A catamaran capsized off the coast of Brighton with rescue crews called to the scene.
The alarm was raised just before noon on Sunday as the vessel was spotted upside down.
Volunteers for the RNLI in Brighton set off on their inshore lifeboat and sailed to the rescue.
But by the time they made it to the water near Hove Lawns Cafe, where the vessel was spotted, the boat was back upright and had headed inshore.
Later that night, shortly after midnight on Monday, the crew launched after a report of a vessel near Brighton Marina.
The 80ft yacht was not moving, but its navigation lights were on to indicate it was in motion - the captain was not responding to radio messages either.
The lifeboat found the yacht anchored just south of the Marina entrance but the crew were not able to wake anyone up on board.
A spokeswoman for the RNLI said: "It did not pose a risk to other water users."
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