A steel-hulled submarine can hold only a certain amount of breathable air. It goes faster when 53 people are crammed into the tight space.

       At some point early on Saturday morning, the life force for the sailors onboard the KRI Nanggala-402, an Indonesian Navy submarine that has been missing since Wednesday, could run out.

       Search crews from the United States, India, Malaysia, Australia and Singapore, along with the Indonesian Navy, have been desperately converging on the waters north of the Indonesian island of Bali, in hopes of locating the submarine and rescuing its crew.

       So far, the Nanggala is nowhere to be found.

       “If the rescue takes longer, the chances get smaller,” said Susaningtyas Nefo Handayani Kertopati, an Indonesian military and intelligence analyst. “The chance of survival is very small. The hope gets thinner.”

       The Nanggala was taking part in torpedo drills before dawn on Wednesday when it requested permission to descend into the Bali Sea. The request was granted, but the submarine failed to make further contact. Emergency pings to the sub went unanswered. The Nanggala was officially lost.

       Hours later, the navy located oil slicks in the waters around where the Nanggala submerged. On Thursday, it discovered some sort of object with a strong magnetic signature in the same area, at a depth of 50 to 100 meters.

       But it is not certain whether either will lead to the submarine or whether there are any survivors.

       The oil slick could be from a fracture in the Nanggala’s hull, said Julius Widjojono, a spokesman for the Indonesian Navy. It could also be a desperate S.O.S. marker sent out by the crew as the submarine sank for some unexplained reason, naval analysts said.

       The Nanggala is built to withstand the pressure of depths up to 500 meters, the navy said. The rough area of the Bali Sea where the submarine was operating has valleys that plunge 700 meters. Naval experts worry that if the Nanggala lost power and plummeted into one of these underwater trenches, its steel hull could have split open like a soft fruit.

       While the main oxygen reserves in the Nanggala are likely to run out at first, submarines are built with backup systems that can provide degraded but still breathable air for some time. The length of time such a system can operate depends on a number of factors, including how well the equipment has been maintained, submarine experts said.

       While 53 people were on the Nanggala when it disappeared, the submarine was built to accommodate 34 crew members, according to specifications released by the Indonesian Navy during an earlier drill. It would not be uncommon for more people to be aboard during drills, but added people would almost surely deplete air supplies more quickly.

       One of those on board the Nanggala as it readied for the torpedo drills on Wednesday is the commander of Indonesia’s submarine fleet.

       Built in 1977 in Germany, the Nanggala was completely refitted in 2012. While the navy maintains that the vessel’s maintenance paperwork is in order, the submarine is 44 years old.

       “Every piece of equipment when it has reached its age, we cannot deny there’s a life cycle cost,” said Connie Rahakundini Bakrie, a defense expert at the University of Indonesia. “However good our maintenance, repair and operations, there’s fatigue.”

       Indonesia’s submarine fleet has another German-made model of the same vintage as the Nanggala, along with three newer South Korean ones. A country of thousands of islands scattered across the Equator, Indonesia is the world’s largest archipelagic nation. But its navy is outdated.

       Submarine accidents are rare, and rescues even rarer. But they do happen. In 2005, seven sailors on board a Russian Navy submarine that had gotten tangled in a fishing net were rescued just a few hours before their oxygen would have dissipated.

       Before the Nanggala, only two submarines have gone missing for extended periods of time in recent memory, one from Argentina and the other from North Korea.

       In another case, in 2000, a Russian Navy submarine, the Kursk, sank to the seabed after an explosion on board. All 118 people died after rescue teams took days to gain access to the submarine, and oxygen ran out for the 23 sailors who had survived the blast.

       On Monday, an Indian ship, which is outfitted with a mini submersible that can conduct underwater rescues, is due to arrive in the Bali Sea to help with the search effort. If the backup air filtration system is fully operational, Indonesian defense experts said, any surviving sailors may be able to last until then.

       “I am optimistic,” said Ms. Bakrie, who is friends with some of the crew members onboard the Nanggala. “But, again, if it’s 700 meters, forget it. Nothing can help.”

       John Ismay contributed reporting.