A United States court has published a severely critical opinion of tactics used by Sea Shepherd against the Japanese whaling fleet, describing the group as the embodiment of piracy.

The group's founder, Paul Watson, is called "eccentric" in the judgment led by the US Ninth Circuit Court of Appeals chief judge Alex Kozinski.

Sea Shepherd ship the Bob Barker collides with the Japanese whaling fleet fuel tanker the San Laurel. Credit:AFP/INSTITUTE OF CETACEAN RESEARCH

"You don't need a peg leg or an eye patch," Judge Kozinski said.

"When you ram ships; hurl glass containers of acid; drag metal-reinforced ropes in the water to damage propellers and rudders; launch smoke bombs and flares with hooks; and point high-powered lasers at other ships, you are, without a doubt, a pirate, no matter how high-minded you believe your purpose to be."