Scientists remain unable to explain a series of recent attacks pods of killer whales have conducted on small boats sailing off the cost of Spain and Portugal, damaging some of the craft to the point they had to be towed back to shore, NPR reports.

At least three boats have been repeatedly struck and damaged by pods of orcas, known commonly as killer whales, a moniker given to them by their skill to hunt seals, dolphins and other prey.  There has never been a fatal attack by an orca on a human recorded, though the attacks on the boats are raising alarms.

In one recent attack, a family was sailing their 37-foot sailboat from France to the Portuguese island of Madeira, off northern Africa, when the craft was jolted by a series of strikes to the hull in the middle of the night.  The family went on deck to see a number of orca making runs at their boat, striking the hull with such force it cracks the boat’s hull and knocked off about 75% of the rudder. 

The attack was similar to at least two other reported incidents that happened in the Atlantic near Europe, where pods of orca disabled two boats by damaging or destroying the boats’ rudders.  Both of those craft were towed back to shore.  No one was injured in any of the attacks, and scientists are not sure if it is one pod of killer whales responsible.

Scientists have speculated that the orca are attracted to the pressure change caused in the water by the boats’ propellers, and that younger orca may be “playing” with the boats without the intent of causing real damage.  Marine biologists note that young orca, particularly males, develop “games” as they age that go in and out of popularity among the whales, and as the orca mature they stop the behavior.

Biologists note that pods of orca in the Pacific have started to play with crab and prawn traps sitting on the sea floor.  Others take a fish they’ve killed and swim with it on their heads for miles.  Both of these behaviors are newly observed.