The Honolulu-based Coast Guard Cutter Walnut was built in 1998 as part of a 16-vessel fleet specifically designed — among other things — to respond to large oil spills. But it's never been called on to sop up a drop of oil. Until now.

The 225-foot cutter left yesterday for a four-month deployment to the Gulf Coast, where it will join a massive and growing Coast Guard response to an oil spill that, in its sixth week, is being called the biggest environmental disaster the U.S. has ever faced.

The cutter is the first vessel from Hawai'i to participate in the cleanup efforts.

"Our primary responsibility will be (as) an oil skimming operation," said Cmdr. Jeffrey Randall, the Walnut's commanding officer. "Every little bit is going to help."

The Walnut can skim as much as 450 gallons of oil from the water per minute. And, though its 50-member crew members haven't helped in an oil spill clean-up before, they do participate in regular training to keep sharp and make sure their equipment's in working order.

"Our goal is to go over there and have an impact," Randall said. "Anytime you get to do this for real, you build your capacity to do it again."

It will take three weeks for the vessel to make the 5,000-mile trip to the Gulf.

The cutter will join a host of other Coast Guard vessels responding to the spill. Another cutter specifically designed for oil spill response — one of the 16 nationwide — also deployed yesterday from California for the Gulf Coast, Randall said.

Walnut crew members heard a week ago that they might be deployed to the spill. But the crew got official word Saturday, after a method known as "top kill" failed to stop the spill and BP said another plan to cap the well wouldn't capture all the crude fouling the Gulf.

Relief wells, seen as a long-term solution to the spill, are being drilled but they aren't expected to be done for at least two more months.

The spill has dumped between 18 million and 40 million gallons of oil into the Gulf, according to government estimates. The leak began after BP's Deepwater Horizon offshore drilling rig exploded April 20, killing 11 people.

Crew members aboard the Walnut were busy preparing for their deployment yesterday, and couldn't speak to the media. But about 15 family members came out to see the ship depart, and several said the surprise deployment would be tough.

They also said they were proud to see a Hawai'i ship assist in the Gulf efforts.

"In our culture, the ocean is sacred," said Big Island resident Janet Cody, whose son, Riken Bautista, is a cook aboard the Walnut. Cody came to O'ahu Saturday to spend the Memorial Day weekend with her son when she found out he was being deployed.

"He had less than 24 hours (to plan). It's disappointing," she said.

Meredith Cratsenberg and her four children, ages 2 months to 7 years, watched the Walnut depart yesterday. Aboard the ship, her husband, Brian Robinson, was overseeing engine operations.

Cratsenberg said the latest deployment will be tough, as they all are, but that the family is always prepared with a "plan A, B, C," so nothing catches them unaware.

"We're just happy he's able to help," she said.