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Steve Eaton, a 57-year-old from Britain, is planning to run 42 kilometres on Princess CruisesGrand Princess to raise money for a UK charity – a feat that is possibly the first marathon run at sea.

The ship will be sailing across the Bay of Biscay, between Southampton and Cadiz, Spain when he takes on the five-hour challenge on September 22 this year.

A seasoned runner, Eaton has clocked up a total of 33 marathons, but this one will be different. At the end of each circuit he will be required to climb a set of stairs at the forward section of the promenade deck. Eaton recently visited the ship to work out just how many circuits he will need to do to cover the required distance.

All money raised will go to Help for Heroes, a UK charity that assists wounded men and women returning from service in Afghanistan and Iraq.

How many circuits do you think Steve will need to complete?