An inventor who built the world's smallest submarine in his garage is refusing to retire from record-breaking inventing.

While other men his age may be looking forward to tending the garden, 71-year-old William Smith is working on his latest design - an unsinkable yacht.

Retired RAF officer William, known as Bill, wants to build the world's first self-righting yacht to cap a decade of success with his one-man mini sub, called the Water Beatle in honour of The Beatles' famous yellow submarine.

He believes if a mast on a yacht is designed to be retractable and is attached to a large weight, it could be used like a pendulum to turn it the right way up if it is ever overturned by rough seas.

He said: "I'm really a 71-year-old schoolboy. I still love to design things.The bug is still there.

"I've already made a few sketches for the unsinkable yacht. It's quite a simple idea really, I'm surprised no one's thought of it before.

"There just aren't enough hours in the day to get all of my ideas done but I'm determined to get the yacht finished."

"I must be the only former RAF man to build a submarine, now I'm probably the only one planning to build a boat. I'd love to know how many Navy men have built aeroplanes.

"My children always thought I was a dotty English eccentric and my grandchildren think I'm a crazy inventor but they all love the submarine.

"I'm really proud the world record for it being the smallest submarine has lasted so long."

Bill, who has three daughters and seven grandchildren, intends to take the Water Beatle for a tenth anniversary run later this summer after he has made some modifications.

Bill spent two years building the sub in the garage at his home in Bognor Regis. It was tested in a local swimming pool and took an 80ft plunge one mile off the coast at Bognor in 1993.

When he finally retires from his undersea adventures, Bill says he may leave the mini sub to the Submarine Museum in Gosport, Hampshire, or just leave it in his garden as a plaything for his grandchildren.

Bill received two Air Vice-Marshall's commendations, among the highest honours possible, for his technical work during his 30-year RAF career.