device to actually start cleaning up what’s known as the Great Pacific Garbage Patch in 2020.

The boats will use GPS to sample the ocean surface for plastic at specific locations and a smartphone app to record large pieces of debris while they travel to the mainland.

About 20 yachts returning to the mainland after racing to Hawaii in the Transpac sailing race are among the participants.

Transpac skippers said garbage has become a big problem for sailors.

Andy Bates, a Kaneohe resident and co-skipper of the 51-foot yacht Adrenalin, said that when he first started sailing the Transpac in 1998, he didn’t see any garbage.

But this year debris damaged his rudder.

"I was horrified at the amount of trash in the ocean," Bates said.

His boat will leave later this week, after its rudder is repaired, to help with the mapping effort.

The Ocean Cleanup, a Holland-based Dutch organization founded by then-18-year-old Boylan Slat, is sponsoring the mapping effort.

The group said the data gathered over the next three weeks on plastics in the Pacific are more than had been gathered in the past 40 years.

Slat, now 21, raised $2.2 million online through social media and crowd- funding to start The Ocean Cleanup, which is based on his high school science project.

Slat was in Honolulu for the launch of the mapping effort.

He said he hopes to build an array of floating barriers, which will use ocean currents to push plastic into a central area, where the plastic can be gathered, compacted and molded and then shipped out of the ocean.

The group hopes to deploy a test array and, if successful, begin cleaning the garbage patch in 2020.

The first step is to map the ocean garbage patch and estimate how much plastic is in the ocean.

Samples taken by the boats that left Hawaii and other boats participating in the effort will be shipped to the Netherlands, where the plastics in the samples will be counted and recorded.

The group hopes to publish a scientific paper detailing its findings next year.

The garbage patch, predicted by the National Oceanic and Atmospheric Administration in 1988, was discovered by Charles J. Moore, returning home through the North Pacific after competing in the 1997 Transpac. Most of the patch consists of very small pieces suspended below the surface.