An explosion and fire on a 21-foot cabin cruiser that killed two youths and injured two other persons late Wednesday occurred after the boats' engine began to miss and its owner began to head for shore, a survivor of the blast said yesterday.

Thomas E. Martin of Triangle said the boat owner, Ralph W. Berry of Woolbridge, had taken the baot out earlier on the Occoquan River to test repairs that had been made.

After the test, Berry headed back to shre to pick up other passengers for a "evening ride." He said the engine had begun to miss on the second trip and Berry was maneuvering the boat back to the marina when the explosion occured about 9:30 p.m.

Fire officials said the boat was engulfed in flames 15 seconds after the explosion.

Martin said the seven passengers jumped off the boat and began to swim to shore, which was about 30 yards away. "We were helping one another. We had everbody calm down."

Martin said when they reached shore, two youths, who were described as good swimmers, were missing.

The bodies of the youths, identified as Richard Dowell, 17, of 124 Poplar Lane, Occuquan and Victor Williams, were discovered in 15 feet of water about 30 yards from shore by divers, a fire official said. They apparently drowned, he said.

Martin, 32, of 3510 Oakdale Dr., and Berry, 35, of 14807 Empire St., were treated for injuries at Potomac Hospital in Woolbridge and released, he said.

Three other passengers on the boat were uninjured and swam safely to shore, the fire official said. They were identified as Steven Schultz, 29, of 13635 Lynbrook Ct., Dean Booth, 24, of 15901 Cardinal Dr., and Gunther Kinney, 27, of 1401 Harris St., at all Woodbridge.

The casue of the explosion is under investigation, but Fairfax officials said naelectrical hort in the motor may have been at fault.