Experts examined a mystery submarine on Friday and called it an "ingenious"homemade job designed for smuggling goods.

The black color and the amateurish construction of the 21-foot unmanned submarine found by surfers off Boca Raton on Wednesday are the best clues to its purpose."It's ingenious work that appears to have been done in a garage by very industrious people who knew what they were doing," said Kevin Peterson, a general manager of Perry Offshore Inc., a submarine manufacturing company in Riviera Beach that produces submarines for clients worldwide.

"Because they painted it black, it obviously was not meant to be seen," Peterson said. Police, surfers and lifeguards originally suspected the submarine might be thelegendary German World War II sub sunk off Boca Raton, when surfers found it bobbing just below the surf at sunset Wednesday. Salvage divers later said it was a barge float. But after the vessel was dragged to the beach at 2 a.m. Thursday, salvagers realized what they had found.

It is not a research vessel, because most research and military submarines are painted yellow, orange or red to stand out under water, Peterson said.

"We can't find any other use for the thing in any commercial, scientific or military application. The only thing it could be used for is to carry contraband in an airtight chamber underwater," he said.

"This sub is pretty unusual. There's not a category for it that we know of," Peterson said.

The submarine must be towed, because it has no engine. It is not designed totransport people, but it can carry 1,000 to 2,000 pounds of cargo at a maximum depth of 150 feet. Wires leading to two air tanks on each side show it probably was raised and submerged by radio control. If law enforcement agencies approached a boat, the submarine accompanying it could be lowered and later retrieved, Peterson said.