It was a great day for sailing, until a submarine surfaced in the middle of it.

But that’s what happened during the International Yachting Week competition, which brought hundreds of competitive sailors to the Argentine coastal city of Mar del Plata earlier this month.

During a Feb. 6 sailing race for 200 kids ages 12 to 15, an Argentine navy submarine surfaced dangerously close to the young sailors.

The submarine didn’t collide with any of the small boats being raced—which are merely eight feet long and can easily capsize—but it did snag an orange buoy used to mark the racing course, forcing the race to be canceled.

The submarine involved, judging by its shape and markings, is the 1,140-ton, 177-foot-long Type 209-class ARA Salta. A diesel-electric submarine, Salta has been in service with the Argentine navy since 1973. She served during the Falklands War and was modernized in 1995. Lately the sub has spent just a few hours per year submerged, owing to funding shortfalls.

It seems silly, but the Salta’s move was fairly dangerous.

“These boats are made for capsizing, and the kids will have to flip them back. If that would have happened with a submarine it would have been very, very scary,” Juan Romero of the Lauderdale Yacht Club told Miami news station WPLG Local 10.

Did the submarine violate any navigation rules? That depends on a reading of the International Regulations for Preventing Collisions at Sea—a set of rules created by the International Maritime Organization—of which Argentina is a member.

In general, sailing vessels must give way to vessels not under their own command, fishing vessels and vessels “restricted in [their] ability to maneuver,” the rules state.

However, power-driven vessels—like the Argentine sub—must give way to sailing ships. The exception to this rule is when a large, power-driven ship is in a narrow channel and can’t maneuver.

In general, submarines are highly maneuverable compared to surface ships—being able to dive, after all. But the Salta could have been limited by her keel depth. That’s naval terminology for the distance between the lowest part of the submarine’s hull and the sea floor.

The waters are apparently fairly shallow with a depth of 62 to 82 feet, according to a racing participant at the Sailing Anarchy forums. (Salta has a draft of 20 feet.) The forumer said the submarine was spotted approaching the race, and the race organizers attempted to warn the Argentine coast guard before she surfaced.

Luckily no one was hurt.

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