AARRRRRGH me hearties! If you see Captain Jack Sparrow, tell him: the Black Pearl may be lying on the seabed of the Sound.

And she's fully primed and ready for battle.

A team of divers has hauled a small cannon out of the waters off Plymouth. They are refusing to say which ship they believe the cannon came from, nor where exactly it was lying.

But the weapon appears to be ready for a spot of close quarters combat.

Howard Jones, a commercial diver and amateur marine archaeologist, found the 200kg cannon after researching local shipwrecks.

When Mr Jones and his colleagues Chris Bird and Ray Ives hauled the heavily encrusted weapon ashore at Mount Batten yesterday, it was found to have a wooden plug, or tampion, in the end of the barrel.

"If they were in danger or under threat, they would put the ball and charge in and then tap in the tampion to keep the powder dry," Mr Jones said.

The men immediately took the cannon to Mr Jones's house, where they immersed it in tanks of fresh water.

"It wouldn't last more than a few hours in the air," Mr Jones said.

He is now appealing for experts from the University of Plymouth and any local archaeological groups to come forward to help with an estimated three-year conservation exercise.

Babcock Marine has offered to X-ray the cannon to see what secrets the tampion is concealing.

Meanwhile, Mr Jones is putting together a bigger team in the hopes of going out to the site again in the autumn to search for the ship itself.

No ship's timbers were visible where they found the cannon, but he hoped a search of the sand would reveal more finds.

Chris Bird said they believed the cannon came from a ship that sank "some time during the 18th century" — a time when the most notorious pirate captains still terrorised the Caribbean and the 'Spanish Main'.

He said there were many other larger cannons on the seabed in the Sound, but it was exciting to discover a new one.

Mr Jones said that at just 4ft 6in long and with a barrel estimated at 3in diameter, this was probably a top deck cannon, used for close-quarter fighting.

"But I'm not an expert," he said. "That's why I'd like some experts to come forward and help us to work on this find."

As with anything found in the sea, the cannon will have to be reported to the Receiver of Wrecks within 30 days.