The skipper of a sailboat that struck a whale off the coast of Nova Scotia says the vessel hit the behemoth with such force that the crew thought they had run aground.

Skipper James Muldoon and his crew were taking part in the Marblehead to Halifax Ocean Race, a biennial, 670-kilometre race from Massachusetts to Halifax.

Muldoon said his 24-metre yacht, Donnybrook, was cruising at 12 knots in the middle of the night. The impact stopped the yacht dead in the water.

"I was tossed out of bed, and everybody woke up, believe me," he said.

The crew immediately started bringing out life rafts, thinking they had run aground.

"They noted the position, they got ready to get on the radio in case we had a need. We had everybody pulling up the floorboards to see if we were taking on water," said Muldoon.

"About the time that we were trying to figure out what happened, a whale's tail, or fin, came up next to the boat and put its tail up in the air and flapped it a couple times and swam off," he said.

Muldoon said it was too dark to have spotted or avoided the whale.

He said it was a large whale, but the crew couldn't identify the species.

The collision delayed the boat for half an hour.

Muldoon says divers later found blubber on the sailboat’s cracked keel.

The boat is now in Rhode Island for repairs.

The Donnybrook finished the Marblehead race, taking second place.

It's not clear what happened to the whale.

The Marblehead to Halifax Ocean Race began in 1905 and runs every two years. More that 100 yachts participate in the 670-kilometre race across five different divisions that runs from Marblehead harbour, Mass., northeast to Halifax.