A charter fishing trip turned exciting Thursday morning when those aboard came within five feet of what they believed to be a surfacing great white shark.

Buddy Vanderhoop, captain of the charter boat Tomahawk III, was taking his charter customers out for a morning of fishing when they came upon the nearly-20-foot shark about a mile offshore, between Aquinnah and Noman’s Land.

“Oh, I know this was a great white shark,” said Mr. Vanderhoop, a well-known charter fisherman out of Menemsha. He estimated that the fish weighed at least 4,000 pounds. The distance between the fin and the tail was at least 10 feet, he said, leading him to believe that the shark was therefore nearly 20 feet in length.

Mr. Vanderhoop’s boat is a 27-foot center console. The shark was not that much smaller than the boat, he said.

“It was the highlight of our summer. It was certainly the highlight of our day,” said Ray Drop, 71, of Edgartown and Park Ridge, N.J., who, along with his son Eric, 25, was out fishing with Mr. Vanderhoop and snapped several photographs.

Mr. Drop said they left the dock around 7:10 a.m., and saw the fish at about 7:30 a.m. “We were on our way to the fishing ground. We saw it in the distance and Buddy turned toward the fish. We tried to get close. It didn’t seem bothered by us at all. It was cruising,” Mr. Drop said. “It was heading westward.”

“We watched it for 10 minutes,” he said.

After they saw the shark, the trio proceeded to visit a number of different fishing spots, before they got into bluefish near the Elizabeth Islands, Mr. Drop said.

Mr. Vanderhoop said he observed ten to 12 gray seals, a popular meal for sharks, on Noman’s Land.

A story about the sighting was posted on the Gazette Web site on Friday, and in an online comment, a reader from West Tisbury questioned the accuracy of the sighting, suggesting that the shark may have been a basking shark. “I saw the photo and immediately thought, basking shark,” wrote Albie Scott.