The dark shapes briefly surfaced near the boat sailing in the darkness off the coast of northern Spain, then there was a thud, followed by an expletive in Spanish.
Orcas had rammed the hull of the Spanish boat, the Urki I, in the early hours of Monday, as the two-person crew tried to divert its course.
But the damage was done: The rudder was broken, and the Urki I had to be towed back to port by a boat from the Red Cross.
Over the past two months, orcas have damaged about a dozen pleasure boats off the Iberian Peninsula from the Strait of Gibraltar to the coast of Galicia, the most northerly point in Spain, baffling marine biologists and sailors.
Although there have been no reports of injuries — at least for humans — scientists and the Spanish authorities have struggled to interpret the interactions.
Were they attacks? Or just friendly encounters from a highly playful mammal that went a little too far?
“Some people are saying they are being playful, and they might be, but they’re playing rough,” said Pete Green, the director of a yacht company who had a boat towed to the Galician port of A Coruña after an orca damaged the rudder this month.
The Urki I was one of three vessels on Monday alone reported to be caught in confrontations with orcas, which are also known as killer whales, according to the Spanish ocean search and rescue agency, which posted videos of the encounters on Twitter.
The hulls and rudders and on all three boats were damaged, and they had to be towed to port.
The largest members of the dolphin family, orcas are highly intelligent and social, with behavior similar to their smaller cousins. Mature orcas can be more than 30 feet long and weigh six tons.
“They love to interact with moving objects, and being near a vessel means wave riding, so there’s speed, there’s interaction,” said Bruno Díaz López, a Galicia-based biologist and director of the Bottlenose Dolphin Research Institute.
“For a calf, the learning process is very important, so play is very important,” he added in a telephone interview on Friday as he was sailing off the coast of northwestern Spain.
Although little is known about their exact migration route, pods of orcas migrate from waters off the Strait of Gibraltar every summer to Western Europe to hunt schools of tuna, so their presence in the area is not uncommon, scientists say. Yet the frequency of the recent incidents with vessels, and the damage they caused, is unusual.
The Spanish Ministry of Ecology said it had recorded 13 encounters since mid-August off the coast of Galicia alone.
The ministry said in an email that it believed the encounters came from a group of four to six young male orcas, which mostly approached medium-size vessels, and always close to the coast.
Victoria Morris, a 23-year-old biology graduate, was crewing on a 46-foot sailboat in the Strait of Gibraltar in July when nine orcas surrounded her vessel. In an interview with The Guardian, she said she went to grab belongings below deck, because she thought orcas would capsize the boat. “The noise was really scary,” she said. “It was so loud that we had to shout.”
Since then, nearly a dozen vessels have encountered orcas, and sailors have described how the interactions made them think that the mammals were trying to lift their boats.
Also in July, Nick Giles was sailing at night when a sudden bang “like a sledgehammer” blow rocked his boat, he told The Guardian. He said his vessel was pushed around for 15 minutes without him being able to steer before the orcas swam away.
Weeks later, members of a Spanish Navy sailing team saw their vessel’s rudder partly dislocated by two orcas, in another tense encounter.
Salvamento Marítimo, the Spanish ocean search and rescue agency, has since then urged vessels to stay away from orcas.
As scientists and the authorities in Spain continue to investigate the phenomenon, Mr. Díaz López said it could be someone else’s problem as the orcas migrated north toward the Bay of Biscay off the coast of Western France. “Let’s see if they cause some damage in French waters soon,” he said.