The Schiedam was a former merchant ship which was captured by pirates, and later sank off the Cornish coast in storm in 1684.
She sank at Dollar Cove, a cove near Helston, Cornwall, and the spot where the wreckage came to rest has recently being exposed by a freak low tide.
Robert Felce went out to the scene and to his shock he found a grenade from the shipwreck.
After more than 330 years in the sea, the grenade was embedded inside an encrusted mass of sediment and pebbles, which are attracted to metal objects in seawater.
Robert, from Mullion, Cornwall, knew what to look for as in previous years he has found two similar grenades from the same wreck.
He said: “I found my third grenade on Dollar Cove at low tide.
“The tide had an influence on the find in that that week was some of the lowest spring tides of the year and it is possible to get out that bit further to examine areas not usually visible.
“The storms and winds have brought lots of changes to Dollar Cove with there being far more rock than sand there now.
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The Schiedam was a former Dutch merchant ship. She was captured by Barbary pirates in 1683 and then seized by the British.
The Schiedam was then used as a transport ship by the Royal Navy and was packed with arms and ammunition from a failed British colony in Morocco.
Split up from the convoy returning home, she found herself trapped in Mount’s Bay and ran aground during a storm on April 4, 1684.
Last November David Gibbins, of Cornwall Maritime Archaeology, shared stunning pictures and video of the wreck site.
The site was rediscovered by David and his colleague Mark Milburn two years ago but this is the first time the site has been so extensively revealed.