The UK Border Agency said it found 10kilos of the drug, worth up to £5million, on the vessel – thought to have come from Colombia – at Hunterston in Ayrshire.
It is understood the haul was seized from the 150,000-tonne Liberian-registered Bulk Australia on Tuesday after its arrival last week.
Authorities say no one has yet been arrested over the haul.
Drug smugglers have been known to weld boxes full of drugs to the hulls of commercial ships, often without the knowledge of crew, and specialist divers are known to have searched this vessel.
The Hunterston haul is dwarfed by the seizure announced earlier this month of 1.2tonnes of cocaine, worth £300m on a yacht in Southampton.
And in November 2008 Irish police found an estimated £500m worth of the drug aboard another yacht.
That shipment is widely believed to have been organised by Glasgow gangland figures.