“I don’t frighten easily and this was terrifying,” skipper David Smith recalls of a late evening in October when his boat was approached by what looked at first like dolphins.

It quickly became apparent that they were much bigger than dolphins. And they were behaving very strangely.

“I looked at this animal – and it was jet black and brilliant white.”

For some two hours, a group of killer whales rammed the underside of the 45ft (13.7m) yacht he was sailing off the coast of Portugal.

“It was continuous,” he says. “I think there were six or seven animals, but it seemed like the juvenile ones – the smaller ones – were most active. They seemed to be going for the rudder, the wheel would just start spinning really fast every time there was an impact.”

David’s job, since he “quit the rat race to sail” back in 2013, is to deliver new boats to where their owners want them moored. In this case, he was part of a team delivering a catamaran from France to Gibraltar.

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An hour before sunset, one of the crew called out.

“He said: ‘It looks like we have some large dolphins,’” recalls David.

The only other encounter he had had with an Orca was more than 20 years ago in a Vancouver aquarium, but he was in no doubt that he was looking at a group of killer whales.

“They were right at the back of the boat”.

Read more: BBC