Six pirates brandishing knives and bayonets boarded a cargo ship under charter to the U.S. Navy in the Strait of Malacca yesterday and stole $19,500 before escaping in a speedboat, Navy officials said yesterday.

The pirates tied up the master of the SS Falcon Countess but did not injure anyone during the nighttime assault, which occurred off the coast of Indonesia. The ship, carrying jet fuel and diesel oil from Bahrain to Navy facilities in Guam, continued on schedule, officials said.

"The pirates followed the usual routine," said one Navy officer. "They came up over the fantail, moved up through the superstructure holding anyone they found at knifepoint until they came to the master's cabin, rifled the ship's safe, tied the master up and left."

Despite the officer's description of the "usual routine," only one other Navy-leased ship has been assaulted by pirates recently, officials said. That attack occurred two years ago and coincided with new Navy instructions to post guards when traveling through the strait that separates Singapore and Indonesia.

Navy officials said the Falcon Countess had traversed the danger zone and had sent its guard below 20 minutes before the pirates boarded. They said that Capt. William Haney, the ship's master, speculated that the pirates had been watching the ship for some time.

The guard would have been armed only with a high-powered water hose that could be used to knock someone off the deck, officials said. They said the Falcon Countess, which is privately owned and leased to the Navy's Military Sealift Command for six months, probably carries no other weapons.

The pirates used bamboo poles with hooks on the ends to climb from their speedboat onto the stern, officials said. The incident has been reported to the U.S. defense attache in Singapore, who will alert marine police. In addition, the Navy said it will send a warning to all U.S. ships in the region.