Work is under way to restore a 17th century naval cannon which is believed to be the only one of its kind in existence.

The iron weapon called a drake was recovered from the wreck of the Swan, a small Cromwellian warship which was lost off Mull while attacking Duart Castle in 1653.

The cannon is being restored by experts from the National Museums of Scotland (NMS) in Edinburgh.

Dr Colin Martin with the iron cannon

The variety and quality of the finds from the wreck have led to it being nicknamed Scotland's Mary Rose.

Historians say the cannon is one of the most significant underwater archaeological finds recently made in Scotland.

It was discovered by Dr Colin Martin of the University of St Andrews, who has been excavating the eroding wreck since 1992 in collaboration with NMS and Historic Scotland.

He said: "At first sight it looks like a very ordinary cannon, with two inscribed marks. One records its weight, totalling 415lbs.

Conservation work

"The other consists of the letters JB set on either side of the touch-hole.

"These are the founder's initials, and are almost certainly those of John Browne, King Charles I's royal gunfounder."

The cannon is now being carefully restored

Layers of dirt which has built up over hundreds of years are now being painstakingly removed from the cannon.

The conservation work so far has revealed that it is probably the only iron example of this type of gun known to have survived into modern times.

NMS conservator Dr Theo Skinner said: "The Swan is a very exciting shipwreck, and there have been many amazing discoveries over the past few years.

"But this cannon is proving to be one of the most important finds so far."

In the 1620s Browne developed a completely revolutionary new type of gun, one which was much lighter for the weight of shot it fired, allowing more to be carried on the king's ships.

King's ship

These stronger and lighter new guns were called drakes, and the secret of their success was a tapered end to their bores, where the pressure of the gunpowder explosion was greatest.

It increased strength at the critical point and allowed the overall weight of the gun to be drastically reduced.

The cannon recovered from the Swan had a bore of 82mm and fired a 4lb shot.

The Swan, built in 1641, was the last and one of the smallest ships to be built for Charles I before he was overwhelmed by the Civil War.