'Ceylon Secret Operation 51' was a British naval attack by Chariot Mk II manned torpedoes on Japanese shipping in the harbour of Phuket island in Japanese-occupied Thailand (28/29 October 1944).

Conveyed to Phuket by the submarine Trenchant, Sub-Lieutenant A. W. C. Eldridge and Petty Officer S. Woollcott boarded one of the 'chariots', and Petty Officer W. S. Smith and Ordinary Seaman A. Brown the other. Their targets were the 5,000-ton Sumatra and 5,272-ton Volpi.

The two 'chariots' were launched at 22.00 on 28 October to cover the final 6.5 miles (10.5 km) to their targets. The two machines arrived at 00.30 but found that the barnacles on the ship prevented the use of magnets to secure their explosive warhead to Sumatra, and Eldridge and Woollcott therefore attached a clamp to the bilge keel and tied the warhead to the clamp. Eldridge and Woollcott set the timer for six hours and began the journey back to Trenchant.

Smith and Brown reached Volpi but found that the ship was resting on the bottom of the harbour and were therefore unable to attach their warhead underneath vessel. Brown boarded Volpi and set explosives in the engine room. Both ships were sunk by the subsequent explosions.

Arriving back at the submarine, the men were ordered to scuttle their 'chariots' by Lieutenant Commander A. R. Hezlet, the submarine’s commander, who believed he had heard the propellers of Japanese ships. The scuttled 'chariots' were uncovered following a tsunami on 1 January 2004, although another report asserted that discovery was made by divers.