A passenger on a whale watching boat captured video of the surprising moment a humpback whale slapped the vessel with its tail.

David Mulder captured video aboard the Zodiac inflatable boat in Nova Scotia showing the moment the humpback whale approached the boat underwater, lifted its tail into the air and slapped the vehicle's bow on its way back underwater.

"It was more of a tap than a hit," Guy Melville, who captains the Zodiac for Brier Island Whale and Seabird Cruises, told CBC News.

"We were shut down and all of a sudden a whale shows up at our front door," Melville said. "The boat was stationary and the whale was in motion. Then it did its handstand and was vertical."