5 ECU SS Great Britain

The SS Great Britain, launched in Bristol in 1843, was the first ocean-going vessel driven by a screw propeller — a technology the Admiralty had dismissed as impractical until a famous tug-of-war trial between HMS Rattler and the paddle steamer HMS Alecto settled the argument conclusively in 1845. The ship was also the first large iron-hulled passenger vessel, making her a genuine technological rupture rather than an incremental improvement.

The ECU denomination — pegged to the European Currency Unit — was never legal tender in the UK but was issued under a loophole permitting commemorative trade coinage. Tower Mint produced several such pieces through the mid-1990s before the ECU was superseded by the euro in 1999.